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Results and discussion

The number of samples taken at each site varied considerably. Unfortunately, some vials broke during transport, causing gaps in the cell count profiles. At all depths, cell abundance varied between sites by about 2 orders of magnitude.

Contrary to most previously described cell count records from other locations that exhibit a logarithmic decline when plotted on a linear depth/log10 cell count scale, many of the PEAT sites exhibit profiles that deviate strongly from this usual pattern. At least in part, these deviations appear to be related to lithologic and sedimentological changes, as described in the “Expedition 320/321 summary” chapter (Pälike et al., 2010). A summary of all cell count data is given in Figure F1.

Site U1331

The uppermost cell count sample is at 3 mbsf in a 7 m thick unit of Pleistocene–Pliocene clay and exhibits relatively low abundances (2 × 104 cells/cm3). The next sample at 8.2 mbsf has a cell abundance that is more than 1 order of magnitude higher, reaching values of 3 × 105 cells/cm3. This sample is in the lithologic unit that reaches to 26 mbsf and consists of lower Oligocene to lower Eocene alternations of oozes with varying clay and calcium carbonate content. All cell count samples in this interval have cell abundances that are similar or higher than the uppermost sample.

A sharp lithologic change at ~26 mbsf marks the Eocene–Oligocene transition, but cell abundances are not affected by this and remain at ~2 × 104 cells/cm3 throughout the rest of the core (Table T1).

Site U1332

At Site U1332, cell abundance close to the sediment–water interface is ~105 cells/cm3, similar to values of the subsurface peak at Site U1331. However, cell numbers decline more linearly with depth than at Site U1331 and reach the minimum detection limit of ~104 cells/cm3 at ~30 mbsf. Below this depth, no cells could be detected. When treating the cell counts that fall below detection as valid data, log10 cell abundance decreases linearly with depth. The depth at which cell counts start to fall below the detection limit roughly coincides with the change from lithologic Unit II to III, which is marked by a steep increase in CaCO3 content to >80%. Unfortunately, several samples from this depth interval were lost, making it impossible to constrain this observation to any greater detail (Table T2).

Site U1334

The cell count record at Site U1334 (Table T3) exhibits the strongest variation from the usual logarithmic decline with depth. Between 3 and 25 mbsf, cell numbers drop steadily with depth from ~105 to 104 cells/cm3. Below this depth, cell numbers alternate between 104 and 105 cells/cm3 with local maxima at ~60 and ~140 mbsf and a minimum at ~120 mbsf. From the deepest local maximum at 140 mbsf downward, cell numbers decline more or less continuously to values of ~5 × 103 cells/cm3 at 200 mbsf.

Except for the local minimum in cell abundance at ~40 mbsf correlating with a minimum in CaCO3 content, the strong deviations from the normally expected decline of cell abundance with depth do not show any correlation with lithology, which mainly consists of nannofossil ooze and some clay in the upper 50 mbsf. The very pronounced greenish gray sediment color between ~140 and 190 mbsf does not show any correlation with cell abundance.

Site U1335

This site exhibits the most “normal” cell count record of all sites drilled during Expedition 320/321. Cell counts are ~5 × 105 cells/cm3 close to the sediment–water interface and drop with sediment depth according to a power-law function, reaching values of ~103 cells/cm3 at ~100 mbsf. Below ~60 mbsf, single samples start to fall below the detection limit. A single replicate of the deepest sample (108.4 mbsf) shows a slight increase in cell abundance, but data are too scarce to make any interpretation. Both the change to continuously high carbonate content and the pronounced sediment color change from brown to greenish gray at 60 and 65 mbsf, respectively, have no influence on the cell count record (Table T4).

Site U1337

The most obvious feature of the cell count record at this site is the lack of a decrease over the upper 22.5 mbsf, with values of 2 × 105 cells/cm3. Below this depth, cell counts decrease to 2 × 104 cells/cm3 at 60 mbsf. Below this depth, log10 cell counts show only a very slight but almost linear decrease with depth, dropping by less than half an order of magnitude over ~100 m.

No change in lithology correlates with either change in the cell count profile at 22.5 and 60 mbsf (Table T5).

Site U1338

Despite some excursions between 30 and 80 mbsf, the cell count profile at Site U1338 follows the expected trend for marine sediment, starting at 106 cells/cm3 close to the sediment–water interface and dropping to 104 cells/cm3 at 150 mbsf (Table T6). The small-scale excursions cannot be related to any particular sedimentological or stratigraphic feature.