Organic compounds in vent fluids from Yellowstone Lake, Wyoming

Hydrothermal fluids were collected over a three-year interval (2016–2018) from two sublacustrine vent fields in Yellowstone Lake, Wyoming: Stevenson Island Deep Hole (YL16 and YL17 series in 2016 and 2017, respectively) and West Thumb (YL18 series in 2018). The TOC content in SI Deep Hole vent fluids varies from 2.7 mg/L to 6.3 mg/L in YL16 and 5.0 mg/L to 16.2 mg/L in YL17. Compared to YL16, the average TOC value of YL17 has more than doubled. The range of TOC is from 7.3 mg/L to 13.7 mg/L in the West Thumb samples. A positive correlation between TOC and dissolved CO₂ concentration is observed in most samples, suggesting their close relationship with sublacustrine heat and mass transfer processes. The organic compounds detected at both vent fields are diverse in molecular structure. In the SI Deep Hole, complex cyclic and unsaturated organic compounds are dominant, with cyclic hydrocarbons having the highest abundance, followed by carboxylic acids, PAHs, and alcohols. At West Thumb vents, propanol is most abundant, followed by alcohols, PAHs, alkanes, and carboxylic acids. This difference in structural complexity and relative quantity of organic compounds may be attributed to the involvement of hydrothermal processes. The vapor-dominated hydrothermal system and extensive fluid-rock interactions in SI Deep Hole facilitate alteration/decomposition of organic precursors. While CO_2(aq) is predominant in vent fluids, dissolved CH₄ is much less abundant. In SI Deep Hole, the carbon isotopic composition of CH₄ in most samples ranges from −20.4‰ to −38.0‰, with values of less than −60‰ in three samples, while CO₂ ranges from −4.4‰ to −10.5‰. A temporal variation of δ¹³C values was obtained for CO₂, with the average increasing from −9.2‰ (YL16) to −5.2‰ (YL17). In West Thumb, the δ¹³C value of CO₂ and CH₄ fall in a similar range, with an average of −10.5‰ and −19.0‰, respectively. There are multiple sources for observed carbon species: the interaction of vent fluids with subsurface rocks and lake sediments is the predominant process, followed by thermal decomposition of organic matter. Microbial activity on the lake floor may contribute to methane formation, while magmatic degassing is the major process that produces CO₂. The connection between vapor-dominated hydrothermal alteration, elevated TOC contents and dissolved CO₂ in vent fluids, and enrichment of ¹³C in CO₂ in YL17 in SI Deep Hole suggests an interplay of geological, chemical, and biological processes and their imprints on vent fluids, all of which make significant contribution to the carbon cycle in the Yellowstone Lake ecosystem.