Surface layer balance of the southern Antarctic Circumpolar Current (prime meridian) used to derive carbon and silicate consumptions and annual air‐sea exchange for CO2 and oxygen

Abstract

A simple model, using concentrations of nitrate and phosphate in austral winter 1992, reveals that the Antarctic Surface Water (AASW) of the southernmost Antarctic Circumpolar Current (ACC) between the Southern ACC Front and the Weddell Front is made up of ∼90% Upper Circumpolar Deep Water (UCDW) and 10% northward flowing AASW from the Weddell Gyre. With a typical timescale of ∼1 year the upwelling velocity was calculated to be as high as 60–100 m yr−1. Knowing the composition of the surface water with respect to its sources, changes due to several processes in the surface layer were deduced for carbon dioxide, oxygen, and silicate. As the timescale of changes in the surface layer of the southern ACC is ∼1 year, this allows us to calculate changes on an annual basis without interference of short‐term variations. Balancing the contributions by upwelling, biological activity, and air‐sea exchange to the concentrations in the surface layer, the area was found to be a large sink for atmospheric oxygen of 6.0 mol m−2 yr−1 (53 μmol kg−1) and a small sink for atmospheric carbon dioxide of 1.0 mol m−2 yr−1 (9 μmol kg−1). The most important cause for the oxygen sink is the upwelling of oxygen‐poor UCDW, which surpasses the oxygen‐elevating effect of primary productivity. This large oxygen sink, in between areas to the north and south which are only a small sink or even a source, conforms with the latitudinal distribution of atmospheric oxygen. The small CO2 sink is largely brought about by biological activity. The annual carbon utilization amounts to 76±22 g C m−2 yr−1, which is relatively high for an open ocean region in the Antarctic. However, it supports recent estimates of primary production of the Antarctic Ocean that are higher than early published values. The annual silicate consumption was calculated to be 126±19 g Si m−2 yr−1. This is considerably higher than the Southern Ocean mean in current estimates. Although the southernmost ACC may be atypical for the Southern Ocean, the current estimate for Southern Ocean silica production may well be an underestimation. The silicate to carbon utilization ratio derived here is 0.53 which aligns with investigations on Antarctic phytoplankton and thus underscores the consistency of our results.