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doi:10.2204/iodp.proc.339.101.2013

Site summaries

Site U1385

Background and objectives

Site U1385 is located at 2585 mbsl on the southwestern Iberian margin (37°34.285′N; 10°7.562′W) near the position of Core MD01-2444 (Fig. F8), which contains a detailed record of millennial-scale variability for the last 190 k.y. (Fig. F9). The overall objective of this site was to recover a late Pleistocene sediment record needed to extend this remarkable record to 1.4 Ma. The acquired record will greatly improve the precision with which marine sediment records of climate change can be correlated to and compared with polar ice cores and European terrestrial records.

Site U1385 was occupied on 25 November 2011. Five holes were cored at this site using the advanced piston corer (APC) and nonmagnetic core barrels (Fig. F10). Four holes were cored to ~150 mbsf and one (Hole U1385C) to just 9.5 mbsf (1 core). A total of 67 cores were collected to obtain 622 m of sediment (103% recovery). The advanced piston corer temperature tool (APCT-3) was deployed 12 times.

Main results

The sediment at Site U1385 is defined as a single lithologic unit. Unit I is a very uniform lithology composed of a Holocene–Pleistocene sequence dominated by bioturbated calcareous muds and calcareous clays that vary in the relative proportion of biogenic carbonate material (23%–39%), as shown by color variation from lighter (i.e., more calcareous) to darker (i.e., more terrigenous) sediment (Fig. F11). Relatively more terrigenous-dominated sediment is present in the upper quarter of Unit I, but its occurrence does not warrant the definition of any additional lithologic units or subunits. No primary sedimentary structures were observed; however, bioturbation is the most obvious secondary sedimentary structure and ranges from sparse to moderate. Other features, such as small-scale subvertical microfaults and contorted lamination, are present at several depth intervals. These features are local and of minor importance and do not seriously disrupt the continuity of the stratigraphic section. The entire section cored at this site is therefore considered to be typical hemipelagic deposits with an average sedimentation rate of ~10 cm/k.y.

The five holes cored at Site U1385 provided ample sediment for constructing a complete spliced stratigraphic section containing no notable gaps or disturbed intervals. A primary splice was constructed using all holes and provides an optimal composite section for all physical and magnetic properties measured using pass-through multisensor tracks (Figs. F10, F11). Two nearly complete secondary splices were also constructed, one using intervals from Holes U1385A and U1385B and the other using intervals from Holes U1385D and U1385E. These alternate splices will maximize the core material available for sampling while adhering to IODP sampling policy.

Biostratigraphy at Site U1385 is based on the shipboard study of calcareous nannofossils and planktonic and benthic foraminifers in core catcher samples from Holes U1385A–U1385D and the five lowermost core catcher samples of Hole U1385E. Nannofossils and planktonic foraminifers are very abundant and relatively well preserved in all samples. Benthic foraminifers are also relatively abundant and diverse; however, ostracods are rare and pteropods were not observed in any samples downhole with the exception of the mudline sample. Pollen and spores are generally abundant and moderately well preserved, providing an excellent opportunity for marine-terrestrial correlations. The chronological framework for Site U1385 is mainly based on calcareous nannofossil and planktonic foraminifer events as well as one benthic foraminifer datum, suggesting a continuous Pleistocene record with a nearly uniform sedimentation rate of ~10 cm/k.y. The age of the base of the section is estimated to be ~1.4 Ma.

Natural remanent magnetization (NRM) intensity ranges from ~10^–5 to ~10^–2 A/m. Within the uppermost 50 m, the intensity is on the order of 10^–2 A/m, but below ~60 mbsf the intensity decreases to ~10^–3 to ~10^–5. The correlation between remanent intensity and magnetic susceptibility suggests that the magnetic minerals that carry the NRM are the same grains that dominate the magnetic susceptibility. Magnetic susceptibility generally varies between 10 × 10^–5 and 50 × 10^–5 SI (Fig. F10). The Brunhes–Matuyama polarity transition (0.781 Ma) as well as the termination and beginning of the Jaramillo Subchron (C1r.1n; 0.988 and 1.072 Ma, respectively) are identified at Site U1385. In addition, a brief normal polarity interval is tentatively interpreted to represent the Cobb Mountain Subchron (C1r.2n; 1.173–1.185 Ma). Postcruise paleomagnetic analysis of samples from Site U1385 will provide a reliable record of variations in relative paleointensity of Earth’s geomagnetic field.

The most notable aspect of all physical property records at Site U1385 is a gradual reduction of magnetic susceptibility values beginning at ~30 mbsf (Fig. F10). Despite the reduction in intensity, magnetic susceptibility displays distinct high-amplitude variability until 50 mbsf, being comparatively low and less variable downhole. This main change between 30 and 60 mbsf seems to correspond to a general change in lithology and/or diagenetic overprint (i.e., mirrored in low natural gamma ray [NGR] counts and high L* values) (Figs. F10, F11). A likely cause for the overall decrease of magnetic susceptibility is the reduction of fine-grained magnetite to iron sulfides within the sulfate reduction zone. High-frequency variations show a close correlation to gamma ray attenuation (GRA) densities, likely reflecting varying relative amounts of clay and carbonate. There is also a notable positive correlation between NGR and magnetic susceptibility below 40 mbsf that is not apparent above this depth, hinting at a change in the factors influencing the sedimentary composition during this interval.

Organic carbon is generally low (<1 wt%) and the C/N ratio indicates that the organic carbon is mainly of marine origin. Diagenesis of organic matter has led to depletion of dissolved sulfate in interstitial waters. In this process, sulfate is consumed and alkalinity, ammonium, and phosphate are produced as by-products. The increase in alkalinity promotes authigenic precipitation of carbonate minerals (e.g., calcite and dolomite), consistent with decreases in magnesium and calcium concentrations in the sulfate reduction zone. Hydrogen sulfide ions produced by sulfate reduction and anaerobic methane oxidation can react with iron to form iron sulfide minerals, which are paramagnetic and have lower susceptibility than magnetite. This process may explain the decrease in magnetic susceptibility observed below ~40 mbsf. Interstitial water oxygen and hydrogen isotope values of show considerable variability in the uppermost 30 m, which is unexpected from a profile that should be dominated by diffusion.

