Abstract
δ13C records from the mid-depth Atlantic show a pronounced decrease during the Heinrich Stadial 1 (HS1), a deglacial episode of dramatically weakened Atlantic Meridional Ocean Circulation (AMOC). Proposed explanations for this mid-depth decrease include a greater fraction of δ13C-depleted southern sourced water (SSW), a δ13C decrease in the North Atlantic Deep Water (NADW) end-member, and accumulation of the respired organic carbon. However, the relative importance of these proposed mechanisms cannot be quantitatively constrained from current available observations alone. Here we diagnose the individual contributions to the deglacial Atlantic mid-depth δ13C change from these mechanisms using a transient simulation with carbon isotopes and idealized tracers. We find that although the fraction of the low-δ13C SSW increases in response to a weaker AMOC during HS1, the water mass mixture change only plays a minor role in the mid-depth Atlantic δ13C decrease. Instead, increased remineralization due to the AMOC-induced mid-depth ocean ventilation decrease is the dominant cause. In this study, we differentiate between the deep end-members, which are assigned to deep water regions used in previous paleoceanography studies, and the surface end-members, which are from the near-surface water defined from the physical origin of deep water masses. We find that the deep NADW end-member includes additional remineralized material accumulated when sinking from the surface (surface NADW end-member). Therefore, the surface end-members should be used in diagnosing mechanisms of δ13C changes. Furthermore, our results suggest that remineralization in the surface end-member is more critical than the remineralization along the transport pathway from the near-surface formation region to the deep ocean, especially during the early deglaciation.
Highlights
The climate experienced significant changes during the last deglaciation
From Last Glacial Maximum (LGM) to Heinrich Stadial 1 (HS1), the simulated Atlantic Meridional Ocean Circulation (AMOC) strength shows a large decrease, which is followed by the recovery during Bølling– Allerød (BA) (Fig. 2a)
At the LGM, the high δ13C associated with North Atlantic Deep Water (NADW) penetrates southward at mid-depth (Fig. 1a), and the simulated δ13C north-south gradient in the Atlantic is in agreement with observations (Gu et al, 2020)
Summary
The climate experienced significant changes during the last deglaciation. One key event is the Heinrich Stadial 1 (HS1) (17.5-14.7 ka BP) when the Atlantic Meridional Overturning Circulation (AMOC) is nearly collapsed (McManus et al, 2004) or significantly weakened (Gherardi et al, 2009). With reduced North Atlantic Deep Water (NADW) formation during HS1 (McManus et al, 2004), an increased contribution of low-δ13C southern-sourced water (SSW; mainly comprised of Antarctic Bottom Water, AABW) in the Atlantic is widely believed to have caused the mid-depth δ13C decrease (Boyle and Keigwin, 1987; Keigwin and Lehman, 1994; Rickaby and Elderfield, 2005; Sarnthein et al, 1994; Zahn et al, 1997). A third view argues that changes in the end-member value of NADW could have caused the mid-depth δ13C decrease (Lund et al, 2015; Oppo et al, 2015). Our knowledge of the water mass composition in the Atlantic during the last deglaciation limits the quantitative assessment of the proposed mechanisms in the Atlantic mid-depth δ13C decrease during the deglaciation
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