Hydrocarbon transformations in sediments from the Cathedral Hill hydrothermal vent complex at Guaymas Basin, Gulf of California – A chemometric study of shallow seep architecture. (February 2021)

• Record Type: Journal Article
• Authors: Dalzell, Connor J.; Todd Ventura, G.; Walters, Clifford C.; Nelson, Robert K.; Reddy, Christopher M.; Seewald, Jeffrey S.; Sievert, Stefan M.
• Is Part Of: Organic geochemistry. Volume 152(2021)
• Publication Date: 2021-02
• Subjects: Guaymas Basin -- Hydrothermal -- Chemometric -- Comprehensive two-dimensional gas -- Chromatography -- GC×GC -- Multiway principal components analysis -- Aromatic -- Hydrocarbons -- PAHs -- Biomarkers -- Diploptene -- Petroleum; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2020.104173
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: 34 hydrothermal oil samples along a push core transect were analyzed by GC × GC. Hydrocarbon matrices were compared using MPCA and HCA of whole oil chromatograms. Sediments were shown to contain migrated and in situ generated hydrocarbons. The molecular results show a high degree of spatial complexity at shallow depths. Abstract: Guaymas Basin is a submarine depression in the Gulf of California marking the northern end of the East Pacific Rise mid-oceanic spreading ridge. The basin receives high input of sedimentary organic matter (SOM) from elevated productivity in the overlying surface waters and runoff from the surrounding continent. This, coupled with high sedimentation rates, produces near-uniform compositions of SOM. Various hydrothermal vent complexes occur along this margin. One of these is Cathedral Hill, a hydrothermal mound with sulfide chimneys surrounded by mats dominated by sulfide-oxidizing Beggiatoa, covering sediments stained by locally produced oil. We collected four push cores along a transect at this site (4 m × 0.21 m) extending from near the vent center to the ambient sediment just outside the microbial mat cover. Porewater temperatures near the mound center were projected to reach 155 °C by 21 cm below the sea floor (cmbsf). Within these conditions, a kinetic model based on vitrinite reflectance equivalence (%Re ) predicts petroleum formation as shallow as 15–18 cmbsf, with metagenesis commencing at <60 cmbsf. Bulk extract data (total lipid extracts as well as polar and apolar fractions), solvent-extracted sediment TOC (herein referred to as protokerogen TOC), and molecular thermal maturation parameters support these generation estimates. In recent years, the application of chemometric techniques to comprehensive two-dimensional gas chromatographic (GC × GC) analyses has allowed comparison of thousands of unique hydrocarbons within oils. Here we reconstruct the shallow subsurface petroleum system by applying multiway principal component analysis (MPCA) and hierarchical cluster analysis (HCA) directly to GC × GC chromatograms. We then compare the resulting multivariate models to a systematic survey of subtracted GC × GC chromatograms, a transect heat map of sample hydrocarbon compound diversities, and profiles of various thermal maturation parameters to elucidate how these hydrocarbon matrices are attenuated by production, migration, and/or thermochemical oxidation. Sample matrices have up to 5800 unique compounds spanning a range of normal and branched alkanes, saturated and unsaturated biomarkers, substituted and unsubstituted polycyclic aromatic hydrocarbons (PAHs), perhydro-PAHs, and benzothiophenes with up to six ring-cycles (i.e., benzoperylenes, dibenzopyrenes, dibenzochrysenes, and indenopyrenes). These matrices display systematic temperature-dependent trends. Generation likely begins in 6–10 and 15–18 cmbsf where sediments are exposed to ∼115 °C vent porewater temperatures, which is shallower than predictions based on our kinetic model. Independent of these sites of generation is ubiquitous staining of the sediments from advected oil that is heavily dominated by PAHs as well as two stratigraphic bands of migrated oil that extend horizontally across the transect at 0–2 and ∼6–10 cmbsf, respectively. The MPCA models along with non-statistical validation techniques show evidence of decreasing diversities and concentrations of alkylated aromatic hydrocarbons concomitant with elevated abundances of dealkylated PAHs and/or the migration of unsubstituted PAHs from deeper basin depths as sediments become exposed to more severe hydrothermal conditions. These results indicate that even at relatively small spatial scales, the petroliferous sediments at hydrothermal vent sites can be highly complex.