The measured geothermal gradient at Site U1385 is ~39.2°C/km, and the estimated heat flow is 47.5 mW/m², which is in the lower half of the normal range for heat flow on the Portuguese margin.

Highlights

Exactly as predicted, drilling at Site U1385 recovered a continuous 1.4 m.y record (lithologic Unit I) of hemipelagic deposits with an average sedimentation rate of ~10 cm/k.y. The multiple-spliced records recovered in five holes provide essential material needed for postcruise studies of millennial-scale climate variability through the Pleistocene and Holocene. Variations in physical properties and color display cyclic changes that reflect changes in the proportion of biogenic carbonate and detrital material delivered to the site. These variations will prove useful to orbitally tune the record at Site U1385, serve as a marine reference section of Pleistocene climate variability, and significantly improve the precision with which marine climate records can be correlated to polar ice cores and terrestrial sequences.

Site U1386

Background and objectives

Site U1386 is located at 561 mbsl on the southern Iberian margin (36°49.685′N; 7°45.321′W) ~25 km south-southeast of the Portuguese city of Faro (Fig. F1). Of all sites drilled during Expedition 339, this site is the most distal from the Gibraltar Gateway, within the depositional sector of the Cádiz CDS. The site targets the eastern end of an elongated mounded and separated drift (the Faro Drift) that has a total length of 100 km, a maximum width of 20 km, and a maximum thickness of ~700 m.

The Faro Drift represents a classic example of middle-slope contourite deposits, which show a well-layered internal acoustic structure with laterally extensive, aggradational to progradational seismic depositional units and widespread discontinuities (Fig. F12). Faro Drift has been developing along the middle slope over the past 4–5 m.y. under the direct influence of MOW and therefore holds a clear signal of MOW through the Gibraltar Gateway, which reopened following tectonic adjustments at the end of the Messinian salinity crisis.

Our primary objective was to recover a key Pleistocene and Holocene sedimentary succession formed under the influence of MU, and hence a clear record of MOW influence on the North Atlantic Ocean. The high rates of accumulation and expanded sedimentary record of this drift site should permit high-resolution examination of past environmental change (climatic and eustatic). This site is complementary to Site U1387, located ~4.1 km southeast, which targets a Pliocene and lower Pleistocene sedimentary record also influenced by MU.

Site U1386 was occupied on 30 November 2011. Three holes were drilled and cored using the APC, extended core barrel (XCB), and rotary core barrel (RCB), achieving the target depth of 526 mbsf in Hole U1386C. Downhole logging was carried out in Hole U1386C using the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Versatile Sonic Imager (VSI) tool strings. Overall recovery at Site U1386 was 351 m (102%) with the APC, 417.6 m (89%) with the XCB, and 82 m (58.5%) with the RCB. The total cored interval at Site U1386 was 954.4 m, and total recovery was 850.6 m (89%).

Main results

The sedimentary succession at Site U1386 extends from the latest Miocene to Holocene (Fig. F13). It is divided into two lithologic units (I and II), distinguished on the basis of inferred depositional process. Unit I is a Pleistocene–Holocene sequence dominated by classic contourite deposition, including nannofossil mud, calcareous silty mud, and silty bioclastic sand lithologies. These three lithologies are generally organized as bi-gradational sequences, the most complete of which coarsen upward from nannofossil mud to calcareous silty mud to silty bioclastic sand and then fine upward through calcareous silty mud into nannofossil mud. Unit I is divided into three subunits (IA, IB, and IC) based on the relative importance of the silty muds and silty sands. Thin turbidite intercalations occur very rarely in the uppermost part of Subunit IA and more commonly in the lowermost 30 m of Subunit IC.

Unit II is a late Miocene–Pleistocene sequence characterized by the deposits of downslope processes interbedded with contouritic and hemipelagic nannofossil muds. The downslope facies include medium- and thick-bedded disorganized turbidites and thick to very thick bedded chaotic debrites. Quartz-lithic–rich turbidites are more common in the uppermost part, whereas debrites and bioclastic (shelly) turbidites are typical of the lowermost part.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, and ostracods) are mostly common to abundant with moderate to good preservation through Unit I but occur more sporadically and with poorer preservation through Unit II. The sedimentary record is continuous for the most part in the Holocene and Pleistocene to ~1.9 Ma, with an average sedimentation rate of 35 cm/k.y. between 0 and 384 mbsf (contourite dominated) decreasing to 15 cm/k.y. between 384 and 453 mbsf (turbidite dominated). Preliminary shipboard dating suggests the presence of a major hiatus before 1.9 Ma, which cuts out much of the early Pleistocene and late Pliocene, and a somewhat discontinuous record through the debrite-dominated section. The lowermost 15 m of the record is most likely of Messinian age (<5.8 Ma).

The observed variability in both benthic foraminifer and ostracod distribution seems to reveal significant changes in depositional processes and bottom water environmental conditions over the last ~5 m.y. In general, marked mixing with shelf-derived taxa in the lower part of the succession is a result of direct input from downslope processes and progressive upward increase in cold-water taxa. Periodic increases in bottom-current energy and ventilation are indicated by both lithologic changes and benthic faunas. Pollen and spores are abundant in all the samples analyzed, along with microcharcoal and dinocysts. Together, these indicate significant changes in terrestrial climate and vegetation comparable with the marine record.

Paleomagnetic measurements identified the Brunhes–Matuyama polarity transition (0.781 Ma) as well as the termination and beginning of the Jaramillo Subchron (C1r.1n; 0.988 and 1.072 Ma, respectively). These events give reliable confirmation of biostratigraphic dating for Site U1386 and confirm the relatively high rates of sedimentation through the contourite succession.

