Carbon Fluxes and Sources of Organic Matter in Late Summer in the Southern Beaufort Sea

IPY2012 Conference Montréal. From Knowledge to Action, 22-27 April 2012, Montréal, Canada
Climate change is expected to deeply affect the Arctic pelagic ecosystems via ice cover reduction and increase of freshwater discharge. Important but mostly unknown consequences are to follow for the downward export of particulate matter and the carbon cycling in general. This situation has caused a growing need of field observations to monitor the complex and rapidly changing Arctic environment and to enlarge the dataset available to modelers. In the framework of the international multidisciplinary project MALINA, an intensive survey in the Beaufort Sea off the Mackenzie River Delta was done during August 2009. The vertical flux of particulate material and carbon was measured and characterized. The sources of the organic carbon pool was also identified by analysis of lipid biomarkers and their δ13C in both suspended particles and surface sediments in order to identify the terrestrial, marine and bacterial sources of carbon and the processes of transport and degradation in the water column. Downward particle and carbon flux was measured directly by means of sediment traps (Technicap PPS3) attached to a drifting line. Traps were deployed at 4 depths (40, 85, 145 and 200 m) along with current-meters. The drifting array was deployed at selected sites of the Canadian Beaufort Sea between 14 and 25 August 2009. Mooring deployments lasted for 28-50 hours and targeted the shelfbreak and the slope along the Beaufort-Mackenzie continental margin, as well as the edge between the Mackenzie Shelf and the Amundsen Gulf. The measured fluxes were relatively low (in the range of 9-56 mg m-2 d-1 for the total mass and 1-15 mg C m-2 d-1 for carbon). Overall, the magnitude and composition of the particulate flux suggests post-bloom conditions during the survey. Particulate organic carbon export was also estimated through natural radionuclide disequilibria (234Th/238U) used as a proxy for POC flux. Water column profiles of total 234Th were obtained from 2 L of seawater (small volume technique) sampled by a CTD-Rosette, and particulate 234Th was measured at selected depths using large volume Challenger in-situ pumps. Derived fluxes were relatively low during the sampled period and consistent with the direct flux measurements. The analysis of molecular and isotopic lipid biomarkers in suspended particulate matter in the water column from the Mackenzie Shelf to the edge of the Amundsen Gulf showed a predominant biogenic and fossil composition, while surface sediments revealed that sinking material was relatively fresh and of marine organic source, mixed with a fossil hydrocarbon background and a small input of terrestrial sources. Altogether, the results highlight that despite the importance of the Mackenzie river outflow in the southern Beaufort Sea, the sinking particle dynamics in the open shelf is largely marine