HR: 0800h
AN: T21C-0539    [Abstracts]
TI: Long-Term Continuous Monitoring of Fluid Chemistry and Flux at the Bush Hill Gas Hydrate Field, Gulf of Mexico Using a New Flow Meter, The MOSQUITO
AU: * Solomon, E
EM: esolomon@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093-0212
AU: Kastner, M
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093-0212
AU: Jannasch, H
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039
AU: Weinstein, Y
AF: Bar-Ilan University, Dept of Geography, Ramat Gan, 52900 Israel
AU: Robertson, G
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093-0212
AU: Aubrey, A
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093-0212
AB: Long-term monitoring of fluid, solute, and methane fluxes that influence marine gas hydrate formation and dissociation has important implications for the seafloor biochemical environment, ocean chemistry, and potentially the atmosphere. Four newly designed flux meters called the MOSQUITO (Multiple Orifice Sampler and Quantitative Injection Tracer Observer) and two temperature loggers were deployed adjacent to the Bush Hill hydrate mound in the northern Gulf of Mexico (GC185) in order to understand how chemistry, physics, biology, and subsurface hydrology dynamically influence the growth and dissociation of the hydrate mound. The MOSQUITO contains a network of osmotic samplers and a tracer injection device, each connected to a titanium capillary tube that penetrates the sediment. The tracer is injected as a point source, and fluid chemistry and tracer concentrations are continuously sampled simultaneously at multiple depths below the seafloor in a three dimensional array with respect to the tracer injection point. Bottom water chemistry is also sampled continuously. Vertical and horizontal flow rates as low as 1 cm/yr are determined by modeling the variability in tracer concentration at each depth over time. MOSQUITOs can be deployed at passive margins, ridge crests, ridge flanks, subduction zones, and lakes. MOSQUITOs were deployed over a period of 430 days from June 2002 to August 2003 and were sampled at weekly resolution. The temperature loggers were attached to the MOSQUITOs and recorded seafloor temperature every 40 minutes. Three MOSQUITOs were deployed within 3 m of the hydrate mound and $\sim$ 5 m apart, adjacent to transient methane seeps; in a mussel field, in a bacterial mat, and in a tubeworm field. The fourth MOSQUITO was placed $\sim$150 m southwest of the hydrate mound to monitor background fluid flow, geochemistry, and temperature. The average bottom water temperature over the 430-day deployment period was $7.94\deg$C, with minimum temperatures occurring every 6-9 weeks and maximum temperatures occurring every 9-12 weeks. The record shows an asymmetric periodicity with a slower rise in temperature followed by relatively abrupt cooling. The hydrology around the hydrate mound is complex with both upflow and downflow of fluid occurring within each of the sub-environments. Within the mussel field, fluid upflow ranges from 1-39 cm/yr and downflow varies from 3-65 cm/yr, with the polarity of fluid flow changing at 1-7 week intervals. In the bacterial mats, upward flow velocities range from 1-100 cm/yr and downward flow velocities range from 4-130 cm/yr. The polarity of fluid flow in the bacterial mats appears to be linked to bottom water temperatures and is seasonal and less variable, with fluid downflow occurring from June to November 2002, as well as during the summer of 2003, and fluid upflow from November 2002 to mid-June 2003. The background site is characterized by minor upward fluid advection. Pore fluid chemistry also varies significantly over short distances and time, and is manifested at the seafloor by sharp interfaces between different benthic biological communities. The relations between bottom water temperature, pore fluid chemical fluctuations, and hydrology are being evaluated. Net chemical fluxes of the major elements and methane, as well as net fluid fluxes, and their relation to gas hydrate formation and dissociation, ocean chemistry, and biology will be addressed.
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
DE: 3094 Instruments and techniques
DE: 1829 Groundwater hydrology
DE: 1050 Marine geochemistry (4835, 4850)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting