HR: 08:45h
AN: OS21A-04    [Abstracts]
TI: Estimates of Vertical Methane Fluxes in Porangahau Ridge Sediment on the Hikurangi Margin, New Zealand
AU: * Coffin, R
EM: richard.coffin@nrl.navy.mil
AF: Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375, United States
AU: Hamdan, L
EM: leila.hamdan@nrl.navy.mil
AF: Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375, United States
AU: Wood, W
EM: warren.wood@nrlssc.navy.mil
AF: Naval Research Laboratory, Stennis Space Center, Stennis, MS 39529, United States
AU: Pohlman, J
EM: jpohlman@usgs.gov
AF: US Geologic Survey, Woods Hole Field Center, Woods Hole, MA 02543, United States
AU: Smith, J
EM: joseph.smith@nrl.navy.mil
AF: Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375, United States
AU: Henrys, S
EM: s.henrys@gns.cri.nz
AF: GNS Science, 1 Fairway Drive, Lower Hutt - Avalon, 5010, New Zealand
AU: Pecher, I
EM: ingo.pecher@pet.hw.ac.uk
AF: Heriot-Watt University, Inst Petroleum Engineering, Edinburgh, EH14 4AS, United Kingdom
AB: Potential gas hydrate deposits were outlined with bottom simulating reflections in seismic data on the Porangahau Ridge, landward of the Hikurangi Channel, along the northeastern coast New Zealand. This expedition, CHARMNZ (CH₄ Hydrates on the AccRetionary Margins of New Zealand; R/V Tangaroa voyage TAN0607), in 2006 was the first survey dedicated to studying gas hydrates on the Hikurangi Margin east of New Zealand. Geochemical data from shallow sediment porewater profiles and vertical fluid migration measured with a heatflow probe were compared with seismic profiles over potential gas hydrate deposits. Spatial orientation of piston cores and heatflow probing was organized to compare vertical methane fluxes along the seismic lines, with focus on predicted areas of concentrated gas seepage or hydrate accumulation. Core and heatflow transects were set on the landward and seaward side of the ridge in order to determine variations in the vertical fluid and gas fluxes. Heatflow data suggested vertical fluid advection on the landward side of the ridge exceeds vertical fluid fluxes on the seaward side. Porewater sulfate and methane profiles showed a range for the depth of the sulfate-methane interface (SMI) between 12.9 m upslope on the landward side of the ridge and 1.84 m near the ridge. Vertical sulfate fluxes in the study area appear to be dominated by diffusion with a range of -4.2 mM m^-2 a^-1 upslope, away from the ridge, and up to -208.6 mM m^-2 a^-1 on the ridge. Porewater sulfide profiles suggest elevated sulfate reduction on the landward side of the mound where elevated vertical fluid flux was measured. Stable carbon isotope analysis (δ¹³C) and gas composition indicated that porewater methane originate from microbial production with δ¹³ below the SMI in the range -60 ppt and -110 ppt VPDB and methane the dominant gas. At this depth a large variation in the methane δ¹³C likely results from variations in the vertical flux rates along the ridge, near surface methanogenesis and variable rates of anaerobic methane oxidation (AOM). Above the SMI the methane δ¹³C was elevated up to -45 ppt and corresponded to low methane concentrations likely resulting from AOM at the SMI. Vertical methane fluxes in this area are within the range observed in other seep areas such as Atwater Valley in the Gulf of Mexico and mid Chilean margin (-9 to -362 mM m^-2 a^-1). Because methane flux estimates can not be attributed to near surface methanogenesis alone, these data suggest deep sediment methane pools are present along the Porangahau Ridge.
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4808 Chemical tracers
DE: 4825 Geochemistry
DE: 4850 Marine organic chemistry (0470, 1050)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting