HR: 0830h
AN: OS51B-0861    [PDF]
TI: Solid and Dissolved Barium Profiles in Gas Hydrate Systems at Blake Ridge (ODP 164) and Peru Margin (ODP 201): Implications for Long-Term Carbon-Cycling in the Deep Biosphere.
AU: * Snyder, G T
EM: gsnyder@rice.edu
AF: Rice University Earth Science Department, MS-126 P.O.Box 1892, Houston, TX 77251-1892 United States
AU: Dickens, G R
EM: jerry@rice.edu
AF: Rice University Earth Science Department, MS-126 P.O.Box 1892, Houston, TX 77251-1892 United States
AU: Castellini, D G
EM: dgc@rice.edu
AF: Rice University Earth Science Department, MS-126 P.O.Box 1892, Houston, TX 77251-1892 United States
AB: While many investigations have focused on quantifying either the amount of gas hydrates in marine sediment sequences or the methane fluxes to and from these systems, relatively little is known about how gas hydrate reservoirs evolve over time. Our research focuses on barium in gas hydrate systems, which may serve as a proxy for the accumulation of organic matter throughout the history of a depositional location, while also providing an indication of certain post-depositional processes. The accumulation of Ba in marine sediment is intimately associated with the supply of organic matter so that the total Ba abundance should reflect the extent to which organic matter has been supplied to a gas hydrate system. Once in the sediment column, the partitioning of Ba is highly correlated to dissolved sulfate concentrations, such that present and past levels of anaerobic oxidation of methane (AOM) and sulfate reduction can be traced with Ba profiles. Prior investigations of Ba cycling in gas hydrate systems generally have been limited by low-resolution profiles of dissolved or solid Ba concentrations. Moreover, there have been no sites with data for both dissolved and solid Ba profiles as well as dissolved sulfate and methane profiles. We present high-resolution profiles of pore fluid and sedimentary Ba in sediments from Blake Ridge and the Peru Margin, locations where profiles of sulfate and methane have also been generated. At both locations, dissolved Ba is low in the shallow sulfate reduction zone. Immediately above the depth of present AOM, dissolved Ba concentrations rapidly rise. These intervals are also marked by a prominent peak in solid Ba, or Ba-front. At Peru Margin, Ba concentrations increase fairly steadily below the depth of AOM to 1200 $\mu$M at 250m depth, among the highest dissolved Ba concentrations ever reported for a marine borehole. At the Blake Ridge, however, Ba concentrations oscillate below the depth of AOM with a series of highs and lows, reaching a maximum of only 44 $\mu$M at 250 m depth, followed by a steady decrease to 10 $\mu$M at 750 mbsf. Although many pore water profiles are similar at the Blake Ridge and Peru Margin, the contrasting Ba profiles indicate some fundamental difference between the two locations. Below 250 m depth at the Blake Ridge, dissolved sulfate slowly rises and barium concentrations are at saturation with respect to barite. The decrease in barium concentrations along this interval may thus result from systematic seawater contamination during drilling, in which case the widely discussed profiles of other species in the Blake Ridge boreholes need to be corrected. Alternatively, unidentified processes in the deep biosphere (e.g., oxidation of sulfide-bearing minerals) release sulfate to pore waters, accounting for the depletion in dissolved barium concentrations. If this is the case, certain biotic processes at great depth are different at Blake Ridge and the Peru Margin.
DE: 4825 Geochemistry
DE: 4851 Oxidation/reduction reactions
DE: 4875 Trace elements
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting