Large delta¹³C gradients in the preindustrial North Atlantic revealed.

The carbon isotopic composition ((13)C/(12)C, expressed as δ(13)C) of fossil foraminifera is the primary tracer used to infer changes in past ocean ventilation, and its variations are interpreted by using the modern oceanic δ(13)C distribution as a framework. However, the present ocean δ(13)C distribution is strongly overprinted by isotopically light anthropogenic carbon dioxide. A correction for this oceanic C-13 Suess effect in the North Atlantic (NA) shows that the pristine NA δ(13)C distribution has a richer and more detailed structure that is more clearly related to water mass distributions. Our results revise some fundamental perceptions regarding glacial-interglacial ocean δ(13)C differences and allow paleo-δ(13)C variations to be understood within the context of modern climate variability.