| Summary: | 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.
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