Sensitivity of δ13C of Southern Ocean suspended and sinking organic matter to temperature, nutrient utilization, and atmospheric CO 2

Abstract

The carbon isotopic composition of organic matter ( δ 13C 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 (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 δ 13C org at the surface were preserved at depth, and also investigated the connection between the dissolved molecular CO 2 concentration ([CO 2(aq)]) and δ 13C org in the SAZ and PFZ, including the relative importance of temperature and biological activity in controlling this relationship. δ 13C org 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 δ 13C org are well correlated with variations in the [CO 2(aq)]. δ 13C org 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 δ 13C org 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 δ 13C org at the surface and δ 13C org 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 δ 13C org encountered here (i.e. between the SAZ and the PFZ) appear to result from temperature driven differences in CO 2 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 δ 13C org changes recorded in sediments. Cooler glacial temperatures are insufficient to maintain the high [CO 2(aq)] necessary to explain observed low glacial δ 13C org. Upwelling of deep-waters can sufficiently further increase [CO 2(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 δ 15N observations suggest that some process must have affected δ 13C org and δ 15N differentially, possibly the influence of increased glacial iron availability on nitrogen metabolism.