SEASONAL AND SPATIAL VARIATIONS IN POREWATER CHEMISTRY IN THE BRANSFIELD AND GERLACHE STRAITS, ANTARCTICA

The southern Bransfield and northern Gerlache straits, along the Western Antarctic Peninsula were sampled to observe seasonal variations in pore water chemistry and its control on foraminiferal community structure and chemistry of the foraminiferal tests. This region exhibits extreme seasonality in primary productivity because of its high latitude and seclusion from nutrient input by sea ice during the winter. Seven sites were sampled in April 2008, following the summer bloom, and once in June 2008, prior to complete sea ice coverage with three replicate multi-corer drops across a productivity gradient. Pore water was extracted from a single core from the first and third drops by centrifugation under a nitrogen atmosphere. Sites 1-3, located in the southern Bransfield Strait at water depths of 500-1200m were located below low productivity water, while Sites 4-7, located in the Gerlache Strait at depths of 600-1200 m, and were under high productivity water. Sites 1-3 showed less remineralization of organic carbon, with lower alkalinity and dissolved inorganic carbon (DIC) concentrations compared to sites 4-7. At Sites 4-7, sulfate concentrations decrease to as low as 25 mM at 30 cm below the seafloor (cmbsf), but no H2S was detected, presumably because of reaction with dissolved Fe2+. Sites 5 and 6, which occur at the deepest portions of the Gerlache basin that were sampled (1150-1250m water depth), have the highest NH4+, alkalinity and DIC concentrations (680μM, 9mM,8mM respectively) and the lightest d13C values (-14‰) of all sites. At Site 5, June cores were from water depths about 50 m shallower than April and have more sulfate reduction and higher DIC, NH4+, and alkalinity concentrations, opposite to expectations based on productivity. At Site 6, both April cores and one June core were sampled at similar water depths and have approximately identical compositions, but the second June core, sampled at a water depth 50m shallower than the others has higher sulfate concentrations and lower alkalinity, DIC, and NH4+ concentrations. The different compositions at these sites appear to be the result of differences in sampling location, particularly in the change in water depths, rather than differences in productivity between seasons.