Based on the physical property data, distinct changes were identified that are commonly related to boundaries between defined lithologic units. In addition, cyclic variation in NGR and magnetic susceptibility values and persistent covariation of both parameters with sediment color appear to track decimeter-scale cyclicity in lithologic character. Especially high magnetic susceptibilities correspond to the turbidite-dominated section between 420 and 445 mbsf. Below 445 mbsf, low values in NGR and magnetic susceptibility conform with a shift in the lithology to debrites and turbidites below a major hiatus.

The interstitial water profile at Site U1386 is dominated by organic matter diagenesis with a shallow sulfate reduction zone in the uppermost 12.5 m and methanogenesis below. High alkalinity associated with sulfate reduction and anaerobic methane oxidation has resulted in authigenic calcite and dolomite formation. Iron sulfide minerals formed as a consequence of sulfate reduction. A maximum in δ¹⁸O and δD coincident with a minimum of chloride in interstitial water suggests the influence of gas hydrates that dissociated upon core recovery.

Downhole measurements were made in Hole U1386C to a total depth of 526 mbsf. Despite a certain degree of borehole rugosity, the combination of logs used closely reflects both lithologic changes and cementation recorded in the recovered cores. This allowed us to infer lithologies from some of the gaps in core recovery. Preliminary inspection also revealed a marked cyclicity from 102 to 346 mbsf through the contourite section, which seems to relate to Milankovitch precession cycles of ~20 k.y. These data will be carefully scrutinized together with cycles observed in both the lithologic and physical property records.

The measured geothermal gradient at Site U1386 is ~34.3°C/km, and the estimated heat flow is 42.1 mW/m².

Highlights

Coring at Site U1386 on the Faro Drift recovered a thick Pleistocene–Holocene succession of mud/silt contourites, as anticipated. These successions showed a continuous record of drift sedimentation over the past 1.9 m.y. at an average sedimentation rate of 35 cm/k.y. for the past 1 m.y. What was not expected was to reach into the latest Miocene at ~515 mbsf.

We confirmed the classic contourite model of meter-scale bi-gradational cyclicity with apparent millennial-scale forcing. There was also evidence of a strong lateral supply of terrigenous material to the bottom currents. We also recognized decimeter-scale cycles characterized by relative abundance and thickness of silty contourites. These are provisionally related to seismic cycles with Milankovitch-scale forcing.

At the base of the main constructional drift, interbedded turbidites and contourites are underlain by a 40 m thick unit of early Pleistocene turbidite sandstones, which we tentatively relate to a phase of increased tectonic activity rather than to deposition during sea level lowstand. These directly overlie a major hiatus of 0.3–2.2 m.y. duration, which we provisionally relate to the aggressive action and downward erosion by bottom currents sometime after the onset of MOW. The early Pliocene succession comprises bioclastic debrites and turbidites, most likely the result of widespread tectonic activity and slope instability. These events at the very beginning of the Pliocene might be related to the final stages in the opening of the Gibraltar Gateway.

Site U1387

Site U1387 is located at 559.1 mbsl on the southern Iberian margin (36°48.321’N; 7°43.1321’W) ~29 km south-southeast of the Portuguese city of Faro (Fig. F1). This site is a close companion to Site U1386 at the eastern end of the Faro Drift and part of the larger Cádiz CDS. Although lying only 4 km southeast of Site U1386, interpretation of the seismic records indicates a slightly reduced Pleistocene section and an expanded Pliocene succession.

The Faro Drift represents a classic example of middle-slope contourite deposits, showing a well-layered internal acoustic structure with laterally extensive aggradational to progradational seismic depositional units and widespread discontinuities (Fig. F6). Faro Drift has been developing along the middle slope over the past 4–5 m.y., under the direct influence of MOW. It therefore holds a clear signal of MOW through the Gibraltar Gateway, which reopened following tectonic adjustments at the end of the Messinian salinity crisis.

Our primary objective at this site was to recover a full Pliocene, Pleistocene, and Holocene sedimentary succession formed under the influence of MU and also to penetrate the Miocene/Pliocene boundary. As at its companion site, the high rates of accumulation and expanded sedimentary record of this site should permit high-resolution examination of past environmental change (climatic and eustatic).

Site U1387 was occupied on 8 December 2011. Three holes were drilled and cored using the APC, XCB, and RCB, achieving the target depth of 870 mbsf in Hole U1387C. Downhole logging was carried out in Hole U1387C using the triple combo, FMS-sonic, and VSI tool strings. Overall recovery at Site U1387 was 97 m (103%) with the APC, 578.4 m (97%) with the XCB, and 409.5 m (70.6%) with the RCB. The total cored interval at Site U1387 was 1270.7 m, and total recovery was 1084.95 m (85%).

Main results

The sedimentary succession at Site U1387 extends for 870 m from the latest Miocene to Holocene (Fig. F14) and is divided into four lithologic units (I–IV) distinguished on the basis of lithologic character and inferred depositional process. Unit I is a Pleistocene–Holocene sequence dominated by classic contourite deposition, including nannofossil mud, calcareous silty mud, and silty bioclastic sand lithologies generally organized as bi-gradational sequences. Thin turbidite intercalations occur more commonly than at Site U1386 and are especially noted in the lowermost 100–150 m of the unit.

Below a more significant unconformity, Unit II is capped by two well-cemented dolomite horizons. For the most part, the unit represents Pliocene sedimentation characterized by a clear cyclic arrangement of lighter colored facies having turbidite and/or contourite affinity and darker colored facies of pelagite affinity (Fig. F14). Close interaction between processes is apparent with more or less continuous bioturbation throughout. In Unit III, the evidence of downslope resedimentation is still more evident, including poorly sorted turbidite sandstones, chaotic debrites, and slump units as thick as 5 m. Shallow-water bioclastic debris is common together with resedimented lignitic material in places. Below ~680 mbsf, a 50 m thick section includes hard calcite-cemented sandstone turbidites and poor core recovery. This unit is mainly early Pliocene in age but may start in the latest Miocene.

