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doi:10.2204/iodp.proc.323.201.2013

Results

Abundance and preservation

In the studied interval, the abundance of calcareous nannofossils is generally very low, with frequent barren intervals. Only Samples 323-U1341B-37H-1W, 75 cm, and 58X-5W, 75 cm, and core catcher Samples 36H-CC, 52H-CC, 54H-CC, 58X-CC, and 61X-CC yield relatively high numbers of calcareous nannofossils (Table T1).

When present, calcareous nannofossils are moderately preserved, with better preservation associated to higher numbers. Dissolution of elements is the most important preservation feature, with no overgrowth observed.

As stated above, barren intervals are distinctive in the record and can be an indication of low calcareous nannofossil production and/or carbonate preservation problems during diagenesis due to high organic matter content in sediments. More data on geochemical proxies and other calcareous microfossil groups is needed to assess this issue that affects all Bering Sea calcareous nannofossil records (see the “Expedition 323 summary” chapter [Expedition 323 Scientists, 2011a]).

Calcareous nannofossil taxonomy

As a general rule, we identified different taxa following Hine and Weaver (1998) and Young (1998). However, taxa considered in this study are mainly included within the families Noelaerhabdaceae (genera Reticulofenestra and Dictyococcites) and the genus Coccolithus (Table T1). For the former, we have adopted the criterion for the open/closed central area respectively, whereas we divided C. pelagicus into C. pelagicus and Coccolithus braarudii, in accordance with Sáez et al. (2003). In both cases, we used readily identifiable features under cross-polarized light, such as central area or coccolith size.

Calcareous nannofossil stratigraphy

Because of low abundances, barren intervals, and, more importantly, the absence of the usual marker taxa (Table T1), it has been impossible to apply any calcareous nannofossil standard zonation schemes such as Martini (1971) or Okada and Burky (1980) to the samples in this study. We have focused instead on narrowing the reversal in abundance between Dictyococcites spp. and C. pelagicus (~2.75 Ma at nearby Ocean Drilling Program Site 883 in the North Pacific; Datum plane A in Sato et al., 2002). Variations in the calcareous nannofossil assemblages shown in Table T1 indicate that this datum lies between 388 mbsf (Sample 323-U1341B-42H-1W, 75 cm; predominantly Dictyococcites spp.) and ~346 mbsf (Sample 37H-1W, 75 cm; predominantly C. pelagicus). Because barren horizons are particularly frequent in this interval, it is impossible to locate this event at higher resolution. However, our results are in agreement with age estimations based on other proxies (see the “Expedition 323 summary” chapter [Expedition 323 Scientists, 2011a]).

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