Lalk Ellen

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Last Name
Lalk
First Name
Ellen
ORCID
0000-0002-9843-9278

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  • Thesis
    The biogeochemistry of methane isotopologues in marine and lacustrine sediments
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-02) Lalk, Ellen ; Ono, Shuhei
    Methane is a globally significant greenhouse gas, energy resource, and it is a product and reactant of microbial metabolisms. Multiple sources and sinks of methane can be challenging to distinguish from each other, thus complicating the understanding of methane budgets and the effects of microbes on mediating Earth’s carbon cycle. The relative abundances of methane isotopologues (e.g., 12CH4, 13CH4, 12CH3D, and 13CH3D) record process-based information about the formation conditions, transport, and fate of methane, and in select environments can serve as a temperature proxy. This geochemical tool is herein applied to methane from marine and lacustrine sediments to test assumptions about prevailing mechanisms of its formation and consumption in these settings. This thesis describes 1) three studies about biogeochemical insights gained by quantifying the relative abundance of clumped methane isotopologue, 13CH3D, in samples from marine and lacustrine sediments, and 2) one foray into method development to improve the quantification of methane in these environments. Chapter 2 presents a global survey of marine gas hydrates where isotope-based temperatures are used to assess whether linkages between methane sources and seepage-associated seafloor features match putative geologic models. Chapter 3 describes two kilometer-scale profiles of methane isotopologues from marine sediments, where the relationship between expected sediment temperature and isotope-based temperature is used to evaluate the temperature limit of microbial processing and abiotic re-equilibration mechanisms. Chapter 4 reports the largest set of methane isotopologue data from ebullition in a single lake basin, which is used to gauge the relative importance of aerobic and anaerobic methane oxidation in the study site and recommend a general sampling strategy to constrain methane source signatures in similar lake settings. Chapter 5 explains the development of a method to quantify the in situ concentration of methane based on ratios of dissolved gases, and its comparison to four other methane quantification methods for surface sediments from marine cold seeps. The findings from this research contribute to ongoing efforts to understand the sedimentary carbon cycle and microbial activity in remote environments.
  • Article
    Clumped and conventional isotopes of natural gas reveal basin burial, denudation, and biodegradation history
    (Geochemical Society, 2023-10-13) Kim, Jihyun ; Martini, Anna M. ; Ono, Shuhei ; Lalk, Ellen J. ; Ferguson, Grant A.g. ; McIntosh, Jennifer C.
    Formation and post-genetic alteration of hydrocarbons provide insights into the dynamic and complex geologic, hydrologic, and microbial history of shallow crustal environments. Clumped isotopologues of methane (e.g., Δ13CH3D) have emerged as a proxy for constraining methane formation temperatures in sedimentary basins. However, unrealistically high apparent temperatures and microbial cycling of methane necessitate further investigation into how the generation and biodegradation of hydrocarbons may modify methane clumped isotopologue signatures. This study analyzed and modeled the clumped isotopologues of methane, in addition to traditional gas isotopes, to provide new insights into the origin, thermal maturity, migration, and biodegradation histories of hydrocarbons in the Paradox Basin in the Colorado Plateau. The basin was deeply buried in the geologic past and has been recently incised, leading to rapid denudation, enhanced meteoric circulation, and microbial activity. δ13CCH4 and CH4/ΣC2+ ratios suggest that most natural gases in various reservoirs throughout the basin are thermogenic in origin with variable thermal maturities. However, signatures suggestive of anaerobic oxidation of ethane and propane, and secondary microbial methane generation, exist. In the northeastern part of the basin, Δ13CH3D values in reservoirs above and below the Paradox Formation source rocks are consistent with thermodynamic equilibrium, indicating that the thermally mature hydrocarbons equilibrated at ≥160 °C during maximum burial over 30–80 Ma. Disequilibrium Δ13CH3D values of natural gas in Paradox Formation reservoirs along the southwestern margin of the basin suggest the presence of low-maturity hydrocarbons consistent with the region’s shallower burial history. Models of Δ13CH3D values based on the exchange rate of hydrogen isotopes between methane and water and the basin thermal history support that meteoric recharge and microbial activity, following incision/denudation over the past few million years, promoted anaerobic oxidation of hydrocarbons (particularly ethane and propane), biodegradation of crude oil, and generation of secondary microbial methane in shallow reservoirs.