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

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 μM, 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 δ13C-CH4 and δ13C-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 d15N and δ13C compared to other activity groups, possibly due to the influx of petroleum or chemosynthetically fixed carbon. Within high activity cores, the δ13C-DIC values were similar to the δ13C-CaCO3 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

Molecular and optical characterization reveals the preservation and sulfurization of chemically diverse porewater dissolved organic matter in oligohaline and brackish Chesapeake Bay sediments.

In this study, we conducted a detailed analysis of porewater downcore chemical properties and porewater dissolved organic matter (PDOM) composition using elemental C, N and S analysis, fluorescence spectroscopy, and ultrahigh resolution mass spectrometry (FT-ICR MS) at two contrasting sites in Chesapeake Bay. The sites, situated in the oligohaline upper bay and in the seasonally hypoxic mesohaline mid bay, receive fundamentally different detrital inputs predominantly from allochthonous and autochthonous sources, respectively. Unsurprisingly, we observed greater molecular oxygenation and degree of aromaticity in downcore PDOM profiles from the upper bay. At the mid bay station, PDOM composition was more indicative of non-aromatic algal-derived material. Unexpectedly, this autochthonous PDOM had lower C:S ratios. Hence, algal-derived organic matter appeared to be readily sulfurized, which was confirmed by quantification of dissolved organic sulfur as well as by qualitative interpretation of FT-ICR MS data. This finding suggests addition reactions of hydrogen sulfide to double bonds in unsaturated, but non-aromatic, organic molecules in autochthonous PDOM. Intriguingly, we also observed increases in humic-like fluorescence and dissolved organic carbon (DOC) concentrations in downcore PDOM profiles from both sites. Given the differences in molecular composition between sites, these results show that humic-like fluorescence can arise from different sources and biogeochemical processes. In the upper bay, we infer that these fluorescence signals reflect solubilization of terrestrially derived organic matter with a high aromatic and polyphenolic composition. By contrast, in the mid bay, these fluorescence peaks correlated negatively with hydrogen sulfide and are more likely linked to bacterial sulfate reduction