Ocean acidification and surface water carbonate production across the Paleocene–Eocene thermal maximum

The addition of massive amounts of carbon to the ocean–atmosphere system at the Paleocene–Eocene thermal maximum (PETM, ∼ 55 Ma) caused deep-ocean acidification, evidenced by widespread dissolution of sea floor carbonate sediments. Because of the strong effect of this dissolution on the preserved record of calcium carbonate it has been difficult to evaluate whether changes in surface water chemistry affected carbonate production at the same time. Here, we investigate the production of biogenic carbonate in surface waters by testing a method which combines fossil calcareous nannoplankton counts with taxon-specific Sr/Ca data, an indicator of coccolithophore production. Reconstructed nannoplankton production at Ocean Drilling Program (ODP) Sites 690 (Southern Ocean), 1209 (Pacific Ocean) and Bass River (New Jersey) did not appear to vary significantly across the PETM indicating that on geological timescales there is no evidence for interruption of phytoplankton carbonate production, despite the major assemblage shifts associated with this interval. Either levels of carbonate chemistry change in surface waters were relatively low, perhaps a function of CO2 emission rates, or calcareous nannoplankton were relatively insensitive to these changes compared with their response to other environmental parameters, namely temperature and nutrient availability.