Temperature-dependent ocean–atmosphere equilibration of carbon isotopes in surface and intermediate waters over the deglaciation

Abstract

High resolution and precise records of the carbon isotopic composition of atmospheric carbon dioxide from ice cores are now providing a valuable constraint on the origin of the rise in CO2 over the last deglaciation. Previous studies using isotopic measurements on the tests of planktonic foraminifera from marine sediments had shown a deglacial minimum in the carbon isotopic composition of seawater carbon in the surface and thermocline waters of the Southern Hemisphere and the tropics, a feature also seen in the atmospheric record. Here we show that this deglacial minimum also can be clearly seen in carbon isotope measurements on planktonic foraminifera from sediment cores in the North Atlantic. There are also new high-resolution and precise records of the carbon isotopic composition of intermediate and mode waters generated using benthic foraminifera which show a history of change that is strikingly similar to that in the atmosphere. Similarities in these new highly-resolved deglacial records suggest that air–sea exchange can effectively globalize the isotopic signature of a release of carbon from the deep ocean. This limits the utility of carbon isotopes for constraining the precise location and mechanism of carbon release, indicating that more direct nutrient tracers are needed. However, given that the equilibrium partitioning of carbon isotopes between the atmosphere and ocean is temperature dependent, the difference between the atmosphere and upper ocean values could potentially provide an independent estimate of sea surface temperature change.