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Sediment and pore water samples from all drill sites of International Ocean Discovery Program (IODP) Expedition 385 were analyzed quantitatively for aliphatic hydrocarbons, petroleum (C9–C44) hydrocarbons, and aromatic and polyaromatic compounds. All hydrocarbon classes showed concentration peaks in deep, hot sediments just above and below deeply buried sills (Sites U1545 and U1546), indicating that they were formed by thermal maturation of buried organic matter in the thermal aureole of sill intrusion and have, to a large extent, remained in situ. Plotting hydrocarbon concentrations against in situ temperature shows a pronounced increase in concentration between 65° and 80°C, the thermal limit of hydrocarbon-degrading microbial populations. A smaller hydrocarbon maximum is associated with surficial sediments: within the upper 4 m of the sediment column, the concentrations of total saturated hydrocarbons and of total petroleum hydrocarbons were almost always higher compared to the next sediment samples in downhole sequence, compatible with biogenic hydrocarbon input that reaches all drill sites in Guaymas Basin. The U-shaped hydrocarbon profiles suggest a biological filter that degrades surficial hydrocarbon input and deeply sourced hydrocarbons as soon as the temperature regime in gradually cooling, slowly accumulating sediments permits microbial activity.
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Sterane and hopane biomarkers capture microbial transformations of complex hydrocarbons in young hydrothermal Guaymas Basin sediments
Abstract In Guaymas Basin, organic-rich hydrothermal sediments produce complex hydrocarbon mixtures including saturated, aromatic and alkylated aromatic compounds. We examined sediments from push cores from Guyamas sites with distinct temperature and geochemistry profiles to gain a better understanding on abiotic and biological hydrocarbon alteration. Here we provide evidence for biodegradation of hopanoids, producing saturated hydrocarbons like drimane and homodrimane as intermediate products. These sesquiterpene by-products are present throughout cooler sediments, but their relative abundance is drastically reduced within hotter hydrothermal sediments, likely due to hydrothermal mobilization. Within the sterane pool we detect a trend toward aromatization of steroidal compounds within hotter sediments. The changes in hopane and sterane biomarker composition at different sites reflect temperature-related differences in geochemical and microbial hydrocarbon alterations. In contrast to traditionally observed microbial biodegradation patterns that may extend over hundreds of meters in subsurface oil reservoirs, Guaymas Basin shows highly compressed changes in surficial sediments.
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- Award ID(s):
- 1829903
- PAR ID:
- 10377458
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Communications Earth & Environment
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2662-4435
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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