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.2019 Nov 5;116(45):22500-22504.
doi: 10.1073/pnas.1905989116. Epub 2019 Oct 21.

Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact

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Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact

Michael J Henehan et al. Proc Natl Acad Sci U S A..

Abstract

Mass extinction at the Cretaceous-Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.

Keywords: Cretaceous/Paleogene boundary; GENIE model; boron isotopes; mass extinction; ocean acidification.

Copyright © 2019 the Author(s). Published by PNAS.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Records of surface ocean foraminiferal δ11B (B), calculated pH (C), andpCO2 (D) across the K-Pg boundary, with high-resolution foraminiferal diversity counts from ref. at the K-Pg Global Boundary Stratotype Section and Point (El Kef) plotted for context (A). pH is calculated assuming our best estimate of K-Pg δ11Bsw, 39.45 ± 0.4‰.pCO2 is calculated from pH along with total alkalinity estimates at each site from a GENIE late Maastrichtian simulation, adjusted for dynamic changes in alkalinity across the K-Pg using LOSCAR simulations from ref. that match observed patterns of carbonate burial. Gray shaded areas are 1-sigma uncertainties, with thin lines representing 1,000 Monte Carlo simulations from the 10,000 that were run. For clarity, we only plot those samples that represent the surface mixed layer, which should be approximately in equilibrium with the atmosphere. Additional data from deep-sea benthic and thermocline-dwelling planktic foraminifera that do not reflect atmosphericpCO2 can be seen in Fig. 2 and inDataset S1. For details of the age models used, the estimation of δ11Bsw, carbonate system calculations, and uncertainty propagation seeSI Appendix.
Fig. 2.
Fig. 2.
Paired benthic–planktic foraminiferal δ11B measurements from 4 time slices following the K-Pg boundary at ODP Site 1209 (Shatsky Rise) are used to calculate the difference in surface and deep ocean pH (∆pH; red diamonds,A). These data show a trend from “normal” patterns of higher δ11B/pH in the surface ocean relative to the deep until 65.92 Ma, where this gradient disappears (see alsoSI Appendix, Fig. S12). Shown for context is the convergence in δ13C from surface waters (bulk CaCO3, in green) and deep ocean waters (benthic foraminifera, navy) (A). Water column δ13C and pH profiles in the cGENIE model the location of Shatsky Rise (see also Fig. 3) are shown inB and C, respectively. A weakened or reversed δ13C gradient can arise in a number of ways (B), but only a shallow remineralization (or “Living Ocean,” red dot-dashed line) model can produce a flat gradient in pH between the surface 80 m and 2,500-m water depth (C). Carbonate data from ODP 1209 include new data and benthic data from ref. . SeeSI Appendix for more details.
Fig. 3.
Fig. 3.
Pacific zonal means of the simulated depth distribution of δ13CDIC in the Maastrichtian ocean for (A) the default (full-strength) biological pump, (B) global export production reduced to 50% of the default, and (C) no biological pump. The approximate paleo-latitude of ODP Site 1209 (Shatsky Rise) is marked by the vertical dashed white line. The action of the solubility pump, which absorbs atmospheric CO2 at high latitudes due to increased solubility of surface waters with respect to CO2 as they cool, imparts an inverse vertical δ13CDIC gradient at low latitudes in the absence of biological productivity (C; see the main text for explanation).
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