Enhanced vertical mixing in the glacial ocean inferred from sedimentary carbon isotopes

Sophie-Berenice Wilmes,J A Mattias Green,Andreas Schmittner

doi:10.1038/s43247-021-00239-y

Sophie-Berenice Wilmes, J A Mattias Green + Show 1 more

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https://doi.org/10.1038/s43247-021-00239-y

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Abstract

Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging. Recent hypotheses suggest that a shoaled Atlantic meridional overturning circulation or increased stratification would have reduced vertical mixing, isolated the abyssal ocean and increased carbon storage, thus contributing to lower atmospheric CO2 concentrations. Here, using an ensemble of ocean simulations, we evaluate impacts of changes in tidal energy dissipation due to lower sea levels on ocean mixing, circulation, and carbon isotope distributions. We find that increased tidal mixing strengthens deep ocean flow rates and decreases vertical gradients of radiocarbon and δ13C in the deep Atlantic. Simulations with a shallower overturning circulation and more vigorous mixing fit sediment isotope data best. Our results, which are conservative, provide observational support that vertical mixing in the glacial Atlantic may have been enhanced due to more vigorous tidal dissipation, despite shoaling of the overturning circulation and increases in stratification.

Highlights

Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging
Sampling Atlantic meridional overturning circulation (AMOC) and tide uncertainties, we have generated an ensemble of LGM model circulation states by modifying the Southern Hemisphere moisture diffusivity[24] and applying three different tidal dissipation cases from Wilmes et al.[14]: present day (PD) with 1 TW (1 TW = 1012 W) global internal tide dissipation, LGM ICE-6G (1.8 TW) and LGM ICE-5G (3.6 TW)
For equivalent AMOC strengths, the AABW circulation cell is enhanced in simulations with stronger tidal forcing

Summary

Introduction

Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging. Recent hypotheses suggest that a shoaled Atlantic meridional overturning circulation or increased stratification would have reduced vertical mixing, isolated the abyssal ocean and increased carbon storage, contributing to lower atmospheric CO2 concentrations. A number of studies hypothesize decreases in vertical mixing during the Last Glacial Maximum (26.5–19 kyr BP; LGM) due to an increase in stratification[1,2,3,4], or a shoaling of the Atlantic meridional overturning circulation (AMOC)[5,6,7], which would have isolated the deep ocean, increased carbon storage there and contributed to lowering atmospheric CO2. Since tidal mixing arises through interactions with topography, it is greatest near the sea floor and decreases towards the surface It has been suggested[6] that a shallower interface between North Atlantic Deep Water (NADW) and Antarctic. The resulting changes in ocean depth and basin shape enhanced open-ocean tidal amplitudes, especially for the dominant semidiurnal tidal constituent M2, and increased global tidal energy dissipation in the deep ocean during the LGM, with the most prominent enhancements in the North and South Atlantic

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