International Ocean Discovery Program, IODP, JOIDES Resolution, Expedition 401, Mediterranean–Atlantic Gateway Exchange, Earth Systems Models, Site U1385, Messinian Salinity Crisis, Messinian, Tortonian, Miocene, Pliocene, hemipelagite, precession, Atlantic, Portuguese margin, Principe de Avis Plateau
Funded by NSF OCE1326927, ECORD, and JAMSTEC
Flecker, R., Ducassou, E., Williams, T., and the Expedition 401 Scientists
Proceedings of the International Ocean Discovery Program Volume 401
R. Flecker, E. Ducassou, T. Williams, U. Amarathunga, B. Balestra, M.A. Berke, C.L. Blättler, S. Chin, M. Das, K. Egawa, N. Fabregas, S.J. Feakins, S.C. George, F.J. Hernández-Molina, W. Krijgsman, Z. Li, J. Liu, D. Noto, F. Raad, F.J. Rodríguez-Tovar, F.J. Sierro, P. Standring, J. Stine, E. Tanaka, M. Teixeira, X. Xu, S. Yin, and M.Z. Yousfi2
1 Flecker, R., Ducassou, E., Williams, T., Amarathunga, U., Balestra, B., Berke, M.A., Blättler, C.L., Chin, S., Das, M., Egawa, K., Fabregas, N., Feakins, S.J., George, S.C., Hernández-Molina, F.J., Krijgsman, W., Li, Z., Liu, J., Noto, D., Raad, F., Rodríguez-Tovar, F.J., Sierro, F.J., Standring, P., Stine, J., Tanaka, E., Teixeira, M., Xu, X., Yin, S., and Yousfi, M.Z., 2025. Site U1385. In Flecker, R., Ducassou, E., Williams, T., and the Expedition 401 Scientists, Mediterranean–Atlantic Gateway Exchange. Proceedings of the International Ocean Discovery Program, 401: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.401.105.2025
2 Expedition 401 Scientists’ affiliations.
Site U1385 (37°34.2849′N, 10°7.5616′W) was first drilled during Integrated Ocean Drilling Program Expedition 339 (Figure F1) to provide a marine reference section of Pleistocene millennial climate variability. Five holes were cored (Holes U1385A–U1385E) to a maximum penetration of 151.5 meters below seafloor (mbsf) using the advanced piston corer (APC) system (Expedition 339 Scientists, 2013) (Figure F2). The record extends to 1.45 Ma (Marine Isotope Stage 47) with an average sedimentation rate of 11 cm/ky (Hodell et al., 2015).
Site U1385 (37°34.0128′N, 10°7.6580′W) was reoccupied during International Ocean Discovery Program (IODP) Expedition 397 (2022) to deepen the sequence to 400 mbsf, just above the Miocene/Pliocene boundary (~5.3 Ma) (Hodell et al., 2023). Five more holes were cored (Holes U1385F–U1385J) ~500 m from the Expedition 339 location (Figure F2) using the extended core barrel (XCB) system. During Expedition 397, Site U1385 was part of a four site depth transect designed to capture and reconstruct water mass changes during the Pliocene and Pleistocene (Figure F3) (Hodell et al., 2023). Results from Site U1385 demonstrate that the Iberian margin yields long, continuous records of millennial Pliocene–Pleistocene climate variability permitting detailed land–sea comparisons (Hodell et al., 2023). The two deeper water sites, U1587 and U1586, also recovered Late Miocene successions (Hodell et al., 2023).
Expedition 401 used the same Site U1385 designation, coring two holes, U1385K and U1385L. We distinguish between the intervals recovered during the different expeditions by prefixing U1385 with the expedition number if the hole letter is not included: Sites 339-U1385, 397-U1385, and 401-U1385. Holes U1385K and U1385L are located ~20 m east of Expedition 397 Holes U1385I and U1385J, respectively (Figure F2). The approximate water depth is 2584 meters below sea level (mbsl), determined by precision depth recorder, placing it in the core of Northeast Atlantic Deep Water (NEADW) today (Figure F3).
Extending Site U1385’s remarkable sediment archive back into the Late Miocene was one of the primary goals of reoccupying this location during Expedition 401. Equivalent-aged sediments recovered at the deeper Site U1587 (Hodell et al., 2023) can be traced upslope on the seismic profile to Site U1385. These Late Miocene sediments at Site U1587 display well-developed precessional cyclicity (Hodell et al., 2024b). However, the poor carbonate preservation at this greater water depth (3480 m) means that high-resolution carbonate-based proxy records cannot be generated (Hodell et al., 2023). The shallower water depth at Site U1385 (2590 m) likely offers better carbonate preservation, making generating these essential proxy records possible.
Site U1385 is the deepest site cored during Expedition 401 (Figure F3). It is located on an elevated ridge, minimizing the chances of disturbance by downslope transport. In combination with Site U1609 and the two deeper water Expedition 397 sites, U1587 and U1586 (Figure F1), the Late Miocene records from Site U1385 complete a depth transect equivalent to the Pliocene–Pleistocene transect generated by Expedition 397 (Figure F3). The objective was to drill a single hole (U1385K) to recover the deeper part of the section below Site 397-U1385 to the late Tortonian (Figure F4) by washing down to 385 mbsf (just 15 m above 400 mbsf) and then recovering a further 503 m (30 + 473 m) of sediment to reach the drilling target depth of 873 mbsf. However, negligible recovery over the Miocene/Pliocene boundary interval in Hole U1385K meant that an additional hole, U1385L, was required to obtain the desired record.
The specific objectives of Site 401-U1385 were as follows:
Holes U1385K and U1385L were cored after drilling down to start coring a little shallower than the 400 mbsf reached at this site during Expedition 397.
The 154 nmi voyage from Site U1610 to Site U1385 (37°34.0099′N, 10°7.6370′W) took 12.8 h at an average speed of 12.0 kt. We lowered the thrusters at 1622 h on 12 January 2024 and switched to dynamic positioning mode at 1652 h. Site U1385 was cored to 151 mbsf during Expedition 339 in 2011 and to 400 mbsf during Expedition 397 in 2022. The APC/XCB bottom-hole assembly (BHA) was assembled with a polycrystalline diamond compact (PDC) bit and lowered toward the seafloor, and a pipe-cleaning pig was pumped down to clean the inside of the drill pipe. The ship was positioned 20 m east of Hole U1385J, with the seafloor tagged at 2595.4 mbsl.
Hole U1385K was started at 0035 h on 13 January and was drilled ahead, reaching 385.0 mbsf at 1615 h. The center bit was retrieved, and we started coring. Cores 2X–6X returned nearly empty, so we ran the bit deplugger to ensure that the bottom of the BHA was free from loose sediment that may have entered during the preceding drill down. Recovery improved for subsequent cores, and we continued coring through Core 21X at 1035 h on 15 January, having reached the age target of 8 Ma. Cores 2X–21X penetrated from 385.0 to 552.5 mbsf and recovered 127.9 m (76.4%) (Table T1). We pulled up the pipe, clearing the seafloor at 1328 h on 15 January and ending Hole U1385K.
The ship was offset 20 m north, and we started Hole U1385L at 1455 h on 15 January 2024, drilling ahead without coring to 376 mbsf. Coring began at 0330 h on 16 January, and Cores 2X–8X penetrated from 376.0 to 443.9 mbsf, recovering 58.4 m (86.1%) (Table T1). We stopped coring at 1645 h because of the high wind and wave conditions forecast for the evening and next day. The bit cleared the seafloor at 1835 h and was pulled up to 332 mbsl while the ship began waiting on weather. After coring Hole U1385K, we had a choice between going directly to the next site or staying to core Hole U1385L; we prioritized collecting core from Hole U1385L, knowing that it would mean waiting on weather the following day.
The ship began waiting on weather at 2315 h on 16 January. At midmorning on 17 January, the average wind speed reached 35 kt, gusting to 65 kt, and then it eased throughout the day. After waiting on weather for 28.25 h, we were able to resume operations at 0330 h on 18 January. The BHA was raised, disassembled, and stowed. The thrusters were raised at 0720 h, and at 0736 h we started the transit to Site U1611, ending operations at Site U1385.
Two lithologies were described for Site U1385: calcareous clay and clayey calcareous ooze (Figures F5, F6, F7; see HANDDRAWN in Supplementary material). Smear slide analysis, additional geochemistry data (e.g., coulometry and X-ray diffraction [XRD]), and physical properties data were integrated with visual core description to aid in lithologic identification and the definition of three lithostratigraphic units and their boundaries (Figures F6, F7). Unit I (385.0–458.5 m core depth below seafloor, Method A [CSF-A] in Hole U1385K and 376.0–442.5 m CSF-A in Hole U1385L) is composed of calcareous clay and clayey calcareous ooze. Subunit Ia (385.0–430.0 m CSF-A in Hole U1385K and 376.0–430.5 m CSF-A in Hole U1385L) is relatively homogeneous with gradational alternations between calcareous clay and clayey calcareous ooze (Figure F8). Subunit Ib (430.0–458.5 m CSF-A in Hole U1385K and 430.5–442.5 m CSF-A in Hole U1385L) is characterized by sharp contacts between calcareous clay and clayey calcareous ooze. Unit II (458.5–519.6 m CSF-A in Hole U1385K only) includes frequent cyclic alternations between calcareous clay (dark gray and olive-gray) and clayey calcareous ooze. Unit III (519.6–552.65 m CSF-A in Hole U1385K only) predominantly consists of calcareous clay (dark gray and olive-gray) with thinner beds of clayey calcareous ooze.
In general, the clayey calcareous ooze is lighter gray or greenish gray, whereas the calcareous clay is a darker shade of gray, olive-gray, or greenish gray. Minor lithologies include clay- and pyrite-rich intervals that exhibit shades of orange-brown and blue-green. In Units II and III, the calcareous clay is a darker shade of greenish gray and/or olive-gray.
