Deep Sediment-Sourced Methane Contribution to Shallow Sediment Organic Carbon: Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico

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 δ CSOC = −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.

Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

“…Some high temperature events in geological time, such as the Paleocene-Eocene Thermal Maximum (PETM) are likely related to substantial methane release due to gas hydrate dissociation (Dickens et al, 1995;Zachos et al, 2005). The latest study shows that the activity of Bacteria and Archaea in seep areas may contribute significant organic matter to the deep ocean (Pohlman et al, 2011;Coffin et al, 2014Coffin et al, , 2015.…”

Carbon isotope forensics for methane source identification

“…With regard to the Deepwater Horizon oil spill that occurred in 2010 in the Gulf of Mexico, Δ 14 C was applied to trace oil carbon into the food web (Chanton et al, 2012). This approach has also been applied to assessment of offshore gas hydrate and petroleum loading (Coffin et al, 2015). These studies do show that application of Δ 14 C can provide accurate, low cost assessment of in situ natural attenuation of organic contaminants.…”

“…Application of carbon isotope analysis to monitor hydrocarbon respiration (e.g., CH 4 and CO 2 ) provides a simple approach for degradation confirmation. Extensive work has been conducted in offshore gas hydrate exploration that applies carbon isotope analysis of CH 4 and CO 2 to assess CH 4 sources and predict deep sediment loading (Coffin et al, 2013, 2014; Coffin, Osburn, Plummer, Rose, & Grabowski, 2015). This capability has also been applied to groundwater and vadose zone gases (Conrad et al, 1997).…”

“…To address this limitation, natural abundance Δ 14 C analysis can provide more in depth assessment of degradation, carbon source(s), and cycling by distinguishing carbon source ages (Bauer, Reimers, Druffel, & Williams, 1995; Bauer, Spies, Vogel, Nelson, & Southon, 1990; Bauer, Williams, & Druffel, 1992; Boyd, Pound, Lohr, & Coffin, 2013; Cherrier et al, 1999; Coffin et al, 2008, 2015). With a 14 C half‐life of ∼5,700 years, petroleum is absent of radiocarbon and provides a clear signature when compared to modern carbon.…”

“…Wells associated with the highest historic and contemporary fuel contamination showed the highest fraction of CO 2 derived from petroleum (fossil) sources. More recently, Δ 14 C has been applied to the coastal assessment of gas hydrate sediment loading (Coffin et al, 2015) and petroleum spill natural attenuation assessment (Chanton et al, 2012).…”