Glacial bottom water oxygen concentrations inferred from benthic foraminiferal interspecies carbon isotope gradients

Abstract:

McCorkle and Emerson discovered in 1988 that the carbon isotope (δ¹³C) gradient between bottom water and pore-water at the anoxic boundary increases with increasing bottom water oxygen concentrations [O₂]. Their observations agree remarkably well with stoichiometric pore-water δ¹³C and [O₂] model predictions, suggesting that such a δ¹³C gradient can be used to calculate bottom water [O₂]. We have refined the novel δ¹³C gradient proxy formulated by McCorkle and Emerson, adding new observations. We show that the δ¹³C gradient between bottom water and pore-water at the anoxic boundary can be approximated by the δ¹³C difference between bottom water and test carbonate d¹³C of benthic foraminifera Globobulimina spp. This species has a habitat adapted to the conditions as the dysoxic/anoxic boundary. The δ¹³C gradient measured by the δ¹³C difference between bottom water and deep infaunal Globobulimina spp. from moderns core-tops compare well with those obtained from bottom- and pore-water measurements, confirming that Globobulimina spp. δ¹³C reflects in situ anoxic boundary pore-water δ¹³C. Furthermore, the δ¹³C gradient between bottom water and pore-water at the anoxic boundary can be estimated from the δ¹³C difference between epifaunal Cibicidoides wuellerstorfi, which records bottom water δ¹³C DIC, and deep infaunal Globobulimina spp.

Application of this δ¹³C isotope gradient proxy on core MD95-2042, from the Iberian margin, provides quantitative constraints on changes in oxygen and respired carbon concentrations in the Northeast Atlantic over the last 150,000 years. Our reconstruction shows that bottom water oxygen concentrations were lower by 55 and 75 µmol/kg during the last and penultimate glacial maxima relative to present day concentrations, and that remineralized organic carbon concentrations were at least doubled. The glacial changes in deep Atlantic oxygen and respired carbon concentrations are due to decreased ventilation linked to ocean circulation change and possibly a globally improved biological pump.