Unit IV is a relatively thinner basal unit of late Miocene age with a very different character (Fig. F14) dominated by middle-slope hemipelagic sediments to nannofossil muds and muddy oozes.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, and ostracods) are mostly common to abundant with moderate to good preservation through lithologic Units I and IV and relatively poorer preservation through Units II and III, where considerable reworking is also apparent. The sedimentary record is continuous for the most part in the Holocene and Pleistocene to ~1.8 Ma, with an average sedimentation rate of 25 cm/k.y. Below the Pliocene–Pleistocene hiatus (1.8–3.2 Ma) at 450 mbsf (Fig. F14), the mean sedimentation rate is ~15 cm/k.y., although a significant proportion of this section comprises instantaneous event beds. The lowermost unit is most likely of Messinian age (<6.3 Ma).

The observed variability in both benthic foraminifer and ostracod distribution reveals significant environmental changes over the last ~5 m.y., closely comparable with those observed at Site U1386. In general, the Pleistocene succession shows typical upper bathyal assemblages indicative of increased organic matter input and reduced ventilation. Marked mixing with shelf-derived taxa can be seen in the Pliocene part of the succession as a result of direct input from downslope processes and progressive upward increase in cold-water taxa, as noted also by planktonic assemblages. Periodic increases in bottom-current energy and ventilation are indicated by both lithologic changes and benthic faunas. Pollen and spores are abundant in most of the samples analyzed, along with microcharcoal and dinocysts. Together, these indicate normal (fresh) supply from Mediterranean forests and grasslands for the Pleistocene; a transitional zone mixed with corroded, reworked forms; and then a Pliocene succession with a notable absence of freshly derived pollen.

Paleomagnetic measurements identified the Brunhes–Matuyama polarity transition (0.781 Ma) but no clear Jaramillo Subchron. Measurements also identified a reliable Matuyama–Olduvai transition (1.778 Ma) toward the base of Unit I. These measurements give reliable confirmation of the biostratigraphic dating for Site U1387 and confirm the relatively high rates of sedimentation through the contourite succession. Cyclic variation in the intensity of NRM and magnetic susceptibility requires further study.

Physical property data show generally high values of NGR, magnetic susceptibility, and bulk density within the uppermost 50 m of lithologic Unit I, with lower values downhole. Cyclic variation in values and the presence or absence of covariation between different parameters (and with sediment color) show a complex track of decimeter-scale cyclicity noted in lithologic character. In some cases, high magnetic susceptibility values correspond to individual turbidites within the Pleistocene section but not those in the Pliocene.

The interstitial water profile at Site U1387 shows a shallow sulfate reduction zone similar to that found at Site U1386, followed by transition to methanogenesis. This shallow zone is assumed to be related to high rates of organic matter accumulation. High alkalinity associated with sulfate reduction and anaerobic methane oxidation resulted in authigenic calcite and dolomite formation. Iron sulfide minerals also formed as a consequence of sulfate reduction.

Downhole measurements were made in Hole U1387C to a bridged-hole depth of 649 mbsf with the first tool string. Further bridging was found by subsequent tool strings at shallower depths. Moderately severe borehole rugosity severely hampered shipboard interpretation, although the combination of logs used generally reflects both lithologic changes and cementation recorded in the recovered cores. Apparent cyclicity in some parts of the section will require further study following an attempt to correct for variation in borehole diameter. The sonic log and vertical seismic profile will be useful in refining our interpretation of key reflectors on seismic profiles at this site. Only one downhole temperature measurement was achieved at the site because of the shallow change to XCB drilling at ~47 mbsf.

Highlights

Although only 4 km apart, Sites U1386 and U1387 on the Faro Drift showed some interesting and significant differences, as well as similarities. The same thick Pleistocene–Holocene succession of mud/silt contourites was found at Site U1387, indicating a relatively continuous record of mounded drift construction over the past 1.8 m.y. at an average sedimentation rate of 25 cm/k.y. Meter-scale bi-gradational contourite cyclicity was common, with evidence of a strong lateral supply of terrigenous material to the bottom currents, especially before ~1.8 Ma. Decimeter-scale cycles characterized by relative abundance and thickness of silty contourites are most evident on physical property and downhole logs. These correlate well between sites and with Milankovitch-scale forcing. The same extended Pliocene–Pleistocene hiatus (~1.4 m.y.) is recognized as being related to a phase of highly active MOW but within drift rather than at the floor of a channel, which was the case at Site U1386.

Interbedded contourites and turbidites characterize the later Pliocene phase of sheeted drift construction with clear evidence of process interaction. The early Pliocene is dominated by resedimented facies, including debrites, slump deposits, and channel-fill turbidite sandstones. This is further evidence of widespread tectonic activity and slope instability at a time close to the opening of the Gibraltar Gateway.

Site U1388

Site U1388 is located at 663 mbsl on the southern Iberian margin (36°16.142′N; 6°47.647′W) ~50 km southwest of the Spanish city of Cádiz (Fig. F1). This site lies within the extensive Cádiz sand sheet, which is in the proximal part of the larger Cádiz CDS, and is the Expedition 339 site drilled closest to the Gibraltar Gateway

The Cádiz sand sheet represents an important example of very poorly studied sandy contourite deposits on a middle-slope terrace. The seismic profiles show a well-layered internal acoustic structure with laterally extensive, aggradational to seismic depositional units; some thin intervals of lateral progradation; and inferred section discontinuities (Fig. F15). This sand sheet has accumulated on a middle-slope terrace over the past 4–5 m.y. under the direct influence of MOW.

Our primary objective at this site was to recover a full Pliocene, Pleistocene, and Holocene sedimentary succession formed under the influence of MOW close to its exit from the Gibraltar Gateway. Site U1388 was intended as our deepest penetration site (1550 mbsf).

Site U1388 was occupied on 18 December 2011. Three holes were drilled and cored using the APC, XCB, and RCB, but a depth of only 226 mbsf was achieved in the third hole (U1388C). The site was terminated prematurely as a result of hole instability; downhole logging was considered unwise. Overall recovery for Site U1388 was 3.6 m (107%) with the APC, 107 m (47%) with the XCB, and 10.4 m (43%) with the RCB. The total cored interval for Site U1388 was 253 m and total recovery was 121 m (47.8%), which was significantly lower than at the other expedition sites.