Individual bed thickness for each primary lithology was calculated for both holes (Figure F7). The top and bottom of each core were excluded because they represent only a portion of the bed and would result in an underestimation of bed thickness. For cores composed entirely of a single lithology, the calculated thickness represents the minimum bed thickness. In many cases, the boundaries between lithologies are gradational and the lithologic contacts are placed within the transitional interval. In these instances, the calculated bed thickness may not strictly correspond to the true bed thickness. The lithologic boundaries can be more accurately determined by integrating visual core description information with physical properties data (e.g., magnetic susceptibility [MS] and natural gamma radiation [NGR]) and reflectance spectroscopy and colorimetry (e.g., red-green-blue [RGB] color; Figure F9).
The trace fossil assemblage at Site U1385 is largely similar throughout; however, intervals with abundant trace fossils (bioturbation index [BI] = 3–4) alternate with intervals associated with laminations where bioturbation is absent (BI = 0). Seven ichnotaxa were recognized at the ichnogenus level, including abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; rare Palaeophycus; and occasional Asterosoma and Spirophyton (Figure F10). Undifferentiated horizontal and vertical structures were also observed. Traces with darker and lighter infill can be related to calcareous clay and clayey calcareous ooze, respectively. Crosscutting relationships are frequent among the ichnotaxa.
Smear slides were taken regularly from Hole U1385K (n = 45) for petrographic analysis (Table T2). Smear slides prepared from Hole U1385L (n = 8) were analyzed to determine the lithology in poorly recovered intervals in Hole U1385K and confirm the presence of the same lithologies identified in Hole U1385K. Based on shipboard analyses for Holes U1385K and U1385L, the total carbonate content in these cores ranges 39%–69% (see Geochemistry). XRD analysis was conducted on 5 squeeze cake residues and 14 spot samples collected from the working halves of Holes U1385K and U1385L to gain a general understanding of the bulk mineralogy of different lithologies and identify any large-scale trends with depth. Representative diffraction patterns of the bulk mineralogy of the two primary lithologies in the three units are shown in Figure F11. Primary minerals identified are quartz; calcite; plagioclase; K-feldspar; and clay minerals including chlorite, illite (or mica), kaolinite, and mixed-layer illite/smectite (I/S) and minor dolomite and pyrite. Distinct peaks suggesting the presence of siderite were found in two samples (401-U1385K-8X-6, 94–95 cm [425.08 m CSF-A], and 12X-3, 127–128 cm [459.75 m CSF-A]). The bulk mineral assemblage is uniform downhole from Unit I to Unit III, with the most distinct changes in the relative peak intensities between calcite and other siliciclastic minerals (Figure F11). Overall, total carbonate content and its variation with depth as determined from XRD are consistent with total carbonate content determined by coulometry (Figure F40) based on samples analyzed from the same stratigraphic interval (Figure F12). Because of the relatively large sampling interval for interstitial water (IW) squeeze cakes and the lithology-driven sampling strategy for XRD analysis, the variations in total carbonate content (Figure F12) do not fully reflect the shorter term and subtler lithologic variations that dominate the NGR data (see Physical properties; Figure F41). The relative abundance of different minerals and mineral groups in all the XRD samples with depth in Holes U1385K and U1385L together with organic carbon content (from Geochemistry) is shown in Figure F13.
The dominant lithologies in Subunit Ia are calcareous clay and clayey calcareous ooze. These lithologies generally vary in color (Figure F8), with calcareous clay having a darker shade of greenish gray (GLEY1 5/5GY and GLEY1 6/5GY) and clayey calcareous ooze having light greenish gray (GLEY1 7/5GY) and greenish gray (GLEY1 6/10GY) colors. Lithologies are commonly distinguishable by color, and gradual color changes throughout Subunit Ia reflect lithologic transitions (Figure F9A). The upward transition from calcareous clay to clayey calcareous ooze is commonly gradational, but the upward transition from clayey calcareous ooze to calcareous clay tends to occur more abruptly.
Because of the subtle and gradational changes observed between lithologies, occasionally physical properties data were used to define lithologic boundaries. For example, calcareous clays have higher NGR and MS values and relatively lower L* reflectance and RGB color values than clayey calcareous ooze (Figures F8, F9A).
This subunit is distinct because of the homogeneous appearance of each of the two lithologies and very gradational changes throughout. The bed thickness in Subunit Ia averages 29 cm (Hole U1385K) to 46 cm (Hole U1385L) for calcareous clay and 42 cm (Hole U1385K) to 62 cm (Hole U1385L) for clayey calcareous ooze. The thickest beds located in the upper part of Subunit Ia were not recovered in Hole U1385K, which may explain the difference in bed thickness between Holes U1385K and U1385L (Figure F7).
Primary structures that are usually too subtle to be easily distinguished in these fine-grained sediments on the sample table can be recognized by careful analysis of the core photographs from the Section Half Imaging Logger (SHIL). Laminae characterized by subtle color variations are present in the calcareous clays in the lower part of Subunit Ia below ~415 m CSF-A (Figures F5, F14C, F14E). Specifically, Core 401-U1385L-6X has parallel and cross lamination, distinguishable by distinct color differences, and Core 401-U1385K-9X has parallel lamination. Overall, Subunit Ia calcareous clay shows color changes throughout and commonly exhibits intercalations between several shades of greenish gray (GLEY1 5/10Y and GLEY1 5/5G_/1), the latter with a slightly greener tint. No distinct sedimentary structures were observed in the clayey calcareous ooze.
Smear slides show that the dominant texture in both calcareous clay and clayey calcareous ooze in Subunit Ia is clay (Figures F15, F16). The sand fraction is very minor (0%–5%), and the maximum grain size differs between lithologies. The clayey calcareous ooze tends to have fine sand-sized foraminifers, whereas these are generally less abundant and finer grained in the calcareous clay (maximum grain size is very fine sand).
Discrete trace fossils in Subunit Ia are sparse to moderate in abundance (mainly BI = 1 to 2 or 3) and occasionally absent (BI = 0) in the laminated intervals (Figure F17). The trace fossil assemblage consists of abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; and rare Palaeophycus. Gradual and subtle color changes throughout Subunit Ia complicate the distinction of lithologic boundaries and the differentiation between darker infilled and lighter infilled traces in the transition between calcareous clay and clayey calcareous ooze (Figure F17). Even so, occasionally dark Zoophycos penetrate around 20 cm from calcareous clay into the underlying clayey calcareous ooze (Figure F18).
Smear slides show that Subunit Ia calcareous clay contains mainly siliciclastic grains, whereas the clayey calcareous ooze contains a smaller quantity of siliciclastic grains and a higher abundance of biogenic carbonates (Figure F19). Subunit Ia calcareous clay consists of 45%–70% siliciclastic grains, 0%–10% detrital and/or authigenic carbonate, and 25%–45% biogenic carbonate. Subunit Ia clayey calcareous ooze consists of 30%–65% siliciclastic grains, 0%–10% detrital and/or authigenic carbonate, and 30%–60% biogenic carbonate. The content of foraminifers varies throughout this subunit, ranging 2%–10%, and tends to be higher in the clayey calcareous ooze (Figure F19). Glauconite and fish bones are present in minor quantities. Pyrite is typically rare, except in areas in or close to pyrite nodules and in intervals with a slightly greener tint (GLEY1 5/5G_/1).
The bulk mineral composition of five samples of clayey calcareous ooze and one sample of calcareous clay from Subunit Ia were analyzed by XRD. In clayey calcareous ooze, carbonate content ranges 53%–59% (averages = 57%) (Figures F12, F13). Quartz content is ~8%, feldspar (including plagioclase and K-feldspar) content is ~8%, and clay minerals make up 27% on average (Figure F13). The calcareous clay sample has lower carbonate content (42%) and higher quantities of quartz (11%), feldspars (10%), and clay minerals (35%). Both clayey calcareous ooze and calcareous clay have relatively low organic carbon content (average = 0.3%).
Subunit Ib comprises similarly alternating calcareous clay and clayey calcareous ooze with variable thicknesses as in Subunit Ia. Subunit Ib is distinguished from Subunit Ia because the lithologies in Subunit Ib commonly have sharp boundaries characterized by distinct color changes. The calcareous clay is greenish gray (GLEY1 5/10Y and GLEY1 6/5GY) and occasionally gray (10YR 6/1), whereas the clayey calcareous ooze is light greenish gray (GLEY1 7/5GY) and occasionally gray (10YR 6/1) (Figures F8, F20). The upward transition from the calcareous clay to the clayey calcareous ooze is gradational, but the upward transition from clayey calcareous ooze to calcareous clay is commonly sharp (Figure F9B).
Lithologic changes in Subunit Ib are reflected by color changes as well as physical properties and color variations. This is particularly well illustrated in Core 401-U1385K-11X (Figure F20), where calcareous clays have higher NGR values and lower L* reflectance and RGB color values than the clayey calcareous ooze (Figures F8, F20). Physical properties data were used to help define the position of lithologic boundaries where lithologic changes are gradational and subtle.
Bed thickness in Subunit Ib averages 57 cm (Hole U1385K) to 54 cm (Hole U1385L) for calcareous clay and 40 cm (Hole U1385K) to 45 cm (Hole U1385L) for clayey calcareous ooze.
The clayey calcareous ooze lacks any distinct sedimentary structures. The darker calcareous clay has occasional color banding with a slightly greener tint (GLEY1 5/5G_/1) and subtle parallel and cross lamination (Figure F14B).
Smear slides show that the dominant texture in both calcareous clay and clayey calcareous ooze in Subunit Ib is clay (Figures F15, F16). Subunit Ib calcareous clay contains 0%–1% sand, 15%–20% silt, and 80%–85% clay, and the clayey calcareous ooze is a little finer grained with 0%–2% sand, 2%–10% silt, and 90%–98% clay. The maximum grain size differs between lithologies; the clayey calcareous ooze tends to contain fine sand-sized foraminifers, whereas foraminifers are typically less abundant and smaller in the calcareous clay where the maximum grain size is very fine sand.
