HR: 1340h
AN: OS13B-0534    [Abstracts]
TI: Porewater Profiles of Dissolved N$_{2}$/Ar Gas Ratios in Sediments From the Gulf of Mexico Continental Margin
AU: * Hartnett, H
EM: h.hartnett@asu.edu
AF: Arizona State University, Depts. of Geological Sciences and Chemistry & Biochemistry, Box 871404, Tempe, AZ 85287 United States
AU: Devol, A
EM: devol@ocean.washington.edu
AF: University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195 United States
AU: Brandes, J
EM: brandes@utmsi.utexas.edu
AF: University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373 United States
AU: Sobolev, D
EM: sobolev@utmsi.utexas.edu
AF: University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373 United States
AU: Chang, B
EM: bchang@ocean.washington.edu
AF: University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195 United States
AB: Dissolved gases in sediment porewaters are useful tracers of the biogeochemical processes that consume organic matter. In autumn of 2003, we collected cores from three stations off the Gulf coast of Texas. The stations ranged from 200 to 1300 meters water depth and represent a range in oxygen exposure time, organic carbon flux to the seafloor, and sediment redox conditions. Porewater profiles of O$_{2}$, NO$_{3}$, NH$_{4}$, and the N$_{2}$/Ar gas ratio were determined at each station. Porewater dissolved O$_{2}$ concentrations decreased rapidly at shallow stations and more slowly at deeper stations; penetration depths ranged from $\sim$5mm at the 200 m station to $\sim$40 mm at the 1300 m station. Nitrate concentrations showed a similar pattern (although over a longer depth scale) with penetration depths ranging from 1.25 cm at the shallow station to 40 cm at the deepest station. We present high-resolution profiles of the porewater N$_{2}$/Ar gas ratio measured in the field by membrane-inlet mass spectrometry (MIMS) using a probe-style inlet. Changes in the N$_{2}$/Ar gas ratio reflect the production of N$_{2}$ gas due to denitrification in the sediments. At all stations the N$_{2}$/Ar gas ratios increased significantly with depth in the sediments and were oversaturated relative to the bottom water N$_{2}$/Ar ratio. The gas ratio profiles increased rapidly with depth as oxygen concentrations went to zero, and maximum values of N$_{2}$/Ar oversaturation occurred at the depth range where NO$_{3}$ concentrations decreased rapidly and thus NO$_{3}$ consumption rates were highest. The maximum in the N$_{2}$/Ar gas ratio occurred at 25 mm at the 200 m station and at more than 70 mm at the 1300 m station, similar to the patterns in the O$_{2}$ and NO$_{3}$ penetration depths. These porewater dissolved gas ratio profiles provide an additional estimate of the total denitrification rate in sediments that can be compared to rates determined from benthic chamber fluxes and NO$_{3}$ profiles.
DE: 4805 Biogeochemical cycles (1615)
DE: 4820 Gases
DE: 4894 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting