A deeper respired carbon pool in the glacial equatorial Pacific Ocean

Atmospheric pCO2 was approximately 80 ppm lower during the last glacial period than during pre-industrial times. Identifying the fate of that carbon has been one of the great challenges in paleoceanography. We present evidence from ten equatorial Pacific Ocean sediment cores to show that the deep Pacific Ocean likely stored more carbon during the last glacial period than the Holocene. The concentration of the redox-sensitive metal uranium (U) in sediments was systematically greater during the last glacial period than during the Holocene, indicating more reducing conditions in glacial-age sediments. Reconstructions of biogenic opal flux indicate that changes in U distribution were not the result of changes in biological productivity and the ensuing rain of organic carbon to the sea floor, which also affects the redox conditions of the sediments. Together, these results lead to the interpretation that bottom water in the equatorial Pacific during the glacial period had significantly lower oxygen concentration than during the Holocene, and a correspondingly greater level of respired CO2. This conclusion is supported by evidence for greater preservation of brassicasterol, a biomarker produced by diatoms, in glacial-age sediments of the eastern equatorial Pacific. The presence of additional respired CO2 in the glacial deep ocean would have lowered atmospheric pCO2 by 1) increasing the total storage of CO2 in the glacial ocean and 2) increasing ocean alkalinity following a transient carbonate dissolution event. Our results are consistent with recently published data from the North Pacific Ocean; this suggests that increased carbon storage in the glacial deep Pacific Ocean was a basin-wide phenomenon, consistent with a large-scale transfer of carbon to the deep ocean during glacial periods.