Main results

The sedimentary succession at Site U1388 extends 226 m from the mid-Pleistocene to Holocene (Fig. F16) and comprises a single lithologic unit notable for its distinctly sandy nature and consequent low core recovery. The principal lithologies include sand, silty sand, and silty mud, all calcareous in nature with 10%–25% carbonate content, which is part bioclastic and part detrital in origin. On the assumption that much of the missing material is unconsolidated sand, the upper 100 m is sandier than the underlying 50 m. The remainder of the unit comprises alternating 20–30 m thick units of sand- and mud-rich material. A range of bi-gradational, normal-graded, and reverse-graded sequences occur, which are interpreted as mainly contouritic in nature, although further work is required to improve our understanding of the depositional process.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, ostracods, and rare pteropods) are much less abundant than at other sites but generally show good to very good preservation. The section is inferred to reach an age of <0.56–0.7 Ma, with a sedimentation rate of 60 cm/k.y. Temperate to subtropical conditions are indicated by the planktonic foraminifer assemblages. Benthic foraminifers show typical upper bathyal assemblages, some mixing with shelf-derived taxa, and variation in ventilation and/or MOW intensity.

Paleomagnetic measurements from all three holes indicate that only the Brunhes normal polarity chron (C1n) is recorded in these sediments, which confirms the biostratigraphic dating. Neither the intensity of NMR nor magnetic susceptibility values are especially high. Physical property data show a correspondence between higher magnetic susceptibility and sandier layers within the more mud-prone sections; otherwise, the data are relatively poor.

The interstitial water profile at Site U1388 shows a relatively deep sulfate reduction zone in which sulfate reduces to zero levels at ~50 mbsf followed by transition to methanogenesis. This corresponds to a level of increased authigenic dolomite recorded in the cores. An anomalous increase in both Na and Cl is apparent downhole, indicating mixing with deeper saline waters. Total organic carbon values of 0.2–0.8 wt% are relatively lower than at other sites

Highlights

Although drilling was difficult, recovery was generally poor, and the site was terminated early because of hole instability, the sediments recovered form a very important part of the MOW story. High rates of sedimentation and a generally sand-rich section testify to intensified MOW flow coupled with abundant sediment supply. These sandy contourites represent the best record we have to date of this facies type, which appears to be significantly different from the downflow counterparts at other drilled sites. Also, the presence of thick mud-rich intervals records variable MOW intensity over the past ~0.6 m.y. during construction of a sand-prone sheeted contourite drift.

Site U1389

Site U1389 is located at 644 mbsl on the southern Iberian margin (36°25.515′N; 7°16.683′W) ~90 km west of the Spanish city of Cádiz (Fig. F1). This site is one of two sites in the “channels and ridges” sector of the larger Cádiz CDS and is perched on a relative topographic high, which is currently elevated 50–250 m above the flanking contourite channels and lies ~4 km northwest of the Guadalquivir diapiric ridge.

We designate this as the Huelva sheeted/patch drift, which is in fact a small remnant of a much larger middle-slope sheeted drift system that has been dissected by several contourite channels. Huelva Channel lies along the northern flank and Guadalquivir Channel along the southern flank of Huelva Drift. Seismic profiles show a well-layered internal acoustic structure with laterally extensive, mainly aggradational seismic depositional units and widespread discontinuities (Fig. F17). A complex erosional-depositional relationship exists between drift and flanking channel. Huelva Drift has been developing in this region over the past 4 m.y., at least, and is presently under the influence of ML.

Our primary objective at this site was to recover a Pliocene, Pleistocene, and Holocene sedimentary succession formed under the influence of ML and to compare this record with those found at Sites U1386 and U1387, which formed under MU.

Site U1389 was occupied on 21 December 2011. Five holes were drilled and cored using the APC, XCB, and RCB, achieving the target depth of 990 mbsf in the fifth hole (U1389E). Downhole logging was carried out in Holes U1389A and U1389E using the triple combo, FMS-sonic, and VSI tool strings. Overall recovery for Site U1389 was 307 m (104%) with the APC, 464 m (90.5%) with the XCB, and 352 m (54%) with the RCB. The total cored interval for Site U1389 was 1463.4 m, and total recovery was 1123.5 m (77%).

Main results

The sedimentary succession at Site U1389 extends for 990 m from the early Pliocene to Holocene (Fig. F18) and is represented by a thick, rapidly accumulated, and very uniform series of contouritic sediment assigned to a single lithologic unit and divided into five subunits (IA–IE). Unit I is dominated by classic contourite deposition, including calcareous mud, silty mud, sandy mud, and silty bioclastic sand lithologies. These are generally organized as bi-gradational sequences and partial sequences, of which base-cut-out, normally graded sequences are more common than top-cut-out, inversely graded sequences. Carbonate content ranges from 21 to 35 wt%, and total organic carbon ranges from 0.3 to 1.8 wt%. The division of Unit I into five subunits is based on subtle changes in the relative abundance of the different lithologies and silt/sand intervals.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, and ostracods) are mostly common to abundant with moderate to good preservation throughout. Pteropod fragments are more common at this site than at any of the other sites, mainly within the uppermost 30 m. The sedimentary record is continuous through the Holocene and Pleistocene to ~2.1 Ma, with a sedimentation rate between 30 and 40 cm/k.y. A relatively short hiatus (2.1–2.4 Ma) occurs at ~640 mbsf, below which the average sedimentation rate is 25 cm/k.y. There is some evidence from lithologic, physical property, and downhole logging data for two other minor hiatuses at ~0.4 and 0.9 Ma. The deepest part of the section is younger than 3.7 Ma.

Distinctive variability in benthic foraminifer assemblages reveals significant environmental changes through the Pliocene–Holocene succession closely comparable with those observed at other sites. The upper 0.9 m.y. of the Quaternary shows typical upper bathyal assemblages indicative of increased organic matter input and reduced ventilation. The remainder of the Pleistocene shows lower nutrient supply, greater influence of MOW, and significant mixing with shelf-derived taxa. Pliocene assemblages suggest high-nutrient, low-oxygen conditions and generally warmer waters. Pollen and spores are abundant in most of the samples analyzed, along with microcharcoal and dinocysts. Together, these indicate normal (fresh) supply from Mediterranean forests and grasslands for the late Pleistocene; a transitional zone mixed with corroded, reworked forms and no pines; and then a Pliocene succession with mostly corroded conifers.

Paleomagnetic measurements identified the Brunhes–Matuyama polarity transition (0.781 Ma), the top and bottom of the Olduvai Subchron (1.778 and 1.945 Ma, respectively), the Matuyama–Gauss transition (2.581 Ma), the Gauss–Gilbert transition, and potentially three minor excursions. These give reliable confirmation of the biostratigraphic dating for Site U1389, although some of the inferred polarity boundaries need further confirmation.

Physical property data show relatively close tracking of magnetic susceptibility and bulk density with more sand-/silt-rich intervals within the Pleistocene succession but a much more complex pattern within the Pliocene. The downhole distribution of porosities shows higher values in an interval of high interstitial water chlorinity, arguing in favor of lateral advection of brine-related fluids through more permeable strata.

The interstitial water profiles at Site U1389 show distinct maxima in several elements at ~530 mbsf, with relatively sharp transitions above and below. This suggests either a barrier to vertical diffusion or enhanced lateral fluid flux. The increase in concentrations is likely due to dissolution of minerals, most likely carbonates. A strong negative correlation also exists between δ¹⁸O and δD, which is characteristic of clay mineral dehydration reactions that take place at temperatures >50°C. This requires that the fresh signal is a result of fluid migration from a deeper, higher temperature source.

Downhole measurements were made in Hole U1389A to 355 mbsf and in Hole U1389E to a bridged-hole depth of 568 mbsf. A good suite of FMS image logs was obtained in Hole U1389A. A distinct change occurs in log characteristics at ~320 mbsf, which correlates closely with a lithostratigraphic boundary and a zone of poor core recovery. This zone appears to be more sand rich on the basis of borehole logs, although no sands were recovered by coring. Distinct cyclicity is apparent in some parts of the section, corresponding to both lithologic and physical property data. This will require further study. Nine downhole temperature measurements were made in the uppermost 100 m of section, indicating a geothermal gradient of 20.9°C/km, which is relatively lower than at other sites of this expedition.

Highlights

We recovered core to a total depth of 990 mbsf at Site U1389, the deepest penetration of the expedition. The site lies under the influence of ML and is perched on a topographic high between contourite channels. For much of its Pleistocene history, it only received sediment from either bottom-current or hemipelagic processes. Especially notable is the extreme uniformity of the succession and its rapid accumulation at rates of 25–40 cm/k.y. The sediment is distinctively contouritic in character throughout, with mixed terrigenous-biogenic composition and characteristic bi-gradational or partial contourite sequences. Significantly, the long-duration hiatus observed at both Sites U1386 and U1387 on the Faro Drift under MU and related to a phase of highly active MOW, is reduced at Site U1389 to 2.1–2.4 Ma. Two other minor hiatuses are inferred at ~0.4 and ~0.9 Ma and are also indicative of enhanced MOW at these times.

Site U1390

Site U1390 is located at 992 mbsl on the southern Iberian margin (36°19.110′N; 7°43.078′W) ~130 km west of the Spanish city of Cádiz (Fig. F1). This site is the second of two sites drilled in the channels and ridges sector of the larger Cádiz CDS and is near the western end of a sheeted drift adjacent to the Guadalquivir contourite channel at 300 m above the channel floor and ~20 km northwest of the of the Guadalquivir diapiric ridge.

This sheeted drift is known as the Guadalquivir Drift, which is part of a much larger middle-slope sheeted drift system that has been dissected by several contourite channels. Site U1389 lies on the Huelva Drift, which is another dissected portion of this same system also under the influence of ML (see “Site U1389”). On seismic profiles, the Guadalquivir Drift displays a well-layered internal acoustic structure with laterally extensive, mainly aggradational seismic depositional units (Fig. F19) and lies unconformably above two earlier buried drifts that have been differentially tilted up toward the north adjacent to the tectonically elevated Guadalquivir Bank.

Our primary objective at this site was to recover the Pliocene–Quaternary sedimentary succession close to and affected by tectonic uplift of the Guadalquivir Bank. By penetrating the unconformities between the surface and buried drifts we have been able to evaluate the timing of this tectonic activity and its effects on drift sedimentation. The secondary objective at this site was to further assess drift sedimentation under the influence of ML.

Site U1390 was occupied on 2 January 2012. Three holes were drilled and cored using the APC and XCB systems, achieving the target depth of 350 mbsf in the first hole (U1390A). The second and third holes were piston cored to refusal. Downhole logging was carried out in Hole U1390A using the triple combo and FMS-sonic tool strings. Overall recovery for Site U1390 was 438 m (98%) with the APC and 248 m (91%) with the XCB. The total cored interval for Site U1390 was 719.5 m and total recovery was 686.3 m (95%).

Main results

Whereas our predrilling interpretation had assumed a relatively low rate of sedimentation above base-Quaternary and mid-Pliocene unconformities, instead we found very high rates of sedimentation under the influence of an active ML. The unconformities noted on seismic records, therefore, are much younger than expected. The basal unconformity of Guadalquivir Drift is 0.9–1.2 Ma, and the bottom of the hole reached to ~1.3 Ma.

