Microbial activity in surficial sediments overlying acoustic wipeout zones at a Gulf of Mexico cold seep

Abstract

[1] Down core concentration gradients of dissolved methane and sulfate; isotope gradients of methane, dissolved inorganic carbon, and authigenic carbonate; and organic matter elemental ratios are incorporated into a vent evolution model to describe spatial and temporal variability of sedimentary microbial activity overlying acoustic wipeout zones at Mississippi Canyon (MC) 118, Gulf of Mexico. We tested the hypothesis that these zones indicate areas where sediments are exposed to elevated fluid flux and therefore should contain saturated methane concentrations and enhanced microbial activity from sulfate reduction (SR), anaerobic oxidation of methane (AOM), and methanogenesis (MP). Thirty surficial cores (between 22 and 460 cm deep) were collected from sediments overlying and outside the wipeout zones and analyzed for pore water and solid phase constituents. Outside the wipeout zones, sulfate and methane concentrations were similar to overlying-water values and did not vary with depth; indicating low microbial activity. Above the wipeouts, nine cores showed moderate activity with gently sloping sulfate and methane concentration gradients, methane concentrations <20 mM, and isotope depth gradients indicative of organic matter oxidation. In stark contrast to this moderate activity, four cores showed high microbial activity where sulfate concentrations were depleted by $50 cm below seafloor, maximum methane concentrations in the decompressed cores were above 4 mM, and down core profiles of d 13 C-CH 4 and d 13 C-dissolved inorganic carbon (DIC) indicated distinct depth zones of SR, AOM, and MP. Bulk organic matter analysis suggested that the high activity was supported by an organic source that was enriched in carbon (C:N $15) and depleted in d 15 N and d 13 C compared to other activity groups, possibly due to the influx of petroleum or chemosynthetically fixed carbon. Within high activity cores, the d 13 C-DIC values were similar to the d 13 C-CaCO 3 values, a result expected for authigenic carbonate recently precipitated. However, these values were dissimilar in moderate activity cores, suggesting that microbial activity was higher in the past. This study provides evidence that the fluid flux at MC 118 varies over time and that the microbial activity responds to such variability. It also suggests that sediments overlying wipeout zones are not always saturated with respect to methane, which has implications for the formation and detection of gas hydrate.

Isotopic composition of sinking particles: Oil effects, recovery and baselines in the Gulf of Mexico, 2010–2015

“…The seafloor at this site hosts a large deep-water coral population, and represents a "mineral-prone seep." A mineralprone seep is in the final stages of seep evolution, where the production of authigenic carbonates has blocked conduits and allowed corals to use these carbonate hard grounds for attachment surfaces (Roberts and Carney, 1997;Lapham et al, 2008aLapham et al, , 2008b. This categorization is consistent with the "self-sealing nature of marine seeps" (Hovland, 2002).…”

Spatial Variation in Sediment Organic Carbon Distribution across the Alaskan Beaufort Sea Shelf

“…For example, in the Gulf of Mexico nearshore the Mississippi River outflow there is a strong terrestrial contribution to surface sediment carbon that shifts to a phytodetritus-dominated contribution over a short distance moving offshore [3,4]. Further offshore, however, on the Texas-Louisiana Shelf, there is significant contribution of CH 4 and petroleum to shallow sediment carbon cycling [9][10][11][12][13]17]. A very different coastal system is the Nankai Trough, off the coast of Tokyo Japan, where deep sediment CH 4 hydrate loading contributes significantly to the shallow sediment carbon cycling [18].…”

“…Terrestrial inputs of OM are primarily from rivers and surface runoff, but in some systems, inputs from groundwater can be significant [6,7]. There can also be additional inputs of OC to certain shelf environments from the upward migration of deep sediment petroleum and methane (CH 4 ) to shallow, surface sediments [8][9][10][11][12][13]. There is a general assumption that phytodetritus is a strong OC contribution to coastal sediment [3].…”

“…Stable carbon isotope analysis (the ratio of 13/12 C) of TOC (δ 13 C TOC ) can be used to help trace carbon cycling and quantify carbon contributions from different sources. Organic carbon derived from marine phytodetritus typically has a range of δ 13 C TOC values from -30 to −17 [24].…”

Hydrate research activities that both support and derive from the monitoring station/sea-floor Observatory, Mississippi Canyon 118, northern Gulf of Mexico

“…Smaller pore-fluid samplers have been deployed using a variety of methods and expand the lateral coverage of pore-fluid geochemistry at the Observatory. Recovered water samples have since been processed yielding valuable data on the pore fluid chemistry representative of its location (Lapham et al, 2008b;. Laboratory experiments demonstrate the pure structure I and structure II hydrate dissolve until conditions reach saturation 2014).…”