Glacial water mass structure and rapid δ18O and δ13C changes during the last glacial termination in the Southwest Pacific

Abstract

Changes in ocean circulation are thought to have contributed to lowering glacial atmospheric CO 2 levels by enhancing deep ocean sequestration of carbon that was returned to the atmosphere during glacial terminations. High-resolution benthic foraminiferal δ 13 C and δ 18 O records from a depth transect of cores in the Southwest Pacific Ocean presented here provide evidence that both wind-and thermohaline-driven circulation drove CO 2 from the ocean during the last deglaciation. Shallow geochemical stratification in the glacial Southern Ocean was followed by a short pulse of rapid δ 13 C enrichment to intermediate water depths during Heinrich Stadial 1, indicative of betterventilated intermediate waters co-occurring with documented wind-driven upwelling in the Southern Ocean. Intermediate depth  13 C enrichment paused at the start of the Antarctic Cold Reversal (~14.7 ka), implying a brief shallow restratification, while deeper layers were progressively flushed of δ 13 C-depleted and δ 18 O-enriched waters, likely caused by the increasing influence of deep waters sourced from the North Atlantic. The coincidence of atmospheric CO 2 increases with these geochemical shifts in both shallow and deep cores suggests that shifts in both atmospheric and oceanic circulation contributed to the deglacial rise of CO 2 .

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

“…This work compares previously published benthic foraminiferal stable isotopic results from two depth transects, one located on the Brazil Margin in the South Atlantic (Lund et al, ) and another from New Zealand in the Southwest Pacific (Sikes, Elmore, et al, ). The principal portion of the Pacific depth transect is composed of five jumbo piston cores collected from the Bay of Plenty, north and east of the North Island of New Zealand, augmented with deeper cores from the Tasman Sea and South Tasman Rise (Figure S1).…”

“…This continued into the early deglaciation, until the ACR at about 14 ka. This appears to have been caused by a shallow pulse of enrichment in the Pacific above 1.6 km during the second half of HS1 that terminated in the ACR (Sikes, Elmore, et al, ). This difference diminished and became slightly positive from the ACR to the mid Holocene.…”

“…The LGM to Holocene isotopic transition across a broad depth profile in the individual basins has been previously described (Lund et al, ; Sikes, Elmore, et al, ). The focus of this paper is to observe Atlantic and Pacific differences during the LGM and last deglaciation.…”

“…Notably, this δ 13 C divide in the Atlantic shoaled to sill depth at the latitude of Drake Passage (~2.5 km) (Curry & Oppo, ). Many lines of evidence, including Δ 14 C, have suggested that the ocean's geochemical divide shoaled in the LGM (e.g., Curry & Oppo, ; Ferrari et al, ; Sikes, Cook, et al, , Sikes, Elmore, et al, ). Modeling results indicate that reduced bottom drag, consistent with a weaker ACC, leads to shallower stratification with the volume of circumpolar transport relatively insensitive to surface wind stress (Marshall et al, ).…”

“…Published South Atlantic‐South Pacific comparisons of LGM δ 13 C indicate larger differences in geochemical signatures between these basins during the LGM (Charles et al, ; Hu et al, ; Ninnemann & Charles, ). Depth profiles suggest that the deep geochemical divide in the Pacific (as indicated by δ 13 C) was around 1000 m shallower than in the Atlantic (Hodell et al, ; McCave et al, ; Sikes, Elmore, et al, ) with Pacific UCDW δ 13 C ~1‰ more depleted than modern and ~1‰ depleted relative to the Atlantic (McCave et al, ; Sikes, Elmore, et al, ). Estimates of paleodensity suggest that these geochemical divides may not necessarily correspond to physical stratification (Roberts et al, ).…”