<p>The final ~0.5m interval of the Cretaceous-Paleogene (K-Pg) boundary at Bidart (France) constitutes the “Deccan benchmark” interval characterized by taphonomic and geochemical proxies of ocean acidification linked with Deccan volcanism. Planktic foraminifera census and morphometric data reveal a concurrence of dwarfed species, thinner test walls, high test fragmentation of planktic foraminifera and increased relative abundance of <em>Guembelitria spp.</em> Together, these evidences point toward severe biotic stress and a likely calcification crisis in planktic foraminifera in the final ~0.5m (~58 ky) of the late Maastrichtian at Bidart.</p> <p>In the sediment-water interface, the benthic foraminiferal assemblage increase to a dramatic >100,000 tests/gram, indicating a sediment-starved horizon at the KPB. Interestingly, a sharp increase in the relative proportion of heavily calcified genera like <em>Cibicidoides spp.</em> (~51%)<em>, Steinsioeina spp. </em>(~10%) and <em>Coryphostoma spp. </em>(~9%)<em> </em>is also recorded at the KPB. The taphonomic angle to such a record is rejected as the benthic foraminifera fragmentation index does not record the ‘acidification’ event as significantly. Similarly, morphometric analysis reveals average sizes of thick-walled genera like <em>Cibicidoides spp., Steinsioeina spp., Gyroidinoides spp., Praebulimina </em>and <em>Coryphostoma spp.</em> increasing at the KPB and ~0.3m below it. A possible explanation for such a biotic advantage for the individuals building heavily calcified tests could be a carbonate super-saturation led by the extinction of pelagic calcifiers at the KPB. In the benchmark, rare occasions of dwarfing and reduced absolute abundances of calcareous benthic foraminifera imply a lower degree of environmental stress. Similarly, census analysis of agglutinated benthic foraminifera records an increased population within the benchmark, indicating a change in community structure.  Whether such a change is a response to acidification or an artifact of preservation is currently under investigation. Our results support an acidification that was restricted to the surface ocean and resulted in severe (planktic) crisis, with limited effect on benthic foraminifera. This is consistent with a lack of benthic foraminifera extinctions across the K-Pg boundary.</p>

<p>The Cretaceous-Palaeogene (K-Pg) boundary interval is marked by the mass extinction of more than 50% of the larger more specialized Cretaceous planktic foraminifera, followed by the extinction of ~33% generalist species (short-term survivors) (Keller and Abramovich, 2009 Punekar et al., 2014). Adaptation strategies identified in Cretaceous planktic foraminifera assemblages within this biotic-stress interval include changes in the community structure through shifts in abundance of species and diversity decline. Changes on a species level are reported as inter- and intra- specific dwarfing, malformation and test-wall thinning. <em>Guembelitria cretacea</em> is typically small sized triserial species identified as the only long-term survivor of this event. Through this study, we test the ocean acidification hypothesis of the late Maastrichtian planktic stress by understanding the link between species carbonate demand and their survivorship at the K-Pg boundary.</p> <p>Four-dimensional X-ray microscopy (FDXRM) scans of pristine Cretaceous planktic morphogroups (the globotruncanids, the rugoglobigerinids and the planoheterohelicids) from pristine late Maastrichtian zone CF4 of DSDP 525A (South Atlantic) yield the most accurate estimation of their respective test calcite volume. The average test weights and the FDXRM reference estimates together suggest that the scaling of calcium carbonate for globotruncanids, planoheterohelicids, rugoglobigerinids, w.r.t. the guembelitrids is 10-269ug, 6-28ug and 9-60ug respectively. This scaling is significant in context of the observed survivorship of these morphogroups across the K-Pg boundary interval. The new results establish a preliminary link between the carbonate demand, ocean acidification related carbonate crisis (especially in the late Maastrichtian biozone CF1) and the survivorship of these morphogroups. However, other detrimental environmental factors in this critical stress interval cannot be ignored.</p>