The relative abundance of discrete trace fossils is sparse to moderate (mainly BI = 1–3) and sometimes absent (BI = 0) where lamination is present. The trace fossil assemblage consists of abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; and rare Palaeophycus. Gradual and subtle color changes throughout Subunit Ib complicate the distinction of lithologic boundaries and the differentiation between darker and lighter infilled traces in the transition between the calcareous clay and the clayey calcareous ooze.
Smear slides show that the clayey calcareous ooze in Subunit Ib contains more siliciclastic grains than that in Subunit Ia (Figure F19). Some calcareous clay samples in Subunit Ib contain a greater proportion of very fine silt-sized authigenic carbonate than those in Subunit Ia, notably in Section 401-U1385K-11X-3 at 64 cm. Subunit Ib calcareous clay consists of 55%–65% siliciclastic grains, 5%–15% detrital and/or authigenic carbonate, and 30%–35% biogenic carbonate. Subunit Ib clayey calcareous ooze consists of 50%–70% siliciclastic grains, 0%–1% detrital and/or authigenic carbonate, and 30%–50% biogenic carbonate. Comparison with XRD and coulometry data suggests that smear slide analysis may have underestimated the amount of carbonate in the Subunit Ib clayey calcareous ooze, as shown by Figure F12. Glauconite, fish bones, and pyrite are commonly present in small quantities.
XRD analysis was conducted on two clayey calcareous ooze samples and two calcareous clay samples from Subunit Ib. The two clayey calcareous ooze samples contain ~67% carbonate, ~6% quartz, ~6% feldspar, and ~20% clay minerals (Figure F13). The composition of the two calcareous clay samples from Subunit Ib is similar to those from Subunit Ia.
Unit II, present in only Hole U1385K, consists of greenish gray calcareous clay, light olive-gray calcareous clay, and clayey calcareous ooze in varying shades of greenish gray (Figure F8). This unit is distinguished from Unit I by the presence of more brownish tones in the light-colored lithologies (Figure F5). Unit II is characterized by a distinct change in dominant colors, with an increased predominance of calcareous clay with a deeper tint of greenish gray (GLEY1 6/10Y, GLEY1 6/5GY, and GLEY1 5/5GY) and brownish tones of gray and light olive-gray (10YR 6/1, 5Y 6/1, and 5Y 6/2), whereas the clayey calcareous ooze is light greenish gray (GLEY1 7/10Y and GLEY1 7/5GY). Several distinctive thin greenish beds (GLEY1 5/5G) are intercalated with the calcareous clays. The clayey calcareous ooze typically has a gradational base and a sharper upper contact with the overlying calcareous clay (Figure F9C).
Where changes between lithologies were subtle and gradational, physical properties data were used to define lithologic boundaries. The greenish gray calcareous clay has higher NGR and MS and lower reflectance (L*a*b*) and RGB than the clayey calcareous ooze (Figure F9C). The gray (5Y 6/1) and light olive-gray (5Y 6/2) calcareous clays typically have intermediate physical properties values between the greenish gray calcareous clay and the clayey calcareous ooze end-members (Figure F9C).
Bed thickness in Unit II averages 75 cm for the calcareous clay and 24 cm for the clayey calcareous ooze (Figure F7). The thickness of the clayey calcareous ooze beds is relatively constant in Unit II, whereas in the upper part of Unit II (458 to ~490 m CSF-A), the calcareous clay beds are thinner than for the lower part of the unit where beds reach a thickness of 2.4 m. This thickness trend is reflected in the proportion of the different lithologies, and calcareous clay becomes less abundant upward (Figure F7).
Overall, the clayey calcareous ooze and gray and light olive-gray calcareous clay do not have any distinct sedimentary structures. The greenish gray calcareous clay, however, does have subtle parallel lamination, low-angle cross lamination, possible lenticular bedding, and color banding in the upper part of Unit II, as in Cores 401-U1385K-13X and 14X (Figure F14A, F14D). These sedimentary structures were not identified in the rest of the unit and are mainly present in the lower portions of the calcareous clay beds directly above the clayey calcareous ooze.
Smear slides show that the dominant texture in both Unit II calcareous clay and the clayey calcareous ooze is clay (Figures F15, F16). Unit II calcareous clay contains 0%–5% sand, 10%–30% silt, and 70%–90% clay, and the clayey calcareous ooze is very similar, with 2%–5% sand, 10%–25% silt, and 70%–90% clay. The maximum grain size differs between lithologies; the clayey calcareous ooze tends to have fine sand-sized foraminifers, whereas the foraminifers are generally less abundant and finer grained in the calcareous clay (maximum grain size is very fine sand).
The bioturbation intensity is generally sparse to moderate (mainly BI = 2–3) and only occasionally absent (BI = 0) in the laminated intervals. The trace fossil assemblage consists of abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; and rare Palaeophycus. Other ichnotaxa, such as Asterosoma and Spirophyton, are occasionally visible. Dark Zoophycos burrows were observed penetrating ~25 cm into the clayey calcareous ooze from the overlying calcareous clay (Figure F18).
Smear slides show that Unit II calcareous clay contains mainly siliciclastic grains, whereas the clayey calcareous ooze contains a slightly smaller quantity of siliciclastic grains and slightly more carbonates (Figure F19). Unit II calcareous clay consists of 35%–75% siliciclastic grains, 1%–10% detrital and/or authigenic carbonate, 15%–50% biogenic carbonate, and 0%–5% biogenic silica. Unit II clayey calcareous ooze consists of 25%–65% siliciclastic grains, 3%–10% detrital and/or authigenic carbonate, 30%–60% biogenic carbonate, and 0%–5% biogenic silica. The content of foraminifers varies from 2% to 8% and tends to be higher in the clayey calcareous ooze. Glauconite, calcareous sponge spicules, diatoms, and fish bones are commonly present in minor amounts. Pyrite is usually rare except where it is concentrated in pyrite nodules and exceptionally in Section 401-U1385K-12X-5 at 4 cm, where it makes up 7% of the smear slide.
XRD analysis was conducted on three clayey calcareous ooze and two calcareous clay samples from Unit II. The clayey calcareous ooze and calcareous clay in Unit II have a similar composition to those in Unit I (Figure F13) and are characterized by variation in the relative abundance of carbonate and siliciclastic minerals. Based on XRD and the coulometry methods applied to 11 samples from Unit II in Hole U1385K, the carbonate content generally increases from the base of Unit II (520 m CSF-A) upward to ~502 m CSF-A, then decreases upward to ~478 m CSF-A, and then increases again upward to the top of Unit II (458 m CSF-A; Figure F12). The general variations in the carbonate content in Unit II can be linked to overall changes in MS, NGR, and reflectance values, especially the L* parameter (Figure F9C). A higher carbonate content corresponds to lower MS and NGR values and higher reflectance values (see Physical properties). Despite the general correlation between the carbonate content and physical properties on a large scale, the higher frequency changes in physical properties were not captured by variations in total carbonate content because of the limited number of samples analyzed through the XRD and coulometry methods.
Unit III is mainly composed of calcareous clay alternating with thinner beds of clayey calcareous ooze (Figures F7, F8). The predominance of calcareous clay results in overall darker hues (greenish gray; GLEY1 5/10Y, GLEY1 6/5GY, GLEY1 6/10Y, and GLEY1 6/5GY) in Unit III compared with the overlying two units. Occasionally, the calcareous clay contains distinctively greenish layers (GLEY1 5/5G). The clayey calcareous ooze is predominantly light greenish gray in color (GLEY1 7/10Y and GLEY1 7/5GY) and typically has a gradational basal contact but a sharper contact at the top (Figure F9D).
Where transitions between lithologies are subtle and gradational, physical properties were used to help define lithologic boundaries. For example, calcareous clay has higher NGR and MS values and lower L* reflectance and RGB color values than clayey calcareous ooze (Figure F9D).
The bed thickness in Unit III has a large range of thicknesses compared to the overlying units, and the calcareous clay has significantly thicker beds than the clayey calcareous ooze (Figure F7). The bed thickness of the calcareous clay ranges 0.4–2.8 m (average = 1.4 m), and the bed thickness of the clayey calcareous ooze ranges 3–28 cm (average = 8 cm). The beds of calcareous clay are probably thicker than the thicknesses calculated here as they may be continuous across sections, but the presence of gaps does not allow the true bed thickness to be accurately determined.
No distinct sedimentary structures were observed in the clayey calcareous ooze and the calcareous clay, but some beds of the darker calcareous clay do contain subtle parallel lamination and color banding in the upper part of Unit III (e.g., in Core 401-U1385K-19X).
Smear slides show that the dominant texture in both Unit III calcareous clay and clayey calcareous ooze is clay (Figures F15, F16). Unit III calcareous clay contains 0%–3% sand, 10%–20% silt, and 80%–90% clay, and the clayey calcareous ooze is very similar, with 1%–5% sand, 10% silt, and 85%–90% clay. The maximum grain size differs between lithologies; the clayey calcareous ooze tends to have fine sand-sized foraminifers, whereas the foraminifers are generally less abundant and finer grained in the calcareous clay, in which the maximum grain size is very fine sand.
Unit III is characterized by generally more trace fossils than Unit II, with moderate bioturbation (mainly BI = 2–3) and occasionally abundant bioturbation (BI = 4). The trace fossil assemblage consists of abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; and rare Palaeophycus. The more conspicuous color change between calcareous clay and clayey calcareous ooze allows for better differentiation between darker and lighter colored infilled traces in the transition between two lithologies (Figure F21).
