The impact of marginal ice zone processes on the distribution of 21OPb, 21OPo and 234Th and implications for new production in the Bellingshausen Sea, Antarctica

Abstract

The vertical distributions of 21OPb, 210Po and 234Th in both dissolved and particulate phases of seawater were measured at five stations along the 85°W meridian in the Bellingshausen Sea, Antarctica. Sea-ice conditions during the expedition ranged from fully ice-covered (fast ice) to open water away from the marginal ice zone. Concurrent primary productivity and algal chlorophyll measurements revealed a band of high productivity at approximately 67°30′S, which remained in a rather static location during ice melt-back. Along the transect a progressive increase in removal (“scavenging”) and sinking of 210Pb, 210Po and 234Th occurred towards the north (open water conditions). Application of a simple, irreversible scavenging model, and particulate organic carbon and organic nitrogen to radionuclide ratios measured on suspended particulate matter, allows the calculation of export production for this region. Under fast ice conditions some removal of phytodetritus from the euphotic zone was observed (1–2 mgC m −2 day −1), representing a low proportion of the primary production ( f-ratio of 0.02). Near the Antarctic continental margin upwelling of regenerated dissolved 210Po was found. At the open water site near the algal band, new production had risen to 27–252 mgC m −2 day −1, but a greater proportion of the original primary production was removed by sinking detritus ( f-ratio of 0.03–0.33). Comparison of 210Po and 234Th as tracers of of export production indicates considerable differences, with 210Po yielding rather lower rates of export flux compared to 234Th Radionuclide estimates of new production are significantly greater than those obtained by traditional sediment trap methods. This may be due to the dynamic nature (short duration and spatial variability) of algal growth in the Southern Ocean.