Porewater Profiles of Dissolved N2/Ar Gas Ratios in Sediments From the Gulf of Mexico Continental Margin
Abstract
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 O2, NO3, NH4, and the N2/Ar gas ratio were determined at each station. Porewater dissolved O2 concentrations decreased rapidly at shallow stations and more slowly at deeper stations; penetration depths ranged from ∼5mm at the 200 m station to ∼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 N2/Ar gas ratio measured in the field by membrane-inlet mass spectrometry (MIMS) using a probe-style inlet. Changes in the N2/Ar gas ratio reflect the production of N2 gas due to denitrification in the sediments. At all stations the N2/Ar gas ratios increased significantly with depth in the sediments and were oversaturated relative to the bottom water N2/Ar ratio. The gas ratio profiles increased rapidly with depth as oxygen concentrations went to zero, and maximum values of N2/Ar oversaturation occurred at the depth range where NO3 concentrations decreased rapidly and thus NO3 consumption rates were highest. The maximum in the N2/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 O2 and NO3 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 NO3 profiles.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMOS13B0534H
- Keywords:
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- 4805 Biogeochemical cycles (1615);
- 4820 Gases;
- 4894 Instruments and techniques