The sedimentary succession at Site U1390 extends for 350 m from the mid-Pleistocene to Holocene (Fig. F20) and is represented by a very rapidly accumulated and uniform series of contouritic sediment assigned to a single lithologic unit and divided into two subunits (IA and IB). Unit I is dominated by classic contourite deposition, including calcareous mud, silty mud, sandy mud, and silty bioclastic sand lithologies. These are generally organized as bi-gradational sequences and partial sequences, of which base-cut-out, normally graded sequences are more common than top-cut-out, inversely graded sequences in Subunit IA; top-cut-out, inversely graded sequences are more common in Subunit IB. Carbonate content ranges from 21 to 35 wt%, and total organic carbon ranges from 0.4 to 1.1 wt%. The division of Unit I into two subunits is based on a marked change at 290 mbsf in the relative abundance and thickness of the silt/sand intervals. These are more prominent in Subunit IB, where the sandy facies makes up ~50% of the total.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, and ostracods) are mostly common to abundant with moderate to good preservation throughout. Pteropod fragments are present sporadically. Two relatively short hiatuses in the sedimentary record are inferred at 0.3–0.6 Ma and 0.9–1.2 Ma, separating intervals with rapid rates of sedimentation averaging 75 cm/k.y. above the upper hiatus and 85 cm/k.y. below. Holocene sedimentation rates are also very high, ~80 cm/k.y. Although age constraints are not yet refined, it appears that the rate below the lower hiatus exceeds 100 cm/k.y. This is the highest rate we know of for contourite sedimentation anywhere.

Distinctive variability in benthic foraminifer assemblages is apparent and reveals significant long-term trends as well as short-term cycles of environmental change. Comparable with other sites, the upper 0.9 m.y. of the Quaternary shows typical upper bathyal assemblages indicative of increased organic matter input and reduced ventilation. The remainder of the Pleistocene shows lower nutrient supply and greater ventilation. The direct role and influence of MOW at this site requires further study. The shipboard palynological study was carried out at a very coarse resolution but, as for other sites drilled, shows dominance of the four main plant ecological groups that characterize this region: Pinus, Mediterranean forest, semidesert, and grasslands.

Magnetostratigraphy proved difficult to resolve closely, in part because of coring disturbance and in part because of the presence of hiatuses at or near critical polarity transitions. The upper 230 m of section is within the Brunhes Chron, but the deeper section requires more detailed shore-based study to resolve. Strong peaks in magnetic susceptibility in these cores appear to correlate with large amounts of diagenetic iron sulfides (presumably including greigite and pyrrhotite).

As was observed at the other sites drilled during Expedition 339, physical property data show relatively close tracking of magnetic susceptibility and bulk density in much but not all of the section. These may correlate or anticorrelate with NGR values and color reflectance (L* and a* values). Both larger scale trends and smaller scale cycles are evident, with some correlation at the small scale with lithology.

The interstitial water profiles at Site U1390 show significantly higher concentrations of many elements, including Na, Cl, Ca, Mg, Sr, and NH₄, than at any of the other sites drilled to this point, including Site U1389, at which the values reached a distinct maximum at ~530 mbsf. However, the high chloride and sodium concentrations are associated with low δD values, which indicates that high salinity may not be the result of in situ salt dissolution. Instead, we suggest that salts were dissolved at depth by interstitial water that had been altered by clay mineral dehydration reactions, which can also affect water sodium concentration.

Downhole measurements were made in Hole U1390A to 350 mbsf with good quality data obtained as a result of good hole conditions, especially in the upper 270 m. A distinct change in log characteristics occurs at ~290 mbsf, which correlates closely with the lithologic boundary between subunits and the change downhole to a more sand-rich lithology. Distinct cyclicity is apparent in some parts of the section, corresponding with both lithologic and physical property data and requiring further study to unravel. Nine downhole temperature measurements were made in the uppermost 110 m of section, indicating a geothermal gradient of 32.0°C/km.

Highlights

At Site U1390, we penetrated a short but very interesting section to a total depth of 350 mbsf. The site lies under the influence of ML close to the Guadalquivir contourite channel and shows very high rates of sedimentation in both the sand-rich and mud-rich sections. With maximum rates of 85 cm/k.y., and perhaps in excess of 100 cm/k.y., these are the highest known rates for contourite drifts anywhere. As a result, some of the contourite muds retain a distinctive lamination, albeit discontinuous in character, whereas the thicker sands are especially clean and well sorted. Two important hiatuses at ~0.4 and ~0.9 Ma reflect episodes of enhanced bottom-current flow, in part related to tectonic adjustment of the local topography. Significantly, these hiatuses are correlative across the Cádiz CDS on the basis of seismic stratigraphy and are recognized as more minor events at the other sites.

Site U1391

Site U1391 is located at 1085 mbsl on the southwest Iberian margin (37°21.532′N; 9°24.656′W) ~50 km northwest of Cape São Vicente (Fig. F1). This site is the most distal of the drilled sites under the influence of MOW and lies on the broad, gently inclined middle-slope region of the southwest Portuguese margin, on which the seismic data indicate an extensive plastered drift that stretches alongslope for ~90 km between the São Vicente and Setubal downslope-oriented submarine canyons.

We consider this plastered drift as part of the larger Cádiz CDS. On seismic profiles it shows a well-layered internal acoustic structure with laterally extensive, mainly aggradational seismic depositional units and gentle thickening of the Quaternary succession toward the axial region of the drift (Fig. F21). Deeper within the section, one or more unconformities are apparent in the seismic profiles and are potentially related to recent tectonic activity. No separate designation of MU and ML is apparent along the Portuguese margin, although in terms of water depth Site U1391 would be closer to ML.

Our primary objective at this site was to recover a Pliocene, Pleistocene, and Holocene sedimentary succession formed under the influence of MOW and to compare this record with those found at sites within the Gulf of Cádiz that are closer to the Gibraltar Gateway. Very little is known about contourite deposition along this margin, so all data will be new and significant in this regard. The record at this site will also provide a direct comparison with hemipelagic sedimentation at Site U1385, which is removed from contourite input and under the influence of NADW.

Site U1391 was occupied on 8 January 2012. Three holes were drilled and cored using the APC, XCB, and RCB, achieving a total depth of 672 mbsf in the third hole (U1391C). Downhole logging was carried out in Hole U1391C using the triple combo and FMS-sonic tool strings. Overall recovery for Site U1391 was 359 m (105%) with the APC, 331 m (91%) with the XCB, and 269 m (81%) with the RCB. The total cored interval for Site U1391 was 1038.1 m and total recovery was 958.6 m (92%).

