Twilight zone organic carbon remineralization in the Polar Front Zone and Subantarctic Zone south of Tasmania

We report on the distribution of excess, non-lithogenic particulate barium (Ba-xs), a proxy for twilight zone remineralization of organic matter, in the Australian sector of the Southern Ocean during the mid-austral summer 2007. This study was part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Mesopelagic particulate excess Ba (Ba-xs) in the twilight zone (focus was on the 100-600 m depth layer) proved to be significantly controlled by the vertical distribution of bacterial activity, with higher Ba-xs contents in situations where significant bacterial activity extended deeper in the water column. However, despite this covariation, the magnitude of the carbon fluxes as estimated from Ba-xs and bacterial activity did not match well, with carbon demand based on bacterial activity largely exceeding organic carbon remineralization estimated from Ba-xs. Possible reasons for this discrepancy are discussed. It appeared that the magnitude of the mesopelagic carbon remineralization flux obtained from Ba-xs was realistic when weighted against primary, new and export productions. Our findings corroborate earlier results indicating that mesopelagic carbon remineralization in the 100-600 m depth layer increases from the Subantarctic Zone (SAZ) toward the Polar Front, and from spring to late summer. We observed furthermore that the iron-replete Subantarctic Zone east of the Tasman Plateau had a higher mesopelagic remineralization efficiency (on average 91 +/- 20% of the carbon exported from the upper 100 m was remineralized before reaching 600 m) compared to the Fe-limited SAZ area west of the plateau and the Polar Front Zone, where mesopelagic remineralization efficiencies ranged between 21 +/- 5% and 64 +/- 24%, respectively. SAZ-East and SAZ-West sites thus differed in their efficiency of carbon sequestration into the deep ( > 600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. (C) 2011 Elsevier Ltd. All rights reserved.