Reconstructing the Carbon-Sulfur Geochemistry of Hydrocarbon Seeps at the Gulf of Mexico from Authigenic Carbonates
Abstract
Hydrocarbon seeps of the Gulf of Mexico (GoM) are characterized by chemosynthetic biological communities and precipitates of authigenic minerals resulting from dynamic geosphere-biosphere interactions. Understanding the carbon-sulfur coupling at hydrocarbon seeps has important implications in developing sedimentary proxies to recognize and interpret present and past seep records as well as the broad biogeochemical implication of these biogeochemical processes. Geochemical investigation using a combination of X-ray diffraction, carbonate petrography, and stable isotope geochemistry was performed on authigenic carbonates recovered from southern GoM (Chapopote Knolls, Campeche Bay) and northern GoM (Green Canyon block 415) collected during research expeditions R/V Meteor cruise M67/2 (2006) and R/V Sonne cruise SO174/2 (2003).
Mineralogically, southern GoM carbonates exhibited aragonitic composition indicative of shallow origin with high sulfate concentrations and Mg/Ca niches, whereas northern GoM samples showed calcitic mineralogies indicative of formation within the sediment column, in low sulfate and Mg/Ca settings. Carbon and oxygen isotope analyses revealed a significant difference in carbon sourcing in these sites. Southern GoM sites showed a significant contribution of non-methane hydrocarbon to the DIC pool for carbonate precipitation, with δ13Ccarbonates averaging about -26‰. Northern GoM samples showed δ13C value averaging about -35‰, suggestive of a relatively higher contribution of methane to the DIC pool. Enriched δ18O values averaging +3.5‰ in both sites suggest potential sourcing from shallow gas hydrates. Unusually depleted δ34S values (as low as to -23‰) for carbonate associated sulfate (CAS) suggest pyrite oxidation during lab preparation or the possibility of sulfide oxidation recorded in the rock record at these sites. The low concentrations of CAS (≤46 ppm) are consistent with high sensitivity to those oxidation effects. Ongoing work will aim to resolve these results using the sulfur isotope composition of the sulfide phases (δ34SCRS, pyrite principally) paired with δ13Ccarbonate values to distinguish the sulfate reduction processes associated with authigenic carbonate precipitation in these seep settings.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMOS43B1716A
- Keywords:
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- 3002 Continental shelf and slope processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3050 Ocean observatories and experiments;
- MARINE GEOLOGY AND GEOPHYSICS