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 (δ13Corg) 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 (47°S), 3080 m under the Subantarctic Front (51°S) and 830 and 1580 m in the PFZ (54°S). We examined whether spatial and temporal patterns of particulate δ13Corg at the surface were preserved at depth, and also investigated the connection between the dissolved molecular CO2 concentration ([CO2(aq)]) and δ13Corg in the SAZ and PFZ, including the relative importance of temperature and biological activity in controlling this relationship. δ13Corg of surface-water organic matter was up to 4.5‰ higher in the SAZ than the PFZ and underwent a seasonal increase of ∼2.5‰ (from ∼−25.5‰ to ∼−23‰) in the SAZ and ∼1.5‰ (from ∼−26.5‰ to ∼−25‰) in the PFZ. These spatial and temporal variations in δ13Corg are well correlated with variations in the [CO2(aq)]. δ13Corg of material collected in deep-water sediment traps was also higher in the SAZ (∼−22‰) than PFZ (∼−24.5‰), with some variability but no clear seasonal change in either region. The δ13Corg of organic matter reaching deep-water sediment traps (>830 m) in the spring was higher than at the surface by ∼4‰ in the SAZ and ∼2‰ 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 δ13Corg at the surface and δ13Corg in the sediment traps was similar in the two regions (1.5‰ and 1.8‰ in the SAZ and PFZ, respectively). The largest differences in δ13Corg 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 δ13Corg changes recorded in sediments. Cooler glacial temperatures are insufficient to maintain the high [CO2(aq)] necessary to explain observed low glacial δ13Corg. 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 δ15N observations suggest that some process must have affected δ13Corg and δ15N differentially, possibly the influence of increased glacial iron availability on nitrogen metabolism.