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doi:10.2204/iodp.proc.320321.219.2016

Results and discussion

Revised CCSF-A depths for both biostratigraphic and magnetostratigraphic data were calculated by adding an offset depth to the CSF-A depth for each core, according to Table T5 in Wilkens et al. (2013). They further refined depths within cores from all three holes at Site U1338 caused by intracore stretching or squeezing and referred to these depths as “Adjusted CCSF.” In Tables AT47, AT52, and AT57, Wilkens et al. (2013) adjusted depths for a total of 843 samples. Of these samples, 232 show no adjustment (0 cm), 281 show an average adjustment of +16 cm, and 330 show an average adjustment of –17 cm. These adjusted depths within cores are generally small but are useful for exceptionally detailed work in need of centimeter precision. Considering the uncertainties involved in the underlying data sets, we did not employ the “Adjusted CCSF” by Wilkens et al. (2013).

Growth factors for each hole are calculated by linear regression (Fig. F1) of CSF-A versus CCSF-A, following the approach from the “Site U1338” chapter (Expedition 320/321 Scientists, 2010b). The CCSF-A depth is divided by the hole-specific growth factor to obtain the corrected composite CCSF-B depth for each biomagnetostratigraphic age-depth indicator from each hole.

Site U1338 biomagnetostratigraphic age-depth data are presented in Tables T1 (calcareous nannofossils), T2 (diatoms), and T3 (magnetostratigraphy). Many individual biohorizons and magnetostratigraphic chron boundaries have been determined from two or all three Site U1338 holes (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]; Baldauf, 2013; Ciummelli, 2013; Backman et al., 2013). It is difficult to determine which hole provides the true or most accurate depth information for individual age-depth indicators. Hence, the midpoint of the deepest and shallowest CCSF-B depths are used for individual chron boundaries and biohorizons in the sedimentation rate plots, taking into account one, two, or all three holes from Site U1338. These midpoint CCSF-B depths show average uncertainties of ±1.23 m (maximum ±3.54 m) for calcareous nannofossils, ±0.86 m (maximum ±8.23 m) for diatoms, and ±0.57 m (maximum ±2.32 m) for magnetostratigraphic chron boundaries.

The entire age-depth data set is presented in Figure F2. These data are composed of 33 geomagnetic chron boundaries, 33 calcareous nannofossil biohorizons, and 57 diatom biohorizons. Magnetostratigraphy is available in three different yet coherent intervals: Pliocene–Pleistocene (3.596–0.781 Ma), early late Miocene (9.987–9.098 Ma), and late middle Miocene (15.160–12.730 Ma).

Age calibrations of diatom biohorizons represent lower resolution equatorial Pacific data generated 23 years ago, which may suggest potential room for improvement (as always in biochronology). Still, the diatom biohorizons generally align well with magnetostratigraphic chron boundaries and calcareous nannofossil biohorizons, presumably because 75% of the diatom biohorizons (Table T2) have been directly calibrated with geomagnetic polarity stratigraphies from the equatorial Pacific (Barron, 1992) in depositional settings similar to that of Site U1338. In the 9.0 through 12.9 Ma interval, however, only 23% (3 of 13) of the diatom biohorizons are directly calibrated with magnetostratigraphy.

In order to show details of the different parts of the sedimentation rate history at Site U1338, the record is divided into three parts: 0–5.2 Ma (Fig. F3), 5–10.2 Ma (Fig. F4), and 9.9–18 Ma (Fig. F5). The proposed rate-determining age-depth indicators, or control points (CPs), and the resulting linear sedimentation rates (LSRs) are presented in Table T4.

It appears clearly from Figure F2 that the 123 age-depth indicators fall into coherent linear intervals, albeit with some minor scatter. Minor variations in sedimentation rates presumably occurred in the presented linear intervals between the nearest chosen age-depth CP couplets. It is beyond the scope of this data report to accommodate for such minor variations, which would require highly resolved cyclostratigraphic or stable isotopic data correlated to sites with independent (magnetostratigraphic) age control to resolve properly. When allowing for minor scatter in these biomagnetostratigraphic data, the suggested LSRs become uniform over relatively long intervals. This is considered preferable in comparison to, for example, placing CPs at each progressively deeper age-depth indicator, which would still result in scatter but also cause artificial extremes in sedimentation rates between closely spaced age-depth indicator couplets.

