Natural Radionuclides as Tracers of the Cycling and Fate of Particulate Organic Carbon on the Mackenzie Shelf: Participation in the Canadian Arctic Shelf Exchange Study (CASES)

0136459 Cochran Of all the world oceans, the Arctic is unique in its persistent cover of sea ice, broad continental shelves that are loci for carbon cycling and strong annual variations in light intensity. Moreover the Arctic is perhaps the ocean most susceptible to showing the early effects of global warming. Although changes such as thinning ice cover may have natural causes, for example related to variations in the gradient of atmospheric pressure in the Arctic, they may also be related to changes in climate brought about by human activities. Climate warming may alter the cycling of carbon on arctic shelves through permanent reductions in ice cover and increases in riverine discharge. In turn these changes may lead to enhanced export of carbon to the slope and deep arctic basins. One of the most distinguishing features of the Arctic is its extensive shelf area, comprising about 30% of the basin area and receiving about 10% of the world river discharge. River discharge in the Arctic is dominated by the great Siberian rivers - the Yenisei, Lena and Ob. The Mackenzie River is fourth in importance after these, but is first in rank in terms of annual discharge of sediment. Moreover, it is the only North American analog of the large Siberian rivers, and in practical terms is the most readily studied. The potential importance of the river-dominated shelves to biogeochemical cycling in the Arctic has lead to an international effort --the Canadian Arctic Shelf Exchange Study (CASES)--organized by Canada to understand the consequences of sea- ice variability on the Mackenzie shelf. Based on prior involvement in Canadian-organized arctic programs in the Northeast and North Water Polynyas, the PI has been invited to participate in the CASES program. The research proposed here for the Mackenzie Shelf will build on the PI's prior efforts in the Arctic and elsewhere to use natural radionuclides as tracers for fluxes of particulate organic carbon and particles. CASES includes three month-long process cruises (in August 2002, 2003 and 2004) on the Mackenzie Shelf and a year-long period between the August 2003 and 2004 cruises during which the research icebreaker will over-winter. This research will be structured to take advantage of the possibility to sample the full annual cycle of productivity on the Mackenzie Shelf. Focus will be on the three distinct zones (an early season inner shelf pool of freshwater dubbed 'Lake Mackenzie' separated by an ice dam from a flaw-lead polynya and the arctic pack ice) that develop in the system, and employ natural radionuclides as tracers for the transport of particles and particulate organic carbon within and between the zones. We hypothesize that significant exchanges between the zones will occur in the spring through summer as the Mackenzie River freshet begins, the ice retreats offshore and productivity increases. We will test this hypothesis through the use of natural radionuclides to determine the presence of freshly input riverine sediments to the inner shelf, the rate of export of POC from the euphotic zone, and through mass balances, as indicators of the transport of particulate material through the zones. The radionuclides we will use include cosmogenic radionuclides (Be-7 and C-14) produced in the atmosphere from the interactions of cosmic rays with atmospheric gases, and radionuclides of the naturally occurring U-238 decay series (Th-234, Pb-210).