Smear slides show that both calcareous clay and clayey calcareous ooze in Unit III contain mainly siliciclastic grains. However, the clayey calcareous ooze contains a greater proportion of carbonate (Figure F19). Calcareous clays comprise 70%–80% siliciclastic grains, 0%–10% detrital and/or authigenic carbonate, and 20%–30% biogenic carbonate, and the clayey calcareous ooze consists of 60% siliciclastic grains, 0%–5% detrital and/or authigenic carbonate, and 35%–40% biogenic carbonate. Comparison with XRD and coulometry data indicates that smear slide analysis underestimated the quantity of carbonate in Unit III clayey calcareous ooze (Figure F12). Minor glauconite and fish remains are common, along with very small quantities of pyrite.
XRD analysis was conducted on two clayey calcareous ooze and two calcareous clay samples from Unit III. The composition of clayey calcareous ooze and calcareous clay in Unit III is similar to those in the higher units. Based on the XRD and coulometry analyses of six samples from Unit III, the carbonate content generally decreases upward in Unit III (Figure F12), which corresponds well to the general increase in MS and NGR values in this unit on a large scale (see Physical properties; Figure F8). The organic carbon content is <1% in all four units and does not appear to vary consistently with lithology (Figure F13).
At Site U1385, the dominant Pliocene lithologies recovered during Expeditions 339 and 397 were calcareous mud and calcareous clay, which were attributed to hemipelagic deposition (Expedition 339 Scientists, 2013; Hodell et al., 2024a) because of the combination of vertical (pelagic) settling and slow lateral advection of sediments across the continental margin. Expedition 401 coring of Holes U1385K and U1385L extended this record into the Messinian and late Tortonian. Clayey calcareous ooze and calcareous clay were identified in Units I–III. These deposits are interpreted to be deeper water sedimentary facies with a finer grain size relative to the Pliocene deposits, corresponding to sedimentation in a more pelagic setting (Hüneke and Henrich, 2011; Stow and Smillie, 2020; de Castro et al., 2021).
The dark calcareous clay of Unit I and the upper parts of Units II and III has subtle parallel lamination, low-angle cross lamination, and color banding (Figure F14). These characteristics are normally present in very fine grained contourites (Gonthier et al., 1984; Stow and Faugères, 2008; Hüneke et al., 2021; Hernández-Molina et al., 2022; Rodríguez-Tovar, 2022), but the deposits in Holes U1385K and U1385L are finer grained than the contourites observed in previous studies. Nevertheless, the influence of bottom currents cannot be excluded because bedload transport and deposition of muds (made of clay- and silt-sized sediments) have been increasingly documented in flume experiments, modern muddy systems, and a growing number of detailed facies studies of ancient mudstone successions (Traykovski et al., 2000; Schieber et al., 2007; Yawar and Schieber, 2017; Li and Schieber, 2018). The observed parallel lamination and low-angle cross lamination in Units I and II (Figure F14) could be indicative of weak traction currents (weak bottom current processes?), which would explain the more common sharp contacts separating the clayey calcareous ooze from the overlying calcareous clay. Postcruise research will evaluate this hypothesis with detailed sedimentological analyses.
The trace fossil assemblage (abundant Chondrites, Planolites, and Zoophycos; frequent Thalassinoides; rare Palaeophycus; and occasional Asterosoma, and Spirophyton) can be assigned to the Zoophycos ichnofacies. The Zoophycos ichnofacies is usually associated with fine-grained, low sedimentation rate, pelagic and hemipelagic, nonturbiditic sediments (MacEachern et al., 2007; Rodríguez-Tovar and Dorador, 2014; Dorador and Rodríguez-Tovar, 2015; Rodríguez-Tovar, 2022). This is consistent with low-energy settings. Variable bioturbation intensity (BI = 4–0) can result from changes in paleoenvironmental conditions. Typically, favorable conditions for macrobenthonic trace fossil maker communities are related to organic matter supply, sedimentation rate, and oxygen concentrations (i.e., in bottom water and at the sediment/water interface; Rodríguez-Tovar, 2022). The absence/scarcity of trace fossils in some intervals suggests some or all of these conditions were not present. The absence of bioturbation in laminated (parallel and cross lamination) intervals may be attributed to short periods of higher energy conditions, although other limiting factors of bioturbation (e.g., changes in sediment characteristics) cannot be fully discarded.
Similar trace fossil assemblages are observed in both the calcareous clay and clayey calcareous ooze, suggesting similar depositional and ecological conditions. The trace fossil assemblage appears to be unaffected by gradational to sharper contacts between lithologies. However, typically the trace fossil abundance is higher below the sharp contact between the clayey calcareous ooze and the overlying calcareous clay (Figure F21). This is particularly reflected by the lower bioturbation intensity in the calcareous clay directly overlying the clayey calcareous ooze, which can be associated with the higher energy conditions during the initial deposition of calcareous clay.
Site U1385 is located on a plateau, the Promontório dos Príncipes de Avis, on the lower continental slope of the southwestern Iberian margin (e.g., Terrinha et al., 2003; Maestro et al., 2015). This plateau represents an area without direct influence of submarine canyons or sedimentary input from slope deposition (see Background; Figure F1), which is generally consistent with the dominant pelagic clayey calcareous ooze and calcareous clay deposits recovered. Precessional-scale climatic variability in the Quaternary sediments may have resulted in fluctuating carbonate and/or siliciclastic input, generating pelagic deposits with variations in carbonate content (Figures F8, F20) (Thomson et al., 1999; Lofi et al., 2016). The higher supply of detrital clays can be attributed to nepheloid layers containing significant amounts of suspended sediment, for example those associated with the Mediterranean Overflow Water (MOW) (Abrantes et al., 2000; Ambar et al., 2002; Magill et al., 2018).
Future research needs to evaluate the influence of regional water mass evolution in the dominant pelagic sedimentation at Site U1385, especially the interplay between the NEADW and the Antarctic Bottom Water (AABW) (van Aken, 2000; Hernández-Molina et al., 2011; Glazkova et al., 2022; Hodell et al., 2024a). This interplay would have been particularly important during the latest Miocene, a key period that marked the reorganization of intermediate and deepwater circulation into a pattern more similar to the one observed at present (Sykes et al., 1998; Billups, 2002; Nisancioglu et al., 2003; Uenzelmann-Neben et al., 2017; Hernández-Molina et al., 2017).
Holes U1385K and U1385L were both washed down to start coring in Early Pliocene sediments. In Hole U1385K, an Early Pliocene assemblage was found in Sample 401-U1385K-2X-CC; however, the next four cores were empty, and no samples were obtained until Sample 7X-CC. An uninterrupted biostratigraphic record was recovered from Samples 7X-CC through 21X-CC, spanning from late Messinian to Tortonian (Figure F22; Tables T3, T4). Hole U1385L recovered the missing part of Hole U1385K, starting with Early Pliocene in Sample 401-U1385L-2X-CC (Table T4).
Samples from all Hole U1385K and U1385L core catchers were prepared and analyzed. Nannofossils and planktonic foraminifers were abundant and relatively well preserved in all samples (Tables T5, T6, T7, T8). Benthonic foraminifers were also analyzed in some of the samples (Table T9). The chronological framework for both Expedition 401 holes at Site U1385 was based on identification of a series of calcareous nannofossil and planktonic foraminifer events (Tables T3, T4). The distribution of calcareous nannofossil and planktonic foraminifer events suggests that the Late Miocene–Pliocene stratigraphy recovered at Site U1385 is continuous (with no hiatuses).
In addition to examining all core catchers from Holes U1385K and U1385L for calcareous nannofossil biostratigraphy, selected samples from Hole U1385K were analyzed to constrain bioevents, paying attention only to marker species. Calcareous nannofossil assemblages are abundant and diverse, and preservation is good. Small placolith species (<3 µm) dominate most of the assemblage. Inorganic input and reworked species are rare in a few of the samples analyzed (Tables T5, T6).
Two nannofossil events were defined in Hole U1385L, and six events were found in Hole U1385K (Raffi et al., 2006 and references therein; Hilgen et al., 2000a). In Hole U1385L, the highest occurrence (HO) of Orthorhabdus rugosus and lowest occurrence (LO) of Ceratolithus acutus were both recorded between Samples 401-U1385L-4X-CC and 5X-CC (mean depth = 409.11 m core depth below seafloor, Method B [CSF-B]). These two events are close in astronomical age (HO of O. rugosus, 5.23 Ma; LO of C. acutus, 5.36 Ma) (Raffi et al., 2006). Postexpedition high-resolution counts will better define their stratigraphic position.
In Hole U1385K, the first event recorded is the HO of Nicklithus amplificus between Samples 401-U1385K-10X-2, 142 cm, and 10X-4, 134 cm (mean depth = 440.47 m CSF-B) (Table T4). The LO of N. amplificus is recorded between Samples 14X-CC and 15X-CC (mean depth = 489.43 m CSF-B). The next recorded event is the top of the Reticulofenestra pseudoumbilicus paracme, a globally recognized event, between Samples 16X-5, 88 cm, and 16X-CC (mean depth = 502.43 m CSF-B).
To better constrain the nannofossil chronology for this site in the latest Tortonian, the LO of Amaurolithus delicatus and the lowest common occurrence (LcO) of Reticulofenestra rotaria were considered (see Biostratigraphy in the Expedition 401 methods chapter [Flecker et al., 2025a]). Comparison of different sections in the Mediterranean suggests that these events are reproducible and reliable in adjacent geographic areas (Morigi et al., 2007; Hilgen et al., 2000a). They have an astronomical age between 7.24 and 7.26 Ma (Hilgen et al., 2000a) and are therefore biostratigraphically helpful for the latest Tortonian interval (Morigi et al., 2007; Hilgen et al., 2000a). Both events are recorded in Hole U1385K, occurring between Samples 17X-2, 62 cm, and 17X 3, 34 cm (mean depth = 506.67 m CSF-B).
Two geologic timescale 2020 (GTS2020) events (Raffi et al., 2020) are also recorded in Hole U1385K: the HO of Minylitha convallis and the base of the paracme of R. pseudoumbilicus between Samples 17X-CC and 18X-CC (mean depth = 517.37 m CSF-B) and between Samples 19X-CC and 20X-CC (mean depth = 537.83 m CSF-B) (Table T4), respectively. These two events give a minimum age for the bottom of the hole of 7.78–8.8 Ma.