Main results

The sedimentary succession at Site U1391 extends for 672 m from the mid-Pliocene to Holocene (Fig. F22) and is represented by a thick, very uniform series of mud-rich contouritic sediment, with rapid rates of sedimentation through the later Quaternary. The succession was divided into two lithologic units (I and II), both of which are dominated by calcareous mud-rich contourite deposition with more minor lithologies including silty mud, sandy mud, nannofossil mud, and biosiliceous mud. Bioclastic silty sand is rare. These lithologies are generally organized as bi-gradational sequences and partial sequences with bioturbated and gradational upper and lower contacts. Carbonate content ranges from 17.5 to 48 wt%, and total organic carbon ranges from 0.5 to 1.8 wt%.

Unit I is divided into Subunit IA, which has a greater number of silty (and sandy) intervals as well as distinct alternation of greenish and reddish units, and Subunit IB, which is more mud rich with mainly greenish gray to dark greenish gray color cyclicity. The Unit I/II boundary occurs at a possible minor hiatus, below which exists a slightly more mixed system, including the base of a debrite, some minor faulting (slump unit?), more biosiliceous material, and a 50 cm thick well-cemented dolomitic mudstone below a probable second minor unconformity. The mud-rich sediment in Unit II has an intermediate contourite-hemipelagite aspect.

Calcareous microfossils (nannofossils, planktonic and benthic foraminifers, and ostracods) are mostly common to abundant with moderate to good preservation throughout. The sedimentary record appears to be relatively continuous through the Quaternary period, with an average sedimentation rate of 27 cm/k.y. for the later Pleistocene and 17 cm/k.y. for the early Pleistocene. However, there may be minor hiatuses present at ~0.7–0.9 and 2.41–2.5 Ma. The average sedimentation rate for the recovered Pliocene section is 13 cm/k.y., although evidence also exists for a minor hiatus at 3.0–3.19 Ma or, alternatively, for much reduced rates of sedimentation at this time.

As found at the other CDS sites, distinctive variability in benthic foraminifer assemblages reveals significant environmental changes through the Pliocene–Holocene succession. In general, the later Quaternary shows typical upper bathyal assemblages that are indicative of increased organic matter input and reduced ventilation. This signature is also evident during some earlier intervals. The remainder of the Quaternary and Pliocene succession shows lower nutrient supply and improved or variable ventilation. Certain signature taxa show somewhat lesser influence of MOW on the Portuguese margin than in the Gulf of Cádiz. Although nannofossils show common reworking in parts of the succession, it is less evident in general than at the Cádiz sites, and there is no evident reworking of planktonic foraminifers. Pollen and spores are abundant in most of the samples analyzed, together with microcharcoal and dinocysts, and show a similar assemblage and pattern to those found at the other drilled sites.

Paleomagnetic measurements indicate that the Brunhes–Matuyama polarity transition (0.781 Ma) occurs below 175 mbsf but is not clear, perhaps because of the inferred hiatus across this boundary. Specific identification of the top and bottom of the Olduvai Subchron (1.778 and 1.945 Ma, respectively) and the Matuyama–Gauss transition (2.581 Ma) was made. These give reliable confirmation of the biostratigraphic dating for Site U1391, although some of the inferred polarity boundaries need further confirmation.

As observed at the other drilled sites, physical property data show relatively close tracking of magnetic susceptibility and bulk density in much but not all of the section. These may correlate or anticorrelate with NGR values and color reflectance (L* and a* values). Both larger scale trends and smaller scale cycles are evident, with some correlation at the small scale with lithology. In lithologic Unit II, much lower NGR variability and very low magnetic susceptibility values are apparent.

The interstitial water profile at Site U1391 shows some significant distinction from that of Site U1385, commensurate with the deposition under MOW rather than NADW. Rates of sedimentation and hence of organic matter accumulation are greater at Site U1391 than at U1385, which makes for a shallower zone of sulfate reduction.

Downhole measurements were made in Hole U1391C to 668 mbsf, almost to the bottom of the hole. The borehole was very rugose with many narrow washouts that affected log quality. Minor changes in log characteristics occur at ~562 mbsf, which correlates closely with a lithologic boundary. The deeper interval has generally lower NGR values and includes two zones with poor core recovery that may be more sand rich on the basis of borehole logs, although no sands were recovered by coring. Distinct cyclicity is apparent in some parts of the section, corresponding to both lithologic and physical property data. Ten downhole temperature measurements were made in the uppermost 146 m of the hole, indicating a geothermal gradient of 14.2°C/km, the lowest of this expedition.

The three holes cored provide a complete composite stratigraphic section to the base of the APC interval at 171 mbsf and a virtually complete section all the way to 354 mbsf. The section below this was cored only in Hole U1391C to a total depth of 671.5 mbsf, with some short gaps between cores and larger gaps in the few instances where core recovery was low. This will be very beneficial for all subsequent paleoceanographic studies.

Highlights

We recovered core to a total depth of 672 mbsf at Site U1391 on the southwest Portuguese margin. The site lies under the influence of ML and penetrates through a relatively complete Quaternary and late Pliocene section of a plastered contourite drift. This is the most distal of our MOW sites and is distinctly more mud rich throughout than those in the Gulf of Cádiz. Nevertheless, sedimentation rates (27 cm/k.y.) for the later Quaternary are as high as those of the Faro Drift, and the contouritic signature of uniformity and bi-gradational sequences is ever present. Important similarities with other sites include an uphole increase in sedimentation rate, the number of silt/sand intervals, and organic matter supply, as well as reduced ventilation uphole. These all support enhanced MOW influence through the Quaternary. Hiatuses, possible hiatuses, or much reduced sedimentation rates are noted at ~0.7–0.9, 2.4–2.5, and 3.0–3.2 Ma, all of which are recognized at one or more of the Cádiz sites. These are interpreted as episodes of enhanced MOW and bottom-current activity.

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