Stable Isotope Constraints on Marine Productivity Across the Cretaceous‐Paleogene Mass Extinction

Abstract

The effects of the Cretaceous-Paleogene (K/Pg) mass extinction (~66 Ma) on marine primary and export productivity remain debated. We studied changes in carbon and nitrogen cycling in eight neritic and upper bathyal sections with expanded K/Pg boundary clay layers in the western Tethys and northeastern Atlantic Ocean, by measuring stable carbon isotopes of bulk carbonate (δ13Ccarb) and organic matter (δ13Corg), nitrogen isotopes in bulk organic matter (δ15N), and selected compound-specific carbon isotopic records (δ13Clipid). Negative carbon isotope excursions (CIEs) in δ13Ccarb, δ13Corg, and δ13Clipid are temporally and spatially heterogeneous as well as decoupled from each other, suggesting that factors affecting the δ13C of dissolved inorganic carbon, as well as isotopic fractionation during carbon fixation across the K/Pg, are more complex than commonly assumed. The negative CIEs in δ13Corg and δ13Clipid at each site are smaller in amplitude and shorter in duration than those in δ13Ccarb, but in most sections both carbon pools recovered to preboundary conditions within the time of deposition of the boundary clay layer (<103–104 Kyr) or shortly thereafter. This rapid recovery is supported by limited δ15N data, which mostly suggests moderate or minor changes in redox conditions (except in Denmark), marine productivity, and phytoplanktonic nitrate utilization in the earliest Danian. Our results indicate that carbon cycling and primary productivity in neritic and upper bathyal regions recovered to preboundary levels faster (<104Kyr) than in oceanic regions (105–106 years), likely sustained by resilient noncalcifying phytoplankton with resting stages, consistent with modeling and proxy studies. ©2019 American Geophysical Union. All Rights Reserved.