Quantifying the roles of ocean circulation and biogeochemistry in governing ocean carbon-13 and atmospheric carbon dioxide at the last glacial maximum

A Tagliabue,D Paillard,J.-C Dutay,N Bouttes,E Michel,M Kageyama,R Alkama,L Bopp,D M Roche

doi:10.5194/cp-5-695-2009

Abstract

Abstract. We use a state-of-the-art ocean general circulation and biogeochemistry model to examine the impact of changes in ocean circulation and biogeochemistry in governing the change in ocean carbon-13 and atmospheric CO2 at the last glacial maximum (LGM). We examine 5 different realisations of the ocean's overturning circulation produced by a fully coupled atmosphere-ocean model under LGM forcing and suggested changes in the atmospheric deposition of iron and phytoplankton physiology at the LGM. Measured changes in carbon-13 and carbon-14, as well as a qualitative reconstruction of the change in ocean carbon export are used to evaluate the results. Overall, we find that while a reduction in ocean ventilation at the LGM is necessary to reproduce carbon-13 and carbon-14 observations, this circulation results in a low net sink for atmospheric CO2. In contrast, while biogeochemical processes contribute little to carbon isotopes, we propose that most of the change in atmospheric CO2 was due to such factors. However, the lesser role for circulation means that when all plausible factors are accounted for, most of the necessary CO2 change remains to be explained. This presents a serious challenge to our understanding of the mechanisms behind changes in the global carbon cycle during the geologic past.

Highlights

Reproducing past changes in the global carbon cycle is a key test of our understanding of the Earth’s climate system and, as such, explaining the documented changes in atmospheric gases and geochemical proxies that occurred during the last glacial maximum (LGM) remains an enduring challenge
We find that LGM dust reduces Southern Ocean (SO) diatom silicification by 12% and silicic acid “leakage” to low latitudes does promote diatom productivity, which reduces the relative export of alkalinity by 7% globally
Our results suggest that LGM dust is the driver of increased export in the South Atlantic, while reduced export in the Antarctic sector of the Southern Ocean is a result of circulation/sea-ice changes

Summary

Introduction

Reproducing past changes in the global carbon cycle is a key test of our understanding of the Earth’s climate system and, as such, explaining the documented changes in atmospheric gases and geochemical proxies that occurred during the last glacial maximum (LGM) remains an enduring challenge. Since the carbon stored in the terrestrial biosphere likely declined (e.g., Bird, et al, 1994; Sigman and Boyle, 2000), the ocean is believed to be responsible for the 80ppm reduction in atmospheric carbon dioxide (pCO2atm) measured in ice cores (Petit et al, 1999) at the LGM. In reproducing this change, only ∼50 ppm needs to be explained by a given hypothesis, as the remainder can be accounted for by subsequent deep-ocean carbonate compensation (e.g., Brovkin et al, 2007).

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Results

Conclusion