Dynamic seasonal cycling of inorganic carbon downstream of South Georgia, Southern Ocean

Abstract

The influence of the island mass effect of South Georgia on the seasonal marine carbon cycle was investigated during austral summer (January–February) 2008. South Georgia (54–55°S 36–38°W) lies on the North Scotia Ridge, strongly influencing the passage of the Southern Antarctic Circumpolar Current Front to the south. Surface waters upstream of the island, in the central Scotia Sea, were characterised by relative high-nutrient low-chlorophyll (HNLC) conditions from winter (September) 2007 to summer, as indicated by satellite and shipboard observations. The fugacity of carbon dioxide ( fCO 2) was slightly supersaturated and the HNLC waters represented a summertime CO 2 source of 2.6±1.5 mmol m −2 day −1. Extensive phytoplankton blooms developed in the Georgia Basin, downstream of South Georgia, in October 2007 and persisted until March 2008. The seasonal depletion in dissolved inorganic carbon (DIC) was 94±1 μmol kg −1 and the Δ fCO 2(sea–air) was –92±21 μatm in the core of the bloom by early February. These conditions created a strong sink for atmospheric CO 2 of –12.9±11.7 mmol m −2 day −1. In contrast, wintertime mixing into DIC-rich sub-surface waters created a strong CO 2 source of 22.0±14.4 mmol m −2 day −1. These processes drive substantial seasonal changes in DIC of up to −0.7 μmol kg −1 day −1 from winter to summer. Similarly to the blooms of Kerguelen and Crozet, the South Georgia bloom is likely to be fuelled by natural iron fertilisation. A DIC deficit of 2.2±0.3 mol m −2 upstream of South Georgia suggested that the relative HNLC waters were more productive than indicated by satellites. The DIC deficit more than doubled downstream of South Georgia (4.6±0.8 mol m −2) to create the strongest seasonal carbon uptake in ice-free waters of the Southern Ocean to date.