Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period

Abstract

The nitrogen-isotope record preserved in Southern Ocean sediments, along with several geochemical tracers for the settling fluxes of biogenic matter, reveals patterns of past nutrient supply to phytoplankton and surface-water stratification in this oceanic region. Areal averaging of these spatial patterns indicates that reduction of the CO 2 'leak' from ocean to atmosphere by increased surface-water stratification south of the Polar Front made a greater contribution to the lowering of atmospheric CO 2 concentration during the Last Glacial Maximum than did the increased export of organic carbon from surface to deep waters occurring further north.The unstratified, nutrient-rich surface waters in the modern highlatitude ocean provide the main conduit for transferring deep-water CO 2 back into the atmosphere. This CO 2 'leak' to the atmosphere is particularly effective in the modern Southern Ocean, because of intensive vertical mixing and low nutrient utilization. Building on this observation, a series of papers 1-6 attributed the glacial lowering of atmospheric CO 2 either to enhanced biological removal of the nutrients and CO 2 in high-latitude surface waters resulting in higher sinking fluxes of organic matter (that is, higher export production), or to a lower supply rate of nutrients and CO 2 from intermediate waters produced by lower vertical mixing. But the extensive deepor intermediate-water anoxia predicted by these models, and the lack of palaeo-oceanographic evidence for increased export production in the glacial Southern Ocean, has led to the questioning of the validity of these models. Here we present new evidence which supports increased stratification south of the position of the modern Polar Front (MPF) during the Last Glacial Maximum as a mechanism that contributed to lower glacial atmospheric CO 2 and deep-water oxygen concentration.The sedimentary record of several palaeoproductivity proxies was recently presented as support for a large increase in the export flux of organic carbon in the Atlantic sector of the Southern Ocean, north of the position of the MPF, which could have contributed to lowering glacial atmospheric CO 2 as a result of Fe fertilization 7 . Here we combine a similar suite of geochemical proxies for palaeoproductivity and its fate with bulk-sediment d 15 N values to better constrain past changes in the nutrient balance of surface waters and its influence on atmospheric CO 2 . Bulk-sediment d 15 N provides a means of evaluating the fraction of surface nitrate utilized by phytoplankton [8][9][10][11] . By combining this information with export flux of nitrogen estimated from palaeoproductivity proxies, we constrain the supply rate of nitrate to surface waters. Our results indicate that, despite higher export flux of organic carbon in the Atlantic sector of the Southern Ocean north of the MPF during the last glacial period, the fraction of nitrate utilized by phytoplankton in this region did not increase, implying a sustained supply of nutrients (and thus CO 2 ) to surface waters. In...

Polar stratification: A critical view from the Southern Ocean

“…Such a shift from a CO 2 source to a CO 2 sink between pre-industrial time and present is documented for the Weddell Sea located in the Atlantic Sector of the Southern Ocean (Hoppema, 2004). According to the first hypothesis, permanent surface water stratification south of the APF, possibly in response to sea-ice expansion, acted as a major contributor to the CO 2 drop during the last glacial period (Francois et al, 1997;Sigman and Boyle, 2000). The stratification of the upper water column would have reduced vertical mixing and thus prevented the ventilation of CO 2 -rich deep water.…”

“…Francois et al, 1997;Crosta and Shemesh, 2002;Robinson et al, 2005). Additionally, Francois et al (1997) and Frank et al (2000) measured authigenic uranium and observed a maximum in authigenic uranium content within sediments deposited during the LGM at site PS1768. The authors interpreted the authigenic uranium peak to indicate a decrease in bottom water oxygen concentration and concluded that during the LGM the water column south of the APF must have been less ventilated by increased stratification.…”

“…Most workers compared proxies for palaeoproductivity ( δ 13 C ratios in diatomaceous organic matter, MAR Opal , Ba bio ) with proxies for nutrient utilization (δ 15 N ratios in organic matter, δ 30 Si ratios in diatom valves) and inferred oceanic stratification for times with low productivity and increased nutrient utilization (e.g. Francois et al, 1997;Crosta and Shemesh, 2002;Robinson et al, 2005). Additionally, Francois et al (1997) and Frank et al (2000) measured authigenic uranium and observed a maximum in authigenic uranium content within sediments deposited during the LGM at site PS1768.…”

“…Francois et al, 1997). Nevertheless, the idea of polar stratification triggering global climate change has persisted and even expanded to a concept of bipolar oceanic stratification: this was proposed as an effective mechanism to lock up atmospheric CO 2 by sequestering carbon in the deep sea and to amplify global cooling under generally cool climate conditions.…”

“…Kumar et al, 1995;Francois et al, 1997;Elderfield and Rickaby, 2000;Frank et al, 2000;Moore et al, 2000;Sigman and Boyle, 2000;Stephens and Keeling, 2000;Schneider-Mor et al, 2005;Abelmann et al, 2006). By means of physico-chemical mechanisms, in particular two competing hypotheses have emerged, which attempt to explain the global atmospheric CO 2 -reduction of 80-100 ppm during late Quaternary glacial periods.…”