Changes in North Atlantic Deep-Water Oxygen across the Middle Pleistocene Transition

We reconstructed changes in deep North Atlantic ventilation across the MPT using a stoichiometric proxy for paleo-oxygen¹, which utilizes the carbon isotope gradient between epifaunal Cibicidoides wuellerstorfi and infaunal Globobulimina affinis (Δδ¹³C_cib-aff). The re-calibrated proxy² is valid when O₂ drops below 235 μmol kg^-1 corresponding to a Δδ¹³C of <2.25 ‰. Here we use core top samples from the Iberian Margin to show the Δδ¹³C accurately predicts O₂ of the Mediterranean Outflow Water which has an O₂ of 170 μmol kg^-1. We also measured the δ³⁴S and δ¹³C of pore water to assess the effect of sulfate reduction on δ¹³C profiles, which can complicate the Δδ¹³C paleo-oxygen proxy¹.

We applied the Δδ¹³C method to nearby Site U1385 for the last 1.5 Ma to investigate deep-water oxygen concentration changes across the MPT. During the 41-kyr world, the Δδ¹³C gradient rarely dropped below 2 ‰ indicating relatively well oxygenated conditions during both glacial and interglacial stages. The first significant decreases in the Δδ¹³C gradient, indicating reduced oxygenation, occurred briefly at 960 ka (MIS 26), at 915 ka (MIS 24/23), and persistently low Δδ¹³C values occurred throughout much of MIS 22 ( 890-864 ka). The Δδ¹³C values during MIS 26, 24 and 22 are less than those observed during some Heinrich events of the last glacial period². The increased frequency of low-oxygen events during glacials after 960 ka is consistent with other studies suggesting the respired content of the deep Atlantic increased across the MPT³. The few CO₂ records that exist beyond 800 ka indicate that glacial CO₂ values were higher prior to 1 Ma than during glacial periods of the last 800 ka^4&5. Our paleo-oxygen results support a scenario of decreasing deep-water oxygen, increased respired carbon storage, and a reduction in glacial atmospheric pCO₂ at 900 ka, which coincided with an important change in ocean circulation⁶ and growth of larger ice sheets⁷.

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