The role of the southern Indian Ocean in the glacial to interglacial atmospheric CO₂ change: organic carbon isotope evidences

Abstract

We use the carbon isotopic composition of sedimentary organic matter from marine cores as a proxy for paleoproductivity and paleo-dissolved carbon dioxide concentrations in surface waters of the Southern Ocean during the last 50,000 years. For the Holocene period, paleo-pCO₂ reconstructions between 44° and 55°S reflect the preindustrial values (270 μatm) and are in the variation range of modern ones. During the glaciation and deglaciation periods, pCO₂ was 50 to 100 μatm higher in surface water than in the atmosphere as recorded in the Vostok ice core. This suggests that the Southern Indian Ocean could have been a potential source of CO₂ for the glacial and deglacial atmosphere. These results, plus those of sedimentary organic carbon content suggest that the biological pump was off and unable to lower atmospheric CO₂ concentration. This indicates that a winter ice covered ocean and stratification of summer surface water caused a reduction of gas exchange with the atmosphere during that period.

Introduction

The storage and transport of carbon in the climate system have received increasing attention in the past years, especially with regard to past CO₂ fluctuations and their relation to natural climate variability. Measurements from ice cores have yielded an impressive record of changes in the concentration of atmospheric CO₂ over the last climatic cycle (Barnola et al., 1987). The CO₂ concentration was low during glacial times and rose rapidly at the onset of interglacial periods. It is generally accepted that the oceans control these changes as their carbon dioxide reservoir is 60 times that of atmosphere. It is argued that the Southern Ocean may have been a potential sink for glacial CO₂ due to its large nutrient reservoir (Sarmiento and Toggweiler, 1984; Knox and McElroy, 1984; Siegenthaler and Wenk, 1984). More efficient use of these nutrients or increased availability of iron coupled with changes in grazing pressure of zooplankton could comprise a biological mechanism to reduce CO₂ (Keir, 1988; Martin et al., 1990; Martin, 1990; Berger and Wefer, 1991; Sarmiento and Orr, 1991; Cullen, 1995). However, studies of sedimentary records of the Southern Ocean do not support the hypothesis of increased of glacial productivity (Labeyrie and Duplessy, 1985; Boyle, 1988; Charles and Fairbanks, 1990; Mortlock et al., 1991; François et al., 1992; Kumar et al., 1993; Shemesh et al., 1994; Shimmield et al., 1994; Singer and Shemesh, 1995; Bareille et al., 1998). More recent studies give a less clear pattern showing that glacial productivity is extremely variable and depends on the specific study locations and the methods used to reconstruct paleofluxes of matter (Anderson and Kumar, 1995; Bareille et al., 1995a; Bohrmann et al., 1995; Bonn et al., 1995; François et al., 1995a, François et al., 1995b; Frank et al., 1995; Kumar et al., 1995; Nürnberg et al., 1995 and Rosenthal et al., 1995). Other hypothesis, including increased efficiency of the solubility pump (Keir, 1993a, Keir, 1993b) and redistribution of ocean alkalinity (see among others: Broecker and Peng, 1989, Broecker and Peng, 1993; Archer and Maier-Reimer, 1994; Opdyke and Walker, 1992, Keir, 1995) have also been proposed to explain glacial CO₂ concentration reduction in the atmosphere. Whatever the mechanism responsible for the glacial pCO₂ drawdown, the role of the Southern Ocean remains poorly defined.

We present paleo-pCO₂ reconstructions for surface waters based on a direct relationship between organic carbon isotopic composition ($\text{δ}{}^{13}\text{C}$) of phytoplankton and the dissolved CO₂ concentrations [CO₂(aq)] established for the dominant autotrophic organisms living south of the Sub-Tropical Convergence in the Indian sector of the Southern Ocean (Bentaleb et al., 1995, Bentaleb et al., 1998). Comparing these results with the Vostok ice core record allows us to determine the past status of atmosphere-ocean equilibrium of the Southern Ocean. As pCO₂ variations are partly linked to productivity, this allows a different approach, independent of estimations of matter paleo-flux, to evaluate the strength of the biological pump and the gas exchange between atmosphere and ocean during glacial times.