Planktonic foraminifers are well preserved in the sedimentary succession recovered with evidence of some fragmentation. Bioevents were identified by analyzing every core catcher with additional samples taken from working halves between core catcher samples to better constrain their position. Because there are no planktonic foraminifer bioevents close to the Miocene/Pliocene boundary, the first event identified in this record is the LcO of Globorotalia margaritae. This event (dated at 5.98 Ma) is recorded between Samples 401-U1385K-10X-3, 119–121 cm, and 10X-5, 119–121 cm (mean depth = 441.79 m CSF-B), and between Samples 401-U1385L-8X-4, 99–101 cm, and 8X-5, 121–123 cm (mean depth = 440.57 m CSF-B) (Table T3). It becomes common in the foraminifer assemblage at 5.98 Ma as reported by Bulian et al. (2023). Although the age for the first occurrence of this species is dated at 6.08 Ma in the GTS2020, we used the age provided by Bulian et al. (2023) for a significant increase in abundance of this species.
In the latest Messinian, neogloboquadrinid coiling was dextral, but some sinistral specimens were found downcore that are related to the successive coiling changes in Neogloboquadrina incompta (Krijgsman et al., 2004; Bulian et al., 2023). The first sinistral to dextral coiling change in N. incompta during the Messinian was recorded between Samples 401-U1385K-12X-4, 134–136 cm, and 12X-5, 99–101 cm (mean depth = 461.90 m CSF-B). This event was not recorded in Hole U1385L because drilling did not reach this depth. Together with N. incompta, abundant specimens of other species such as Neogloboquadrina atlantica were found. Influxes of this species with sinistral and dextral coiling were found in some samples that could potentially be used in future biostratigraphic studies. The highest common occurrence (HcO) of Globorotalia miotumida was also recorded at the same position as the coiling change in N. incompta. This was expected because the events are almost contemporaneous. Indeed, the best way to identify the first coiling change of N. incompta is the identification of frequent specimens of G. miotumida in the same sample because after this point their abundance is drastically reduced. Abundant sinistral specimens of N. incompta were recorded in Sample 401-U1385L-8X-5, 121–123 cm (441.43 m CSF-B). This peak in abundance of sinistral N. incompta must be related to some of the fluctuations in coiling after the first coiling change because this sample does not contain G. miotumida.
The Tortonian/Messinian boundary, which is coincident with the LcO of G. miotumida, is recorded between Samples 401-U1385K-17X-2, 114–116 cm, and 17X-3, 117–119 cm (mean depth = 507.37 m CSF-B). The age for this event is 7.24 Ma. Immediately below the LcO of G. miotumida, Globorotalia menardii 5 drastically decreases in abundance, although the low-resolution sampling did not allow us to identify this event. This species usually coexists with G. miotumida in the northeast Atlantic during a short period of less than a single precession cycle. We did not use this bioevent here because it occurred only a few thousand years later than the LcO of G. miotumida. The LcO of G. menardii 5, typically with dextral coiling (dated at 7.36 Ma), is recorded between Samples 17X-4, 114–116 cm, and 17X-5, 114–116 cm (mean depth = 510.27 m CSF-B). The depth of this event may not be very precise; quantification at higher spatial resolution is needed to locate it better.
The HcO of G. menardii 4 (7.51 Ma) is located between Samples 401-U1385K-18X-1, 119–121 cm, and 18X-3, 19–21 cm (mean depth = 515.87 m CSF-B) (Table T3). Better quantification with a higher resolution biostratigraphic study is also needed to identify this event more precisely.
No other planktonic foraminifer bioevents are identified below 7.51 Ma. However, the low sedimentation rates and large number of (probably precessional) cycles visible in the physical properties data indicate that the bottom of Hole U1385K is likely to be Tortonian, probably around 8.8 Ma.
A total of 10 core catchers from Hole U1385K and 4 core catchers from Hole U1385L were processed and studied for the abundance of more than 64 species of benthonic foraminifers. The level of preservation of the benthonic foraminifers was noted along with their abundance with respect to planktonic ones.
The >150 µm fraction of washed samples was analyzed for calcareous microfossil content. Although preservation of the foraminifers is good in all the samples analyzed, benthonic foraminifer specimens are rare. Benthonic foraminifers are <4% of the total foraminifer population in most samples, indicating that the site is deeper than 1200 m throughout the interval recovered, consistent with the water depth today of 2585 m. The benthonic foraminifer fauna mainly belong to the genera Cibicidoides, Globocassidulina, Gyroidinoides, Lenticulina, Melonis, Nuttalides, Oridorsalis, Pullenia, Sigmoilopsis, Stilostomella, and Uvigerina in varying proportions. The absence of shallow-water benthonic species in the samples indicates that the benthonic fauna is autochthonous and indicative of bottom water conditions.
The abundance of the elevated epibenthonic species like those of Cibicidoides spp. and Textularia spp. is between 7% and 25% in most samples with a high of 32% in Sample 401-U1385K-12X-CC (Figure F23; Table T9). Study of recent benthonic foraminifer assemblages from the Gulf of Cádiz region have shown that the abundance of elevated epibenthonic species is positively correlated with bottom water current velocities, and the elevated epibenthonic assemblages in the Gulf of Cádiz region have been used as an indicator for MOW (Schönfeld, 1997, 2002a, 2002b; Schönfeld and Zahn, 2000). The low abundance of the elevated epibenthonics at Site 401-U1385 suggests the site was not directly under the effect of MOW or any high-velocity current in the studied interval.
Globocassidulina subglobosa is a species that thrives in oligotrophic environments with episodic input of phytodetritus (Gooday, 1993; Szarek et al., 2007). It makes up to 17%–23% of the benthonic fauna between Samples 401-U1385K-14X-CC (mean depth = 484.58 m CSF-B) and 19X-CC (mean depth = 532.49 m CSF-B) (Figure F23; Table T9).
Uvigerinids have been associated with high surface water productivity and low bottom water oxygen content by previous workers (Zhu, 2001). Uvigerina peregrina and Uvigerina hispida are the common Uvigerinids at Site 401-U1385, and their abundances are low compared to the other Expedition 401 sites (ranging 2%–10% in most samples). However, they become more abundant (24%–26%) in Samples 401-U1385L-5X-CC (mean depth = 414.21 m CSF-B) and 401-U1385K-16X-CC (mean depth = 503.98 m CSF-B) (Figure F23; Table T9).
The presence of Nuttalides umbonifera is noted in all samples, although it is common (>17% abundance) only in Sample 401-U1385L-3X-CC (mean depth = 393.715 m CSF-B) (Figure F23). N. umbonifera has been associated with AABW in the modern ocean (Hodell et al., 1985).
The present study is quite low resolution, but preliminary results indicate that with higher resolution sampling it will be possible to identify periods of oxygen-rich/-poor bottom waters and eutrophic/oligotrophic environment and evaluate the likely impact of AABW/North Atlantic Deep Water at the site.
Paleomagnetic investigation of 16 cores from Hole U1385K and 7 cores from Hole U1385L focused on demagnetization of the natural remanent magnetization (NRM) of archive-half split core sections and discrete samples of the working-half split core sections. Pass-through paleomagnetic measurements were performed using the superconducting rock magnetometer (SRM) to investigate the NRM on a total of 132 archive halves (91 archive halves from Hole U1385K and 41 archive halves from Hole U1385L). No pass-through measurements were made on core catcher sections. Alternating field (AF) demagnetization was performed on the SRM by applying stepwise peak fields of 5, 10, 15, and 20 mT, with measurement of the remaining magnetization taken at 2 cm resolution. SRM measurements were sporadically disturbed by the occurrence of artificial flux-jumps; these results were removed from the database. Next, the SRM data were processed to remove all measurements collected from anomalous peak intensity intervals and all measurements that were made within 5 cm of the section ends, which are biased by measurement edge effects. A moving window of 1 m was used for smoothing the inclination values to suppress the influence of data points that deviate significantly from the expected geocentric axial dipole (GAD) inclination (57°) at the site latitude (37.3°N). The processed NRM, inclination, and intensity data after 20 mT peak field AF demagnetization are shown in Figures F24 and F25 and listed in PMAG in Supplementary material and Tables T10 and T11.
A total of 117 discrete oriented cube samples were collected from the working-half sections using an extruder. For each core, 3–7 discrete samples were taken, avoiding visually disturbed intervals. First, the anisotropy of magnetic susceptibility (AMS) and bulk MS were measured on all samples using the MFK2 KappaBridge unit. Next, the NRM of 61 cube samples (47 samples from Hole U1385K and 14 samples from Hole U1385L) was measured on the AGICO JR-6A spinner magnetometer. Stepwise AF demagnetization was performed at successive peak fields of 0, 5, 10, 15, 20, 30, 40, 50, 60, and 70 mT up to a maximum of 80 mT for the Hole U1385K samples and up to a maximum of 40 mT for the Hole U1385L samples where the magnetic signal was weaker (similar to the equivalent upper part of Hole U1385K). When magnetization became erratic, demagnetization was stopped. The inclination data after 20 mT peak field AF demagnetization of the discrete samples are displayed in Figures F24 and F25 and listed in Tables T12 and T13.
AMS results for both Holes U1385K and U1385L show an overall vertical direction of the K min axis in agreement with a sedimentary fabric of horizontal strata and a vertical drill hole (Figure F26).
