The Southern Ocean at the last glacial maximum: A strong sink for atmospheric carbon dioxide

Analysis of satellite ocean color, sea surface temperature, and sea ice cov er data reveals consistent patterns between biological production, iron ava ilability, and physical forcings in the Southern Ocean. The consistency of these patterns, in conjunction with information on physical conditions duri ng the last glacial maximum (LGM), enables estimates of export production a t the LGM. The LGM Southern Ocean experienced increased wind speeds, colder sea surface and atmospheric temperatures, increased deposition of atmosphe ric dust, and a greatly expanded winter sea ice cover. These variations had strong effects on Southern Ocean ecology and on air-sea fluxes of CO2. The seasonal ice zone (SIZ) was much larger at the LGM (30 million km(2)) than at present (19 million km(2)). The Antarctic Polar Front (PF) likely marke d the northern boundary of this expanded SIZ throughout the Southern Ocean, as it does today in the Drake Passage region. A large northward shift in t he position of the PF during glacial times is unlikely due to topographic c onstraints. North of the PF, the increased flux of aeolian dust during glac ial times altered phytoplankton species composition and increased export pr oduction, and as a result this region was a stronger sink for atmospheric C O2 than in the modem ocean. South of the PF, interactions between the biota and sea ice strongly influence air-sea gas exchange over seasonal timescal es. The combined influence of melting sea ice and increased aeolian dust fl ux (with its associated iron) increased both primary and export production by phytoplankton over daily-monthly timescales during austral spring/summer , resulting in a strong flux of CO2 into the ocean. Heavy ice cover would h ave minimized air-sea gas exchange over much of the rest of the year. Thus, an increased net flux of CO2 into the ocean is likely during glacial times , even in areas where annual primary production declined. We estimate that export production in the Southern Ocean as a whole was increased by 2.9 - 3 .6 Ct C yr at the LGM, relative to the modern era. Altered seasonal sea ice dynamics would further increase the net flux of CO2 into the ocean. Thus t he Southern Ocean was a strong sink for atmospheric CO2 and contributed sub stantially to the lowering of atmospheric CO2 levels during the last ice ag e.