Age estimates of late early Miocene through Pleistocene geomagnetic chron boundaries are considered well constrained (Lourens et al., 2004), yet we have permitted a few such boundaries in the older/deeper part of the record to fall slightly off the proposed interpolated rate lines (Fig. F5) so as to not introduce extreme, shorter variations in sedimentation rates caused by less well constrained depths or age estimates of a few chron boundaries.

With these caveats, this data report may be taken to provide a basic biomagnetostratigraphic reference framework for future attempts to develop highly resolved age models for Site U1338 based on, for example, carbon isotope stratigraphy or astronomically tuned cyclostratigraphy.

We are fully aware of that the relatively few (11) interpolated linear sedimentation rate intervals encompassing the past 18 My at Site U1338 may be drawn differently, yet we consider that the proposed sedimentation rate history represents a reasonable interpretation of the age-depth distribution of available magnetostratigraphic, diatom, and calcareous nannofossil data. All depths below are on the CCSF-B scale.

Sedimentation rate lines between 0 and 5 Ma

Pliocene–Pleistocene sedimentation rates are shown in Figure F3. The youngest rate line is determined by the top of the Site U1338 sediment sequence, placed at 0.00 m and 0.00 Ma, and the Chron C1n/C1r boundary (base Brunhes; CP1).

CP2 is the Chron C2n/C2r.1r boundary (base Olduvai), and CP3 is the Chron C2An.3n/C2Ar boundary (base Gauss). There are two diatom and three calcareous nannofossil biohorizons clearly falling to the left of the proposed rate line between CP2 and CP3, suggesting that these represent paleoecologically driven disappearances prior to their genuine extinctions.

The three nannofossil extinctions represent Discoaster species, two of which have known problematic abundance histories toward the end of their ranges in the equatorial Pacific (Backman and Shackleton, 1983). It remains uncertain why the diatom biohorizons at 4.7 Ma (top Fragilariopsis cylindrica) and 4.9 Ma (base Nitzschia jouseae) deviate from the proposed interpolated line controlled by CP4 (base Asteromphalus elegans at 4.2 Ma) and CP5 (top Ceratolithus acutus at 5.04 Ma).

Sedimentation rate lines between 5 and 10 Ma

A constant sedimentation rate is suggested over 4 My from CP5 in the earliest Pliocene to CP6 (base Chron CAn) in the early late Miocene (Fig. F4). Two nannofossil biohorizons are clearly off the line, one being the top of the absence interval (paracme) of Reticulofenestra pseudoumbilicus (7.09 Ma) and the other the base of Amaurolithus spp. (7.39 Ma). Both biohorizons appear unproblematic in terms of abundance patterns (Backman et al., 2013). The former biohorizon does not represent the evolutionary first appearance of the species but rather its reappearance after having been virtually absent from the assemblages for about 1.7 My for reasons unknown. Its position suggests a reduced absence interval of about 0.4 My at its upper end in the region of Site U1338, as the proposed interpolated line suggests a reappearance at 7.5 Ma rather than 7.1 Ma. Base Amaurolithus spp. seems to appear at ~0.3 My earlier (7.7 Ma rather than 7.4 Ma) to the Site U1338 region compared with its calibrated first appearance from both the western equatorial Atlantic and two sites in the eastern equatorial Pacific (Backman et al., 2012). In Monte dei Corvi section on the Adriatic Sea coast, base Amaurolithus spp. occurs in the middle part of Chron C4n.1n at an estimated age of 7.57 Ma (Di Stefano et al., 2010), which would bring this biohorizon to within 0.1 My from the proposed interpolated line. These data suggest that this biohorizon is diachronous, perhaps up to 0.3 My, as suggested at Site U1338.

The interval between CP6 and CP7 is controlled by geomagnetic chron boundaries, one of which only marginally fits the proposed rate line. Top Catinaster coalitus at 9.70 Ma disappeared from the Site U1338 region well prior to its calibrated extinction. Not surprisingly, the final part of its range is characterized by low and discontinuous abundances (Backman et al., 2013).

Sedimentation rate lines between 10 and 18 Ma

The interval between geomagnetic chron boundaries CP7 and CP8 encompasses 3.2 My (Fig. F5). Even if this interval is divided using one or two intermediate biostratigraphic CPs, biohorizons will still be scattered around the shorter interpolation alternative.

