Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in 13 C (to ؊19‰ ␦ 13 C), the most 13 C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The ␦ 13 C values are highly variable, but most are within the range ؊12‰ to ؊19‰. This variability may be diagnostic for identifying cold-seep-hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to ϳ1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in 13 C (␦ 13 C ؍ ؊46‰ to ؊58‰), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane.Keywords: 13 C-depleted bivalve shells, cold seep, methane hydrate, mud volcano, California margin basin. Figure 1. Bathymetry of California borderland and location of seep-hydrate site at 800 m water depth on northern slope of Santa Monica Basin and control site at 550 m depth; 100 m contour interval. ODP-Ocean Drilling Program.
This study, one in a series, reports bulk chemical and mineralogical compositions, as well as petrographic and outcrop descriptions of rocks collected from three measured outcrop sections of the Rex Chert member of the Phosphoria Formation in SE Idaho. The three measured sections were chosen from ten outcrops of Rex Chert that were described in the field. The Rex Chert overlies the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, the source of phosphate ore in the region. Rex Chert removed as overburden comprises part of the material disposed in waste-rock piles during phosphate mining. It has been proposed that the chert be used to cap and isolate waste piles, thereby inhibiting the leaching of potentially toxic elements into the environment. It is also used to surface roads in the mining district. The rock samples studied here constitute a set of individual chert beds that are representative of each stratigraphic section sampled. The informally named cherty shale member that overlies the Rex Chert in measured section 1 was also described and sampled. The upper Meade Peak and the transition zone to the Rex Chert were described and sampled in section 7. The cherts are predominantly spicularite composed of granular and mosaic quartz, and sponge spicules, with various but minor amounts of other fossils and detrital grains. The cherty shale member and transition rocks between the Meade Peak and Rex Chert are siliceous siltstones and argillaceous cherts with ghosts of sponge spicules and somewhat more detrital grains than the chert. The overwhelmingly dominant mineral is quartz, although carbonate beds are rare in each section and are composed predominantly of calcite and dolomite in addition to quartz. Feldspar, mica, clay minerals, calcite, dolomite, and carbonate fluorapatite are minor to trace minerals in the chert. The mean concentrations of oxides and elements in the Rex Chert and the cherty shale member are dominated by SiO 2 , which averages 94.6%. Organic-carbon contents are generally very low in the chert, but are up to 1.8 wt. % in cherty shale member samples and up to 3.3% in samples from the transition between the Meade Peak and Rex Chert. Likewise, phosphate (P 2 O 5) is generally low in the chert, but can be up to 3.1% in individual beds. Selenium concentrations in Rex Chert and cherty shale member samples vary from <0.2 to 138 ppm, with a mean concentration of 7.0 ppm. This mean Se content is heavily dependent on two values of 101 and 138 ppm for rocks from the transition zone between the Meade Peak and Rex Chert. Without those two samples, the mean Se concentration would be <1.0 ppm. Other elements of environmental interest, As, Cr, V, Zn, Hg, and Cd, are generally near or below their content in average continental shale. Stratigraphic changes (equivalent to temporal changes in the depositional basin) in chemical composition of rocks are notable either as uniform changes through the sections or as distinct differences in the mean composition of rocks that compose the upper and lower ha...
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