Abstract
Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane energy contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. Shallow sediment and porewater samples were collected from Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico near a seafloor mound feature identified in geophysical surveys as an elevated bottom seismic reflection. Geochemical data revealed off-mound methane diffusion and active fluid advection on-mound. Gas composition (average methane/ethane ratio ~11,000) and isotope ratios of methane on the mound (average δ13CCH4(g) = −71.2‰; D14CCH4(g) = −961‰) indicate a deep sediment, microbial source. Depleted sediment organic carbon values on mound (δ13CSOC = −25.8‰; D14CSOC = −930‰) relative to off-mound (δ13CSOC = −22.5‰; D14CSOC = −629‰) suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. Porewater and sediment data indicate inorganic carbon fixed during anaerobic oxidation of methane is a dominant contributor to on-mound shallow sediment organic carbon cycling. A simple stable carbon isotope mass balance suggests carbon fixation of dissolved inorganic carbon (DIC) associated with anaerobic oxidation of hydrate-sourced CH4 contributes up to 85% of shallow sediment organic carbon.
Highlights
Sediment organic carbon (SOC) composition and provenance have been extensively studied in theGulf of Mexico (GoM)
The Δ14CCH4(g) values measured in gas pockets of near-mound cores C3 and C8 taken close to the primary mound core are presented in Figure 2 and Table 2
Subsequent diagenesis of organic matter with different contributions of 14C-depleted CH4 between core locations is evident with differences in conventional radiocarbon age (CRA) between on-mound and off-mound cores. δ13C data in this study provides an estimate of the dissolved inorganic carbon (DIC) fixation driven by the anaerobic oxidation of methane (AOM) of deep sourced CH4 in shallow sediment carbon cycling
Summary
Sediment organic carbon (SOC) composition and provenance have been extensively studied in theGulf of Mexico (GoM). Sediment organic carbon (SOC) composition and provenance have been extensively studied in the. Inputs of terrestrially-derived organic carbon (OC) dominate near shore sediments and decrease with distance offshore [1,2,3,4,5]. Terrestrial-derived OC in surficial sediments of the GoM shifts from C3 plant material near shore to highly degraded, soil-derived material offshore [1,2,3]. Surface sediments on the slope (365–2270 m water depth) have Δ14C values between −309.1‰ and −228.6‰ corresponding to 64% to 78% modern carbon [3]. Carbon isotope signatures typical for marine phytoplankton have been measured in surface sediments at water column depths from 74 to 2250 m (mean δ13C value = −20.8‰; [6]).
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