Enhanced δ13C and δ18O Differences Between the South Atlantic and South Pacific During the Last Glaciation: The Deep Gateway Hypothesis

Enhanced vertical gradients in benthic foraminiferal δ13C and δ18O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North-Atlantic sourced interior waters; nonetheless our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high-resolution depth transects of Cibicidoides spp. δ13C and δ18O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between ~1.0 and 2.5 km was more than 1 ‰ enriched in δ13C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ18O was ~ 0.5 ‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a ‘deep gateway’ effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east-west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern-sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.