Sensitivity of δ13C of Southern Ocean suspended and sinking organic matter to temperature, nutrient utilization, and atmospheric CO2

The carbon isotopic composition of organic matter (delta(13)C(org)) was measured for particles suspended in surface waters (from six north-south transects across the subantarctic (SAZ) and polar frontal zones (PFZ) of the Southern Ocean south of Australia between September 1997 and March 1998), and obtained from sediment traps deployed during the same period at 1060, 2050 and 3850 m depth in the SAZ (47degreesS), 3080 m under the Subantarctic Front (51degreesS) and 830 and 1580 m in the PFZ (54degreesS). We examined whether spatial and temporal patterns of particulate delta(13)C(org) at the surface were preserved at depth, and also investigated the connection between the dissolved molecular CO, concentration ([CO2(aq)]) and gamma(13)C(org) in the SAZ and PFZ, including the relative importance of temperature and biological activity in controlling this relationship. delta(13)C(org) of surface-water organic matter was up to 4.5parts per thousand higher in the SAZ than the PFZ and underwent a seasonal increase of similar to2.5parts per thousand (from similar to -25.5parts per thousand to similar to -23parts per thousand) in the SAZ and similar to 1.5 (from similar to -26.5parts per thousand to similar to -25%) in the PFZ. These spatial and temporal variations in delta(13)C(org) are well correlated with variations in the [CO2(aq)]. delta(13)C(org) of material collected in deep-water sediment traps was also higher in the SAZ (similar to -22parts per thousand) than PFZ (similar to -24.5parts per thousand), with some variability but no clear seasonal change in either region. The delta(13)C(org) of organic matter reaching deep-water sediment traps (> 830 m) in the spring was higher than at the surface by similar to4parts per thousand in the SAZ and similar to2parts per thousand in the PFZ, suggesting that preferential export of some components of surface organic matter may occur or that the extent of remineralisation of sinking materials varies seasonally. However, the seasonally averaged offset between delta(13)C(org) at the surface and delta(13)C(org) in the sediment traps was similar in the two regions (1.5parts per thousand and 1.8parts per thousand in the SAZ and PFZ, respectively). The largest differences in delta(13)C(org) encountered here (i.e. between the SAZ and the PFZ) appear to result from temperature driven differences in CO2 Solubility rather than differences in biological production. We applied these results to quantify the relative contributions of temperature, nutrient utilization, and atmospheric equilibration to glacial-interglacial delta(13)C(org) changes recorded in sediments. Cooler glacial temperatures are insufficient to maintain the high [CO2(aq)] necessary to explain observed low glacial delta(13)C(org). Upwelling of deep-waters can sufficiently further increase [CO2(aq)], provided extensive sea-ice cover restricts air-sea equilibration, and provided nutrient utilisation is not much larger than current values. Lowered phytoplankton growth rates could also partially contribute. Reconciling these results with high glacial delta(15)N observations suggest that some process must have affected delta(13)C(org) and delta(15)N differentially, possibly the influence of increased glacial iron availability on nitrogen metabolism. (C) 2003 Elsevier Ltd. All rights reserved.