Benthic foraminiferal assemblages across the Cretaceous Paleogene boundray of ODP Site 208-1262 on Walvis Ridge, eastern South Atlantic

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Sediments recovered at ODP Site 1262 on Walvis Ridge (eastern South Atlantic Ocean, paleodepth ~2500–3000 m) offer an opportunity to look into the nature and cause(s) of the benthic foraminiferal turnover across the Cretaceous/Paleogene (K/Pg) boundary at abyssal depths at a location relatively remote from the location of the asteroid impact on the Yucatan peninsula. Late Maastrichtian assemblages were diverse and heterogeneous, reflecting oligo- to mesotrophic conditions at the sea floor. As at other locations, there was no significant extinction of benthic foraminiferal species at the K/Pg boundary, but the diversity and heterogeneity of the assemblages decreased precipitously. The percentage of infaunal buliminid taxa decreased rapidly, but the total percentage of infaunal taxa remained essentially unchanged. Benthic foraminiferal accumulation rates (BFAR) dropped at the boundary itself, and fluctuated strongly later in the Danian. After the K/Pg boundary, opportunistic taxa increased in relative abundance, and diversity and heterogeneity strongly fluctuated. There thus is no clear agreement between three proxies which have been used to indicate a high food supply to the sea floor: percentage buliminids, percentage infaunal taxa, and BFAR. This discrepancy might indicate that the food supply changed in character (e.g., type of food, temporal variability in flux), but did not show a long-term, large net change in total amount. The strong fluctuations in BFAR, heterogeneity and diversity indicate that environmental instability started at the K/Pg boundary, and persisted through planktic foraminiferal zones P* and P1a. In lower planktic foraminiferal zone P1b, agglutinated species of the genera Spiroplectammina and Clavulinoides, thought to be infaunal and indicative of increasing levels of food supply, increased in relative abundance while the relative abundance of buliminids remained low. Possibly, agglutinated taxa took over at least part of the infaunal niche in the Paleocene, as a result of the rise in the calcium carbonate compensation depth. Towards the upper part of the studied interval (lower planktic foraminiferal Subzone 1c), benthic foraminiferal assemblages stabilized, with heterogeneity almost back to Maastrichtian values, although diversity and BFAR did not fully recover.

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