Strong thermodynamic imprint on Recent bottom-water and epibenthic δ13C in the Weddell Sea revealed: Implications for glacial Southern Ocean ventilation

Abstract

Paleonutrient proxies are widely used to reconstruct the geometry of deep-water masses during the Last Glacial Maximum (LGM). Epibenthic δ13C provides best spatial coverage, and artifacts are well investigated. Discrepancies between reconstructed LGM-circulation patterns derived from models or different benthic nutrient proxies can partly be resolved by varying air–sea signatures of δ13C, i.e. δ13Cas. However, there are very few data available to calculate a δ13Cas of modern bottom water dissolved inorganic carbon (DIC) δ13C, and to document how this signal is recorded in benthic foraminiferal δ13C. Here I show that today bottom water in the Atlantic sector of the Southern Ocean is 13C enriched with δ13CDIC values between 0.4 and 1.0‰ and δ13Cas values >0.4‰, and that this signal is recorded in live and dead epibenthic δ13C. This is in contrast to a uniform modern Antarctic circumpolar δ13CDIC of rather 0.4‰, which hitherto is used as modern framework to compare to low LGM δ13CDIC of southern sourced bottom-water and glacial inter basin differences. I conclude that a potential reduction of the strong Recent thermodynamic imprint during bottom-water generation in glacial times could explain depleted circum Antarctic 13CDIC without associated CO2 enrichment and anoxia in Antarctic bottom waters. The present synoptic compilation of δ13CDIC and live benthic foraminifera δ13C is in support of hypotheses that explain low LGM δ13C by a depletion of southern end-member 13CDIC due to extensive sea-ice formation with low δ13Cas-brine rejection and diminished air–sea gas exchange.