HR: 16:45h
AN: OS24A-04    [Abstracts]
TI: Applying Ra isotopes to measure water exchange rates in permeable sediments
AU: * Colbert, S L
EM: scolbert@ups.edu
AF: University of Puget Sound, Department of Geology 1500 N. Warner, Tacoma, WA 98416, United States
AU: Hammond, D E
EM: dhammond@usc.edu
AF: University of Southern California, Department of Earth Sciences 3651 Trousdale Blvd., Los Angeles, CA 90089, United States
AB: In the coastal ocean, waves and currents generate pressure gradients at the seafloor, which drive a circulation of seawater through sandy seafloor sediments. Biological activity in the sediments converts inflowing organic matter and oxidants into CO2 and nutrients, which are returned to the water column. To evaluate the impact of benthic organic matter cycling on ocean chemistry, the fluid flow rate through permeable sediments must be known. At Huntington Beach, CA, between the shoreline and 3 km offshore, we have collected sediment cores and in situ pore waters. Samples were analyzed for oxygen, nutrients, and four naturally-occurring radioisotopes: 222Rn, 223Ra, 224Ra, and 228Ra. We have calculated fluid circulation rates within sediments using a reaction- transport model of the radioisotope pore water distributions. Pore waters in the top 25-55 cm of sediments were flushed every 4-10 days. 228Ra profiles identified slower exchange that occurred deeper in the sediments. Dissolved 228Ra increased with depth in sediments and accounted for a significant fraction of the pore water 224Ra. The resulting 224Ra emanation gradient must be included in models to accurately calculate the irrigation rate. 223Ra profiles were similar to 224Ra profiles, suggesting a similar relationship with 227Ac. The irrigation rate can then be applied to the pore water distribution of nutrients to calculate the flux of nutrients returned to the water column and estimate the supply of organic carbon to the seafloor.
DE: 3021 Marine hydrogeology
DE: 4804 Benthic processes, benthos (0408)
DE: 4845 Nutrients and nutrient cycling (0470, 1050)
DE: 4860 Radioactivity and radioisotopes
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting