Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release

Julia Gottschalk,Sönke Szidat,Elisabeth Michel,Alain Mazaud,Alfredo Martínez-García,Anja S Studer,Lena M Thöle,Samuel L Jaccard,Nicole Schmid,Martin Butzin,Adam P Hasenfratz

doi:10.1038/s41467-020-20034-1

Abstract

Past changes in ocean 14C disequilibria have been suggested to reflect the Southern Ocean control on global exogenic carbon cycling. Yet, the volumetric extent of the glacial carbon pool and the deglacial mechanisms contributing to release remineralized carbon, particularly from regions with enhanced mixing today, remain insufficiently constrained. Here, we reconstruct the deglacial ventilation history of the South Indian upwelling hotspot near Kerguelen Island, using high-resolution 14C-dating of smaller-than-conventional foraminiferal samples and multi-proxy deep-ocean oxygen estimates. We find marked regional differences in Southern Ocean overturning with distinct South Indian fingerprints on (early de-)glacial atmospheric CO2 change. The dissipation of this heterogeneity commenced 14.6 kyr ago, signaling the onset of modern-like, strong South Indian Ocean upwelling, likely promoted by rejuvenated Atlantic overturning. Our findings highlight the South Indian Ocean’s capacity to influence atmospheric CO2 levels and amplify the impacts of inter-hemispheric climate variability on global carbon cycling within centuries and millennia.

Highlights

Past changes in ocean 14C disequilibria have been suggested to reflect the Southern Ocean control on global exogenic carbon cycling
We assessSST variability independently of the uncertainties inherent to each proxy based on the first principal component (PC1) of all three datasets (Fig. 3), which accounts for 77% of the data variance
Applying the most recent Δδ13C[O2] calibration[36], we find that bottom water [O2] during the last glacial maximum (LGM) was lowered by 100 ± 40 μmol kg−1 from present-day concentrations (~220 μmol kg−1; Fig. 6)[37], which is consistent with higher d14RB-Atm values, increased sedimentary authigenic U (aU) enrichments and higher foraminiferal U/Mn ratios (Fig. 6)

Summary

Introduction

Past changes in ocean 14C disequilibria have been suggested to reflect the Southern Ocean control on global exogenic carbon cycling. Intervals of deglacial CO2,atm rise coincide with cold spells in the northern-hemisphere, i.e. Heinrich stadial (HS) 1 and the Younger Dryas (YD), as well as a concomitant rise in Antarctic air temperature (the thermal bipolar seesaw; Fig. 1) Both phases are interrupted by stagnating CO2,atm levels, and warm climate conditions in the North Atlantic, i.e. the Bølling Allerød (BA). Current (ACC) with local bathymetry in regions often referred to as upwelling hotspots[11,12] (Fig. 2) One of these important regions is located in the South Indian Ocean, where the ACC impinges on the Kerguelen Plateau (Fig. 2, Supplementary Fig. 1), causing elevated vertical mixing rates, enhanced cross-frontal exchange and efficient export of subsurface waters to the north[11,12]. Limited carbonate preservation, low abundances of foraminifera and/or uncertainties related to temporal changes in surface-ocean reservoir ages[13] can pose

Methods

Results

Conclusion