The Marine Carbon Cycle After the Extinction at the End of the Cretaceous

Abstract:

The oceanic carbon isotope gradient between surface and deep waters, as observed in δ¹³C in surface and deep ocean calcifiers, disappeared or reversed in the aftermath of the Cretaceous/Paleogene asteroid impact and associated mass extinction. This lack in vertical gradient has been interpreted as indicating a near-complete collapse of marine productivity for several hundred thousand to millions of years (a ‘Strangelove’ Ocean), or a decline in functioning of the biological pump, ranging from minor to catastrophic (‘Living Ocean’ model). A long-term cessation or catastrophic decline in the transfer of organic matter to the deep sea does not agree with the observed absence of significant extinction of benthic foraminifera. We investigated by how much the biological pump may have weakened sufficiently to interpret the carbon isotopic data using an Earth system model, and taking into account the role of the oceanic solubility pump, imprinting a gradient opposite in sign to that generated by the biological pump. We find that a weakening, which is considerably less than a complete collapse in organic matter export to the deep sea, can explain the carbon isotope record, in agreement with the lack of benthic extinction. The oceanic carbon isotope record can account for much of the magnitude of observed atmospheric excursion. In addition, we obtain new insight in the mechanistic workings of the ocean carbon cycle: significant organic carbon rain to depth continued even when carbonate mineral ballast production was effectively shut down due to the mass extinction of pelagic calcifiers. Oceanic ecosystems, thus the composition of the exported material and mode of export may have changed profoundly, but the strength of the biological pump was not dramatically disrupted for geologically long periods.