The late Maastrichtian calcification crisis in Bidart (France): a benthic environment perspective

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 Guembelitria spp. 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.

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 Cibicidoides spp. (~51%), Steinsioeina spp. (~10%) and Coryphostoma spp. (~9%) 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 Cibicidoides spp., Steinsioeina spp., Gyroidinoides spp., Praebulimina and Coryphostoma spp. 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.