Constraining oceanic carbonate chemistry evolution during the Cretaceous-Paleogene transition: Combined benthic and planktonic calcium isotope records from the equatorial Pacific Ocean

The Mesozoic-Cenozoic transition is a period of biogeochemical cycle perturbations. The strongest of them is the Cretaceous-Paleogene boundary (K-Pg) crisis, characterized by one of the most important extinctions of planktonic marine calcifiers in Earth’s history. One of the primary drivers of this biocalcification crisis is thought to be the increase in atmospheric CO₂ concentration and ocean acidification triggered by the Chicxulub Impact, and/or Deccan volcanism. Because it reflects changes of the calcium cycle and/or depends on parameters of the carbonate system, the Ca isotope composition of carbonate minerals precipitated from seawater (Ca) offers the potential to reconstruct some of the environmental changes that occurred. Here we present new high-resolution planktonic and benthic foraminiferal Ca, O, C, and Sr/Ca records across the K-Pg transition from Shatsky rise (Leg 198; ODP Site 1209, Hole C). The Ca record displays a succession of rapid shifts of ca. ‰ across the K-Pg transition. They are similar though not identical between the planktonic and benthic records. These shifts took place on a timescale significantly shorter than the residence time of Ca in the oceans and are therefore unlikely to result from global disequilibrium in the oceanic Ca budget. Instead, changes in the fractionation factor between carbonate minerals and seawater in response to changes in precipitation rates may explain the observed Ca and Sr/Ca record. The benthic and planktonic Ca records show a late Maastrichtian and an early Danian negative excursions best explained by a succession of episodes of ocean alkalinity increase related to increased continental weathering caused by CO₂ emissions from Deccan volcanism and the aftermath of the K-Pg biocalcification crisis. Carbonate compensation via carbonate sediment dissolution, biological carbonate compensation via reduction of biocalcification, and/or an increase in continental weathering must have occurred to offset the excess CO₂, ultimately resulting in rapid changes in ocean carbonate chemistry, in combination with reduced surface alkalinity export in response to the early Paleogene planktonic biomineralization crisis.