The NRM intensity measured on the SRM in Hole U1385K ranges from about 1.0 × 10−4 to 1.0 × 10−1 A/m and exhibits values between 455 and 535 m CSF-B, that are two orders of magnitude higher than the overlying and underlying cores (Figure F24). The high-intensity signal is most likely related to the presence of magnetite in the sediment and different paleoenvironmental conditions on the seafloor. Based on the SRM inclination records after 20 mT demagnetization, Hole U1385K shows several polarity reversals. At least four normal polarity intervals and five reversed polarity intervals are easy to identify (Figure F24). NRM intensities of the discrete samples from Hole U1385K range 1.3 × 10−5 to 5.1 × 10−2 A/m, and demagnetization diagrams (Z-plots) are mostly straightforward to interpret (Figure F27). Detailed analyses showed that a normal polarity overprint component was generally removed between 0 and 10 mT, which is probably related to an overprint caused by the core drilling (Acton et al., 2002). After demagnetization at 20 mT, a second component remained with inclinations clustering approximately around normal and reversed polarities. We interpret this to be the characteristic remanent magnetization (ChRM) component. The ChRM component of most samples was successively demagnetized at higher applied magnetic fields up to 80 mT and generally shows a linear decay in magnetization, suggesting magnetite is the main magnetic mineral (Figure F27). The inclinations of the discrete samples at 20–40 mT are generally coherent and confirm the polarities suggested by the SRM inclination values (Figure F24).
SRM measurements for Hole U1385L show NRM intensity ranging 1 × 10−4 to 5 × 10−3 A/m, similar to the upper part of Hole U1385K. The inclination record at 20 mT shows only shallow (between −30° and +30°) values, indicating that the primary magnetic components are not properly resolved (Figure F25). NRM intensities of the discrete samples from Hole U1385L are also relatively weak and range 7.1 × 10−5 to 1.6 × 10−4 A/m. Demagnetization of these samples shows less stable components in the Z-plots and was discontinued after 40 mT (Figure F27). The inclinations of the discrete samples at 20 mT indicate a normal polarity interval at 384–404 m CSF-B enclosed between reversed polarity intervals (Figure F25).
Inclination values of both SRM measurements after 20 mT and JR-6A reveal the magnetostratigraphic pattern in Holes U1385K (Figure F24) and U1385L (Figure F25), which were then combined and correlated to the most recent geomagnetic polarity timescale (GPTS 2020; Raffi et al., 2020) to obtain magnetostratigraphic age constraints for the observed polarity reversals and the core sections and to calculate sediment accumulation rates (Table T14).
The JR-6A discrete sample data for the upper part of Hole U1385L show negative inclinations in Core 401-U1385L-2X and positive inclinations in Cores 3X and 4X (384–404 m CSF-B). In the following interval (404–444 m CSF-B), negative inclinations are dominant in both the Hole U1385L and U1385K records, even though inclination values of the SRM records do not approach the expected antipodal GAD inclination at the coring site (−57°), and the results of the discrete samples are also somewhat scattered (Figures F24, F25). This interval is also poorly resolved in the partly overlapping SRM data of Holes U1385F–U1385H and U1385J (Expedition 397; Hodell et al., 2023). Knowing from the previous results of Expedition 397 that we started coring in the lowermost Pliocene, the reversed–normal–reversed polarity sequence most likely corresponds to Chrons C3n.3r, C3n.4n (Thvera), and C3r, respectively. Below this, four intervals with normal polarity and three intervals of reversed polarity are well established in Hole U1385K and demagnetization results from discrete samples are straightforward. The upper two normal polarity intervals at 444–458 and 468–484 m CSF-B correlate to Chrons C3An.1n and C3An.2n, respectively. The next normal polarity interval at 502–508 m CSF-B most likely correlates to the Chron C3Bn–C3Br.1n interval in which the Tortonian/Messinian boundary is defined (Hilgen et al., 2000b; see Paleomagnetism in the Expedition 401 methods chapter [Flecker et al., 2025a]). This is in good agreement with the determination of the LcO of the G. miotumida group between 506.6 and 508.1 m CSF-B in Hole U1385K (Table T3), the planktonic foraminifer bioevent that closely coincides with this stage boundary. The lowermost normal polarity intervals (520–522 and 524–540 m CSF-B) correlate best with the Chron C3Br.2n–C3n.2n interval, indicating that the multiple short (20–30 ky) polarity intervals in the latest Tortonian part of the GPTS are not well resolved, probably because the resolution of discrete samples is too low and as a result of poor determination of magnetic components in the SRM records. The reversed polarities of the discrete samples of the lowermost Core 401-U1385K-21X imply that the base of Hole U1385K is older than 8.2 Ma. The interval with high-intensity NRM signals is dated between 7.8 and 6.3 Ma.
The magnetostratigraphic correlation of the observed polarity pattern to GPTS 2020 was used to calculate sediment accumulation rates for the combined record of Holes U1385K and U1385L (Figure F28). This curve indicates an average accumulation rate of ~5 cm/ky for the Messinian part of the record.
Headspace gas analysis was performed as part of the standard protocol required for shipboard safety and pollution prevention monitoring. A total of 16 headspace samples were analyzed from Hole U1385K, 1 sample per recovered core (Figure F29; Table T15). Methane (C1) at 23,688 parts per million by volume (ppmv) and ethane (C2) at <1 ppmv were detected in the shallowest Sample 401-U1385K-2X-1, 0 cm; thereafter, only lower levels of methane were detected. These concentrations of volatile hydrocarbons do not present a drilling concern. Headspace analyses were not repeated in Hole U1385L, which targeted a depth range that overlapped Hole U1385K. Comparison with earlier drilling of shallower stratigraphy in Expedition 397 Hole U1385G (Hodell et al., 2024a) shows the overlying zone of methanogenesis by microbial degradation of organic matter (Figure F29).
IW was extracted from a total of 13 whole-round samples from Hole U1385K at a sampling resolution of 1 sample per XCB core (Tables T16, T17). Samples for IW analyses were not collected in Hole U1385L, which overlapped the depth range of Hole U1385K. Data from Hole U1385K are presented in comparison to those from Expedition 397 Hole U1385G (Hodell et al., 2024a), which recovered IW samples from 0 to 395 m CSF-A. Some IW analyses were also performed during Expedition 339 in Holes U1385A and U1385B over the uppermost 150 m (Expedition 339 Scientists, 2013), but these data overlap with the records from Hole U1385G and are therefore not presented here. No mudline water or bottom water samples were available, so geochemical properties are compared to the International Association for the Physical Sciences of the Oceans (IAPSO) seawater standard where relevant. Figure F30 shows comparisons between several of the IW analyses.
Alkalinity decreases from a maximum of 4.8 mM at 419.6 m CSF-A (Section 401-U1385K-8X-2) to a minimum of 1.8 mM at 519.5 m CSF-A (Section 18X-4) (Figure F31; Table T16). Comparison with data from shallower stratigraphy in Hole U1385G (Hodell et al., 2024a) shows that the initial trend of decreasing alkalinity in Hole U1385K is the tail of the decline in alkalinity below approximately 250 m CSF-A before values stabilize around 2.4 mM. We interpret the decrease in alkalinity to be predominantly caused by formation of authigenic silicates, which consume sodium, magnesium, and potassium, despite net carbonate dissolution as indicated by the increase in calcium during this interval (Figure F30).
Values for pH for Hole U1385K range from a minimum of 7.5 to a maximum of 7.7 (mean = 7.6) (Figure F31). The pH data are also best understood in context with the overlying trend reported from Hole U1385G (Hodell et al., 2024a), with a continuous decline in pH below approximately 50 m CSF-A, where the sulfate–methane transition zone (SMTZ) is located (Figure F30). Salinity is 32 throughout Hole U1385K (Figure F32; Table T16).
Major ion concentrations (Na+, Cl–, Ca2+, Mg2+, K+, SO4 2−, and Br−) of IW were analyzed by ion chromatography (IC) (Table T16). Sodium and chloride are constant; the mean values are 494.7 and 583.6 mM, respectively (Figure F32). The Na/Cl ratio range is 0.84–0.85. Comparison to shallower IW samples in Hole U1385G (Hodell et al., 2024a) shows consistent Na/Cl ratios but an unexplained offset of approximately 5% in both sodium and chloride concentrations between the data sets from the two different expeditions. It is likely that these offsets, which are also visible in other conservative elements (e.g., Br; Figure F33), are an artifact of analytical methods (e.g., poor pipette calibrations or evaporation during preparation of solutions) during sample or standard preparation. The artifact is probably present for nonconservative elements as well, although the much greater relative changes for reactive elements largely obscure the small (~5%) effect that is most easily observed in the stable concentration profiles of the most conservative elements. For comparisons of IW concentration data between Holes U1385G and U1385K, normalization to chloride concentrations would effectively remove this discrepancy; the data presented below are not normalized in this manner. The decreasing trend in Na/Cl downcore likely reflects a sodium sink in clays or zeolites.
Calcium, magnesium, and potassium concentrations continue trends seen in the overlying IW samples collected from Hole U1385G (Figure F34; Hodell et al., 2024a). Calcium increases with depth in Hole U1385K from a minimum of 9.0 mM at 419.6 m CSF-A (Section 401-U1385K-8X-2) to 13.6 mM at 550.2 m CSF-A (Section 22X-5). This calcium increase with depth in Hole U1385K coincides with a slight decrease in pH and likely reflects net carbonate dissolution (Figure F30). Magnesium shows a slight decrease from 33.1 mM at 419.6 m CSF-A to 27.5 mM at 550.2 m CSF-A. Potassium concentrations in Hole U1385K are relatively constant, ranging 6.2–7.0 mM. The variations in magnesium and potassium are likely caused by minor amounts of authigenic clay and dolomite formation.
Sulfate is consistently less than 1.0 mM from the uppermost sample to 477.7 m CSF-A (Section 401-U1385K-14X-2) and then increases slightly to 2.6 mM at 536.0 m CSF-A (Section 20X-2) (Figure F33). Low sulfate concentrations indicate that the cored interval is deeper than the SMTZ at ~50 m CSF-A (Expedition 339 Scientists, 2013; Hodell et al., 2024a). Bromide shows a constant value of 1.0 mM in Hole U1385K, approximately 0.04 mM higher value than at the bottom of Hole U1385G (Hodell et al., 2024a). The offset between Br concentrations in Holes U1385G and U1385K likely stems from the same procedural or analytical errors reflected in Na and Cl concentrations. The increase in Br concentrations in the shallow part of the profile stems from the degradation of organic matter; otherwise this ion behaves conservatively.