Between CP8 and CP9, four geomagnetic chron boundaries do not fit the proposed linear interpolation. Less good core conditions began to affect the splice and composite section toward the deep end of the sequence (see “Site U1338” in the “Expedition 320/321 summary” chapter [Pälike et al., 2010]), which presumably affected the positions of some of the deepest geomagnetic chron boundaries. For example, in Holes U1338B and U1338C (Table T3), the series of successively older geomagnetic chron boundaries shows a corresponding successive increase in depth. When combining the data from the two holes to calculate midpoint depths of individual chron boundaries from their lowermost and uppermost occurrences, the obvious progression of depth in the individual holes dissolves, suggesting that the two holes are not perfectly correlated in their deepest parts. The position of, for example, the C5ADr/C5Bn.1n boundary in Hole U1338B at 14.781 Ma occurs above (354.00 m) the younger (14.581 Ma) C5ADn/C5ADr boundary at 356.44 m in Hole U1338C (Table T3). We therefore used only data from Hole U1338C for the four deepest geomagnetic chron boundaries in order to maintain a logical depth progression of each successive chron boundary.

The lowermost part of the Site U1338 sedimentation rate history is based on nannofossil biostratigraphy. The short overlap between two nannofossil biohorizons at 15.69 Ma (Discoaster deflandrei decreases to <30% of the total Discoaster assemblage) and 15.73 Ma (base Discoaster signus) indicates that CP10 can be confidently placed at the latter biohorizon. The base of common Sphenolithus heteromorphus provides CP11, which has a consistent first common occurrence in Chron C5Dr in the Indian Ocean, Atlantic Ocean, and Mediterranean Sea (Backman et al., 1990, 2012; Di Stefano et al., 2015).

Sedimentation rates versus age

The sedimentation rate history at Site U1338 shows distinct variability and some general trends (Fig. F6; Table T4). When plotting the sedimentation rate history using linear interpolation between specific age-depth indicators, rates change stepwise at the CPs. We assume that the rate history was more smooth than depicted in Figure F6 and that the changes did not generally occur at the CPs but more likely as transitions between the CPs.

Rates were modest (11 m/My) during the first 2 My of sedimentation at Site U1338, when the site was located ~100 nmi south of the Equator (see Fig. F5 in the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). Sedimentation rates thereafter increased to 34 m/My by ~13.2 Ma, when the site began to approach the Equator from the south. This high rate was maintained for about 3 My until the rate was halved to 17 m/My between 10.0 and 9.1 Ma.

This major yet temporary decrease in sedimentation rate coincides with intense carbonate dissolution at Site U1338, also known as the “carbonate crash,” in the eastern equatorial Pacific (Farrell et al., 1995; Lyle et al., 1995). The most intense dissolution occurs over a 0.72 m thick interval within Section 321-U1338B-22H-2, with a minimum carbonate content of 5% and an average of 12% between 198.85 and 199.57 m (Lyle and Backman, 2013) corresponding to a 42 ky long interval according to the present age model, beginning at 9.618 Ma and ending at 9.576 Ma.

Carbonate content and the sedimentation rate increased after this early late Miocene carbonate crash at ~9.6 Ma to 30 m/My for a 4.1 My long interval lasting into the earliest Pliocene. This late Miocene rate regime changed in the early Pliocene between 5.0 and 4.2 Ma into a new Pliocene–Pleistocene sedimentation rate regime with an average of 14 m/My representing a decrease of 56% compared with the late Miocene regime. Exactly when this major decrease occurs is unclear. If extrapolating the late Miocene regime upward and the early Pliocene regime downward, these will meet at ~4.8 Ma, when the site was located ~80 nmi north of the Equator (see Fig. F5 in the “Expedition 320/321 summary” chapter [Pälike et al., 2010]).

Pliocene–Pleistocene sedimentation rates are, however, not uniform, but they show a stepwise decrease from 18 m/My (5.0–4.2 Ma) to 15 m/My (4.2–3.6 Ma), followed by 13 m/My (3.6–1.9 Ma) to 12 m/My (1.9–0.8 Ma) and finally 11 m/My (0.8–0.0 Ma), as Site U1338 is slowly moving away from the Equatorial high productivity region.

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