Southwest Pacific Vertical Structure Influences on Oceanic Carbon Storage Since the Last Glacial Maximum

Abstract

Lower atmospheric CO2 concentrations during the Last Glacial Maximum (LGM; 23.0–18.0 ka) have been attributed to the sequestration of respired carbon in the ocean interior, yet the mechanism responsible for the release of this CO2 during the deglaciation remains uncertain. Here we present calculations of vertical differences in oxygen and carbon isotopes (∆δ18O and ∆δ13C, respectively) from a depth transect of southwest Pacific Ocean sediment cores to reconstruct changes in water mass structure and CO2 storage. During the Last Glacial Maximum, ∆δ18O indicates a more homogenous deep Pacific below 1,100 m, whereas regional ∆δ13C elucidates greater sequestration of CO2 in two distinct layers: enhanced CO2 storage at intermediate depths between ~940 and 1,400 m, and significantly more CO2 at 1,600 m and below. This highlights an isolated glacial intermediate water mass and places the main geochemical divide at least 500 m shallower than the Holocene. During the initial stages of the deglaciation in Heinrich Stadial 1 (17.5–14.5 ka), restructuring of the upper ~2,000 m of the southwest Pacific water column coincided with sea-ice retreat and rapid CO2 release from intermediate depths, while CO2 release from the deep ocean was earlier and more gradual than waters above it. These changes suggest that sea-ice retreat and shifts in Southern Ocean frontal locations contributed to rapid CO2 ventilation from the Southern Ocean's intermediate depths and gradual ventilation from the deep ocean during the early deglaciation.