Ammonium and phosphate concentrations in IW were analyzed by spectrophotometry (Figure F35; Table T16). Ammonium concentrations decrease downcore from a maximum of 2527 µM at 419.6 m CSF-A (Section 401-U1385K-8X-2) to a minimum of 1808 µM at 550.2 m CSF-A (Section 21X-5). Phosphate concentrations show minor variation between 3.60 and 4.12 µM in Hole U1385K. Ammonium shows good continuity with the data from Hole U1385G (Hodell et al., 2024a), with the continuous decrease in ammonium with depth likely reflecting downward diffusion below the zone of methanogenesis. Phosphate concentrations show an apparent discontinuity between 8.2 µM at 394 m CSF-A (Section 397-U1385G-42X-5; Hodell et al., 2024a) and approximately 4.0 µM below 419.6 m CSF-A (Section 401-U1385K-8X-2). This discontinuity between the analyses conducted during different expeditions is likely an artifact of data standardization and analytical methods, although the greater magnitude of the difference in phosphate concentrations (Hole U1385K concentrations are <50% of the values at the bottom of Hole U1385G) indicates a different source of the offset than for the much smaller effect observed in Na, Cl, and Br concentrations. Internal data consistency was confirmed for both data sets, and data for Hole U1385K were rerun and verified to be robust. It is possible that the conditions or timing of IW sampling resulted in differential mobilization of P-bearing phases during the two expeditions, as the good agreement between the amount of organic C in the two holes suggests that sudden changes in the availability or degradation of organic matter that could affect IW phosphate concentrations are unlikely.
Trace element concentrations in IW were analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Table T17). All trace elements exhibit continuous trends from the overlying samples collected from Hole U1385G (Figures F36, F37, F38; Hodell et al., 2024a). Small (~5%) offsets may be present given the behavior of the conservative elements, but the scale of changes of these chemically reactive elements, as well as the general scatter in many of the profiles, obscures these smaller analytical artifacts. Lithium increases from 84 µM at 419.6 m CSF-A (Section 401-U1385K-8X-2) to 262 µM at 550.2 m CSF-A (Section 21X-5) (Figure F36), likely reflecting clay mineral diagenesis. Silicon decreases from 211 µM at 419.6 m CSF-A to <150 µM below 450 m CSF-A, and the similarity to the depth profile for alkalinity (Figure F36) suggests that authigenic silicate formation is occurring at depth at this location.
Barium concentrations decrease from a maximum of 80 µM at 419.6 m CSF-A to a minimum of 8 µM at 535.9 m CSF-A in Hole U1385K (Figure F37). Combined with barium data from Hole U1385G (Hodell et al., 2024a), barium shows a clear response at the SMTZ and continues to respond to sulfate concentrations through the mechanism of barite (BaSO4) solubility. Boron shows a gradual decrease from 492 µM at 419.6 m CSF-A to 322 µM at 550.2 m CSF-A with some fluctuations, matching the trend of silicon and likely reflecting a decrease in clay dissolution. Strontium shows an increase from 581 µM at 419.6 m CSF-A to 1460 µM at 550.2 m CSF-A, which accompanies the increase in calcium and indicates calcium carbonate dissolution and/or recrystallization.
Iron concentrations in Hole U1385K were below the detection limit of the ICP-AES methods, whereas manganese ranges 0.8–1.6 µM. Higher concentrations of Fe were recorded in Hole U1385G (10.9 µM at 394.9 m CSF-A; Hodell et al., 2024a), but the apparent difference in Fe concentrations may be related to the same analytical artifact tentatively identified in phosphorus concentrations. A precipitous change in redox conditions is highly unlikely, but it is possible that Fe-phosphorus phases were mobilized in different ways during sampling for the two expeditions.
Sediment samples were collected for analysis of solid-phase geochemistry at a resolution of one sample per recovered core in Hole U1385K (Table T18). These samples came from a fraction of the IW squeeze cake residue (n = 13 samples). Some additional samples were taken from both Holes U1385K (n = 6 samples) and U1385L (n = 3 samples) to identify the geochemical variation across lithologic color cycles identified by the shipboard sedimentologists.
Weight percent CaCO3 varies from 38.6 to 68.9 wt% (Figure F39; Table T18) with a mean of 52.4 wt% and a standard deviation of 8.5 wt%. CaCO3 continues the same amplitude of variability seen in shallower sediments collected in Hole U1385G during Expedition 397 (Hodell et al., 2024a), suggesting that the same range of production and mixing may be occurring and that the variability is likely cyclic in nature. The highest carbonate concentrations presumably reflect higher carbonate productivity and/or lower dilution or dissolution. The range in the CaCO3 measurements represents random sampling of the higher frequency lithologic cycles, detected visually in color variations (see Lithostratigraphy) and in measurements of physical properties of the cores (see Physical properties).
The determination of carbonate by coulometry and XRD analysis agrees very well (Figure F40). A negative correlation (R 2 = 0.58) between the CaCO3 weight percent and adjacent NGR measurements shows that the cycles in NGR are controlled by the balance of carbonate and clay deposition at this site (NGR itself is controlled principally by the concentration of clay minerals) (Figure F41). A weaker negative correlation (R 2 = 0.33) between the CaCO3 weight percent and adjacent MS measurements suggests that the carbonate sedimentary fraction also dilutes the magnetic mineral fraction in the sediments, though, being a minor sediment component, the relationship is not as clear as for that with NGR (Figure F41; Physical properties). The CaCO3 range and cyclicity is aliased by the low resolution and random shipboard sampling across the lithologic cyclicity (similar to previous sites; e.g., Figure F41) and could be refined with higher resolution analyses. In XCB cores, whole-round IW sampling was selected from core sections with more intact lithology, avoiding sediments visibly disturbed by drilling rather than sampling specific lithologies to evaluate their carbonate content. Consequently, some bias may be incorporated into the reported CaCO3 weight percent because part of the natural range may be unsampled.
Total organic carbon (TOC) is calculated to be 0.18–0.97 wt% (Figure F39; Table T18) based on the difference between total carbon (TC) and total inorganic carbon (TIC) (TOC = TC − TIC). Weight percent nitrogen (Figure F39; Table T18) was measured downhole and ranges 0.008–0.048 wt%. The C/N mass ratio (using TOC and total nitrogen [TN]) is used to distinguish the origin of organic matter (marine versus terrestrial) in sediment (Emerson and Hedges, 1988; Meyers, 1997) and may also be influenced (decreased) by the degradation of organic matter (Ruttenberg and Goñi, 1997). The C/N data set ranges 4.8–62.6, with a mean C/N ratio of 13.7 and a standard deviation of 12.6, indicating that the organic carbon is dominantly of marine origin with sporadic input of terrestrial organic matter (Figure F39; Table T18). There is no significant downhole trend in the data, suggesting that marine production and terrestrial inputs as well as organic degradation are broadly consistent across the drilled interval, with variability likely cyclic at frequencies higher than that sampled here. Variability in the C/N ratio may be related to terrestrial inputs or marine productivity changes in discrete samples. However, some component of the variability is likely analytical scatter due to the analytical challenge of obtaining TOC from TC minus TIC, and one outlier with high C/N ratio is associated with a very low measured TN concentration of 0.008 wt%.
Physical properties at Site U1385 were determined in Holes U1385K and U1385L at 2.5 cm spacing on the Whole-Round Multisensor Logger (WRMSL) and the Section Half Multisensor Logger (SHMSL) and at 10 cm spacing on the Natural Gamma Radiation Logger (NGRL) (see Physical properties in the Expedition 401 methods chapter [Flecker et al., 2025a]). Additionally, all cores from Hole U1385L were scanned on the Special Task Multisensor Logger (STMSL) to stratigraphically correlate them to the physical properties data from Expedition 397 and to Hole U1385K. P-wave velocity measurements of the split core section (working half) were determined at two locations in each core section. One or two discrete cylindrical samples were taken from each core for moisture and density (MAD) measurements.
The physical properties data show pervasive cyclicity throughout Holes U1385K and U1385L (Figure F42). Regularly spaced high-amplitude cycles characterize the central zone (from ~330 to ~500 m CSF-A) of the recovered interval, with more variable amplitude in the cycles above and below that zone. A prominent rise in MS and a decline in lightness (L*) (Figure F43) at 458 m CSF-B, corresponds to the Lithostratigraphic Unit II/III boundary (see Lithostratigraphy). This boundary is also conspicuous as a two orders of magnitude downhole increase in magnetic intensity (see Paleomagnetism; Figure F24).
Three large-scale trends define the MS data set (Figure F42). The interval from 380 to 458 m CSF-B exhibits ~1 m wavelength cycles with no variation in baseline level with depth. The amplitude abruptly increases at around 417 m CSF-B. From 458 to 520 m CSF-B, Lithostratigraphic Unit II is characterized by a shift to higher MS values that subsequently decline downcore. MS values increase abruptly at ~520 m CSF-B and decline downcore to 535 m CSF-B, where they return to lower values with lower amplitude variability.
The high MS values correspond to calcareous clays, and low values correspond to clayey calcareous oozes (Figure F8A, F8B, F8D).
The Unit I/II boundary (458 m CSF-B) coincides with both a sharp rise in MS values and the onset of a brief interval (458–482 m CSF-B) of inverse phase relationship between the MS and NGR cycles, where high MS values correspond to low NGR values and vice versa (Figure F44). Below and above this interval, the MS NGR values appear to be in phase, with high and low MS values associated with calcareous clays and clayey calcareous oozes, respectively (Figure F8D). The out-of-phase interval of higher MS values is in the upper part of Unit II, which includes brown calcareous clays (Figure F6), and the peaks in MS, L*, a*, and b* (Figure F43) correlate with thin light brownish (10YR 6/1) layers.
