61st Annual Report on Research 2016

Introduction

Salt domes represent significant liquid petroleum and natural gas traps. Despite the domes ability to promote trapping, localized microbially mediated reactions lead to hydrocarbon degradation. As this hydrocarbon is degraded, authigenic carbonate minerals can form representing a hallmark 'geobiologic' system. Supported by PRF, my lab uses geochemical techniques to more specifically identify the microbial processes involved in hydrocarbon degradation and carbonate formation.

New Findings

            It is well established that hydrocarbon degradation occurs in proximity to salt domes in the Gulf Coast. Although speculated in the literature, the specific mechanisms facilitating degradation are poorly understood. Upon degradation, hydrocarbon carbon is oxidized to form dissolved inorganic carbon (DIC) species in pore waters near salt domes. This DIC reacts with dissolved calcium to produce calcium carbonate minerals that accumulate along the flanks and crests of many Gulf Coast salt domes. Geochemical analyses of these carbonates provide insight into 1) the hydrocarbon degradation mechanisms and 2) the nature of the original hydrocarbon.

            In order to address these issues, my lab has been conducting integrated carbon and sulfur geochemical analyses on samples derived from six Gulf Coast salt domes. Carbonate carbon isotope compositions (d¹³C_carb) range from ~ –55 permil (VPDB) to near neutral, with most samples plotting below the liquid hydrocarbon (oil) end member (Figure 1).  These d¹³C_carb values suggest that these carbonates received a significant amount of carbon from the degradation of methane. Whereas these data provide insight into carbon sources (addressing #2 above), they do not reveal methane (or hydrocarbon in general) degradation mechanisms. Complimentary sulfur isotope analysis of trace sulfate incorporated into the carbonate lattice (so called carbonate-associated sulfate, CAS) allows the determination of sulfur-related degradation mechanisms and in particular sulfate reduction and/or sulfide oxidation processes. Analysis of CAS is relatively novel and until now had not yet been applied to salt dome carbonates. We find CAS sulfur isotope compositions (d³⁴S_CAS) that range from ~ +10 to ~ +70 permil (VCDT) (Figure 1). These values exceed the d³⁴S values of local, salt-hosted anhydrite and gypsum that are thought to represent the original source of dissolved sulfate in these systems. Such high d³⁴S values primarily result from closed-system sulfate reduction facilitated by microbes. When considered together, the carbon and sulfur isotope compositions suggest that the anaerobic oxidation of methane (AOM) acts to degrade methane, reduce sulfate and ultimately promote the precipitation of carbonates (and sulfur minerals) through the following reactions:

CH₄ + SO₄^2– = HCO₃^– + HS^– + H₂O              (AOM)

Ca²⁺ + HCO₃^–= CaCO₃ + H⁺                                     (calcite precipitation)

The discovery of AOM in salt dome systems is very intriguing, primarily as it is most widely reported from seafloor sediments associated with methane seeps, far removed from the subsurface depths at which salt domes occur. In addition, AOM is a microbial process, implying that salt domes host similar communities that live in the so-called 'deep-biosphere'.

Figure 1: Salt dome carbonate geochemical data. Samples that plot in the lower right quadrant indicate AOM. Those that plot outside of that quadrant may also form via AOM, however the data are non-distinct. All data aside from one point indicate closed-system sulfate reduction.

In order to better constrain the carbonate precipitation environment, we have also collected preliminary organic carbon isotope data (d¹³C_org) and carbonate clumped isotope data (D₄₇). Similar to modern methane seep systems (mentioned briefly above), d¹³C_org values of organic matter contained within salt dome carbonates are ¹³C-depleted with values extending down to ~ –50 permil. As with carbonate carbon, this likely reflects a significant methane carbon source although isotope fractionation associated with fixation pathways must also be considered when interpreting organic carbon isotope compositions. The carbonate clumped isotope paleothermometer has been applied to many different systems in the last ten years, including diagenetic systems like salt domes. Preliminary clumped isotope compositions suggest carbonate precipitation temperatures ranging from ~15 to ~50 degrees Celsius which are broadly consistent with shallow subsurface precipitation (as has been hypothesized for salt dome carbonates). However, recent research indicates that AOM may yield clumped isotope signatures inconsistent with precipitation temperatures, confounding interpretations (Figure 2). Future work will include collection of more data and comparison with modern depth distributions that may provide insight into controls on clumped isotope compositions.

Student Involvement

This research has funded three undergraduates and one gradate student working toward theses. The three undergraduate students include Lucas Lu, Shawn Colby and Bayne Westrick-Snapp and these students completed undergraduate theses over the summer. Masters student Kylie Caesar also completed her thesis over the summer and intends to submit a manuscript in the near future. All students have presented their research at multiple meetings (total of nine abstracts). Kylie was awarded an oral presentation at the most recent Southern California Geobiology Symposium at Caltech, a particularly important achievement, as oral presentations are highly competitive at this venue.

Incoming graduate student John Hill and undergraduate student Kaelin Andelin intend to pursue salt dome-related projects. Future goals include 1) publication submittal and 2) application of other geochemical analyses to further our understanding of this interesting system.

Figure 2: Salt dome carbonate clumped isotope compositions and comparison to AOM carbonates recovered from methane cold seeps. Recent work has revealed that seep AOM carbonates yield clumped isotope signatures inconsistent with ambient formation temperatures. Notice that salt dome carbonates overlap with modern and ancient seep carbonates.

Advancement of the PIs Career

This funding has provided an excellent platform onto which I have and will continue to build a new research directive. As a young faculty member, I intend to use this directive as a cornerstone to reach tenure. Aside from new discovery and the opportunity to work with students, this research has promoted external collaboration. This work has allowed continued collaboration with a long time colleague (Dr. Tim Lyons, UCR) and development of a new collaboration (Dr. Rick Kyle, UT Austin). I look forward to expanding this research (including beyond the PRF funding window), involving more students and collaborators, and to publishing results for dissemination to the broader scientific community.