The higher MS values between 458 and 530 m CSF-B coincide with a prominent interval of high magnetic intensity (Figure F24) that extends below the Unit II/III boundary. Postexpedition sampling and analysis is required to determine the precise change in magnetic mineralogy.
Gamma ray attenuation (GRA) density values vary from ~1.77 to 2.01 g/cm3 for Holes U1385K and U1385L. As at the other sites of the expedition, GRA density is affected by core diameter: cores with smaller diameter give low bulk density estimates compared with cores that fill the core liner. MAD bulk density values are more reliable (Figure F45). Cores 401-U1385K-16X and 20X have higher GRA density values than cores immediately above and below, suggesting that core quality is also a factor affecting GRA density values.
The GRA density record consists of thin cyclic variation (~0.5–2 m cycles) superimposed on longer changes and trends in amplitude (Figure F42). Unit I (376–431 m CSF-B), consists of low-amplitude ~1 m thick cycles, with a subtle increase in density values with depth. Unit II is differentiated from Unit I by a downhole reduction in cycle amplitude and a higher baseline value. GRA values generally increase with depth in this unit, probably reflecting a typical sediment compaction trend.
The prominent MS peak at the Unit II/III boundary is absent in the GRA density record (Figure F42). The compaction trend of increasing bulk density continues in Unit III.
NGR fluctuates between 15 and 45 counts/s in Holes U1385K and U1385L (Figure F46). The record can be divided into three intervals (Figure F42): an upper interval (380–417 m CSF-B) characterized by cycles of varying amplitude and thickness centered around 30 counts/s; a central interval (417–504 m CSF-B) dominated by regular high-amplitude cycles; and a lowermost interval (504 m CSF-B to the bottom of Hole U1385K), where thinner (~1 m or <1 m at the bottom part of the NGR record) low-amplitude cycles are superimposed on lower frequency (~10 m) variations.
The three components of NGR, potassium, uranium, and thorium, were derived from the total NGR counts using the MATLAB code developed by De Vleeschouwer et al. (2017). The spectral NGR signal shows similar long-term variations to total NGR counts (Figure F46).
Lightness (L*), green–red (a*), and blue–yellow (b*) color is highly cyclic, consisting of regular cycles with thicknesses ranging ~0.5–1.5 m. The L* record shows the same high-amplitude cyclic pattern as the NGR (Figures F42, F43), whereas a* and b* data show a similar abrupt rise in values at 458 m CSF-B, coinciding with the lithologic color change at the Unit I/II boundary. Only in the upper part of Unit II (458–490 m CSF-B) do the a* and b* records show regular high-amplitude cyclicity. This corresponds to an interval of very well developed color-specific lithologic alternations with relatively sharply defined boundaries (see Lithostratigraphy). Elsewhere, the variability is smaller scale and more irregular, possibly reflecting the gradational nature of many of the lithologic contacts.
At Site U1385, thermal conductivity was measured in triplicate at one position per core, typically on Section 2 of each core, using a half-space probe (H51033). When Section 2 was cracked or deformed, the measurement was made on another section. The average value for thermal conductivity is slightly higher in Hole U1385L (1.93 W/[m·K]) compared to Hole U1385K (1.61 W/[m·K]) (Figure F45), with minimum and maximum values ranging 1.03–2.48 and 1.36–2.19 W/(m·K) for Holes U1385K and U1385L, respectively, except for one anomalous value that yielded a thermal conductivity of 5.53 W/(m·K) at ~435 m CSF-B. Holes U1385K and U1385L show a slowly increasing trend of thermal conductivity values with depth. This trend matches with and continues the thermal conductivity measurements of previously drilled splices at this site during Expedition 339 (Hole U1385A) and Expedition 397 (Holes U1385F and U1385G) (Expedition 339 Scientists, 2013; Hodell et al., 2024a). Thus, when comparing these holes, it is evident that deeper and older sediments have higher thermal conductivity values. Thermal conductivity slowly increases downhole in Holes U1385K and U1385L, except for two outliers present in Subunit IIb: 2.49 W/(m·K) at 436 m CSF-B in Hole U1385K and 5.53 W/(m·K) at ~433 m CSF-B in Hole U1385L (Figure F45), which are anomalous based on thermal conductivity of similar sediments. Unit III shows a top-down increasing trend with some value dispersion in Core 401-U1385K-14X at ~480 m CSF-B.
MAD bulk density slowly increases through Units I–III in Hole U1385K. The general trend for Site U1385 is that thermal conductivity smoothly increases downhole, followed by a similar increase in GRA and MAD bulk densities. Porosity decreases downhole because of compaction, which also explains the depth-increasing density trend. Grain density shows a scattered pattern within a narrow range, with no obvious trend.
P-wave velocities were measured with the WRMSL (P-wave logger [PWL]) for Holes U1385K and U1385L (Figures F42, F47). Unlike the highly scattered values from Sites U1609 and U1610, the PWL of the WRMSL at Site U1385 yielded apparently reliable P-wave velocity values, probably because of minimal gaps between the sediments and the core liners.
Excluding a few outliers, PWL values range 1550–1900 m/s for Holes U1385K and U1385L. Values generally increase with depth and show notable and very similar cyclicity to the MS, NGR, and GRA density data sets. Similar to NGR and MS, the amplitude of PWL cycles is lower in Subunit Ia compared to Subunit Ib. The cycles in Unit II are 1.1–1.5 m thick, and they shift toward thinner cycles in Unit III (0.5–1.1 m thick).
A separate set of discrete P-wave velocities (P-wave caliper [PWC]) were measured on working halves. As shown in Figure F47, PWC and PWL velocities increase with depth. For Hole U1385K, the lowest value (~1584 m/s) is at ~420.9 m CSF-B and the highest value (1855 m/s) is at 542.6 m CSF-B. For Hole U1385L, the lowest value (~1585 m/s) is at ~420.26 m CSF-B and the highest value (1728 m/s) is at 440.36 m CSF-B. Generally, PWC velocities show similar absolute values to PWL velocities. Both sets of velocity measurements likely underestimate the in situ V P velocities, but because there was no downhole logging at this site (Rider, 1985), the extent of this underestimation is not determined.
A total of 35 discrete cylindrical samples were taken from the working halves of each core for Holes U1385K and U1385L for bulk density measurements. As shown in Figure F45, MAD bulk density values are generally higher than GRA values in intervals where the XCB system was employed for coring. However, because discrete samples are free from the negative influence of sediment-liner gaps, MAD bulk density measurements are considered more reliable than GRA densities.
Bulk density calculated from MAD measurements typically varies between 1.84 and 2.08 g/cm3 for Hole U1385K and between 1.85 and 1.98 g/cm3 for the short upper interval recovered in Hole U1385L (Figure F45). Grain density typically ranges 2.71–2.83 g/cm3 for Hole U1385K and 2.72–2.76 g/cm3 for Hole U1385L (Figure F45). Generally, both grain density and porosity decrease with depth (Figures F45, F48, F49).
Discrete porosity values range 39.4%–54.8% in Hole U1385K and 44.6%–51.8% in Hole U1385L (Figure F45). The lowest porosity (39.4%) and highest bulk density value (2.08 g/cm3) measured at ~385 m CSF-B in Hole U1385K correspond to a calcareous mud sample, potentially indicating that in the lithologic cycles, calcareous mud layers have higher bulk density than the calcareous ooze layers (see Lithostratigraphy).
Expedition 401 Holes U1385K and U1385L include cyclic variations in physical properties data that can clearly be correlated from hole to hole (Figure F50) within sediments that date from the Tortonian to the Early Pliocene (see Biostratigraphy). Stratigraphic correlation was first made from Hole U1385L to the short overlapping depth interval cored during Expedition 397 (Holes U1385F–U1385H and U1385J) to make a consistent composite depth scale for these holes. Holes U1385K and U1385L overlap by approximately 30 m (Cores 401-U1385K-7X through 10X correspond to Cores 401-U1385L-5X through 8X), allowing for stratigraphic correlation (Table T19) and the construction of a splice record for Site U1385 (Table T20; Figure F50). However, because most of the Miocene stratigraphy at Site U1385 is recovered in a single hole (either Hole U1385K or U1385L but not both), there are inevitable gaps in stratigraphy that complicate the alignment of the two holes. To gauge how much stratigraphy is missing in each of the core gaps, after the expedition we made a visual correlation between Expedition 401 Site U1385 and Expedition 397 Site U1587 (not shown; Hodell et al., 2024b), which has a complete stratigraphic record and similar stratigraphic patterns despite being at a deeper water depth. Based on this correlation, we adjusted the thickness of the core gaps in Holes U1385K and U1385L in the composite depth scale to reflect the thickness of missing stratigraphy. The affine and splice tables presented here were finalized in May 2024 (Tables T19, T20) and will be revised further to adjust the spacing between cores based on cyclostratigraphic interpretation.
The age model is composed of nannofossil (n = 9), foraminifer (n = 7), and paleomagnetic (n = 12) age control datums between 5.0 and 8.8 Ma (Tables T14, T3, T4). Gaps in age tie points are less of a concern at this site as cyclicity provides evidence for consistency of sedimentation rates, albeit complicated by gaps in recovery. There is a good correspondence between the bioevents and magnetostratigraphic chron boundaries at Site U1385 (Figure F51). These indicate that the sedimentation rate is fairly constant (~50 m/Ma) from just above the Miocene/Pliocene boundary (409 m CSF-B) to 510–520 m CSF-B. Below this there are some biostratigraphic indications that the sedimentation rate declines slightly to the bottom of the hole. This is supported by the cycle thickness measurements made on both the NGR and MS data, which show a shift to thinner beds at this level (Figure F51; Table T21). These lower sedimentation rates are consistent with a low sedimentation rate at Site U1609 in the Tortonian relative to the overlying sediments.
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