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doi:10.2204/iodp.proc.331.201.2016

Results

n-Alkanes

Representative ion chromatograms showing the n-alkane content of extracted bitumen are shown in Figure F1. Distinct end-members are identified. At Site C0013, located proximal to the vent and hydrothermal mound, low carbon number n-alkanes predominate (C16–C22). At Site C0017, distal from the vent and from likely sources of heating, n-alkanes have a biological signature in the form of an odd over even predominance—likely reflecting terrestrially derived plant-organic matter present within hemipelagic sediments (Meyers and Ishiwatari, 1993). Samples from the deeper sections of holes drilled at Site C0014 contain n-alkanes in which carbon numbers ranging from C21 to C31 are the most prominent and for which the odd over even carbon number preference is much reduced. The shallowest samples seen at Site C0014 are similar to those seen at Site C0017 (the least hydrothermally altered).

Terpanoid biomarkers

The classic terpanoid biomarkers found in oil derive from their biological precursor via a complex series of chemical reactions that generate a number of structural intermediaries (e.g., steranes derived from sterenes that ultimately derive from sterols [Mackenzie et al., 1982]). Typically, only the fully saturated terpanes (e.g., steranes and hopanes) are found at oil window levels of thermal maturity because unsaturated counterparts are not thermally stable (Peters et al., 2004).

Both pre-oil window and oil window biomarkers are present in samples, and in a few instances both types of biomarker are observed, indicating mixing of mature and thermally immature compounds. Figure F2 compares 217 and 215 m/z ion chromatograms that illustrate the carbon number distribution of steranes and sterenes present at Sites C0017 and C0014. Sterols are the chemical precursors of sterenes and steranes, and despite their ubiquity in sediments the relative proportions of a particular carbon-numbered homolog can provide a fingerprint and useful paleoenvironmental information (Huang and Meinschein, 1979). The distribution of carbon numbers is similar (e.g., the C28 sterane and sterene homologs are proportionally least abundant) for both steranes and sterenes; thus, some bitumen components within zones of hydrothermal alteration are correlated to adjacent thermally immature sedimentary organic matter. This correlation strongly suggests that the bitumen obtained from hydrothermally altered samples was generated from the hemipelagic muds found within the hydrothermal system and not from a source or reservoir distal from the Iheya area.

Patterns of thermal alteration, similar to that observed for n-alkanes, were observed for geohopanes (Fig. F3). Adjacent to the sulfide mound at Site C0013, a low proportion of thermally unstable hopane isomers is found (e.g., low proportions of 17β,21β(H) and reduced 17β,21α(H) hopanes relative to 17α,21β(H) hopanes; see Farrimond et al., 1998). Downslope from the hydrothermal mound at Site C0014, where hypothermal alteration is present at depth but absent at the surface, subsurface samples have an absence of thermally less stable 17β,21β(H) hopanes and greater abundances of 21β(H) hopanes. However, shallower samples at Site C0014 possess both pre-oil window and thermally mature oil window biomarkers (e.g., both 17β,21β(H) and reduced 17α,21β(H) hopanes). Samples obtained from the drill site most distal from the hydrothermal mound (Site C0017), including samples of thermally unaltered hemipelagic mud, contain the highest proportions of 17β,21β(H) hopanes (pre-oil window biomarkers).

Despite the apparent broad correlation between the level of alteration of sedimentary organic matter and different zones of hydrothermal alteration, there are clear instances where pre-oil window and oil window biomarkers are found in the same sample (Sample 331-C0013E-5H-1, 96–104 cm, contains very small amounts of 17β,21β[H] hopanes). This complicates assigning a single level of thermal maturity to a sample, as in many cases samples can be shown to contain a compounds with contrasting thermal stabilities and origins.

Polynuclear aromatic hydrocarbons

Polynuclear aromatic hydrocarbons (PAHs) were detected in samples closest to the hydrothermal mound, although their abundance varies significantly and they were not readily detected in Site C0017 samples. Figure F4 presents an ion chromatogram on which the phenanthrene compounds used to estimate thermal maturity in the most thermally mature samples are identified and provide a %VRE. Because of the varied abundance of higher ring number PAHs (e.g., pyrene and other PAHs with four or more aromatic rings), these compounds are sporadically present and were not considered further.

At Site C0014, a range of thermal maturities is found, with compounds that are stable under pre-oil window or oil window conditions found in stratigraphically adjacent samples. All samples appear to be more thermally altered than can be accounted for by present-day downhole temperatures and reasonable durations of heating (e.g., >1 My). Samples exhibiting oil-window levels of thermal maturity are explained as hydrothermally generated petroleum that has migrated through the subsurface—although there is still the possibility that the bitumen is indigenous and particularly so for bitumen within the relatively impermeable hemipelagic mud. This is not the case for samples that have relatively high but nonetheless pre-oil window levels of thermal alteration. Even for these samples, the apparent level of thermal maturity is greater than can be accounted for by present-day bottom hole temperature (Fig. F5). The implication of this thermal maturity is that although present-day downhole temperatures may indicate a suitable habitat for hyperthermophilic organisms, paleotemperatures were probably much hotter in the past.

n-Alkanoic acids

n-Alkanoic acids form a quantitatively small fraction of the bitumen (parts per million) but are potentially significant because of their ability to aid solubilization of bitumen in saline fluids (Meredith et al., 2000) and also as components of biomass and the lipid content of sedimentary organic matter (Killops and Killops, 2005). The differences between the most thermally altered and least altered sites (C0013 and C0017) are shown in ion chromatograms in Figure F6. In all instances, the most abundant n-alkanoic acid was hexadecanoic acid with the proportion of unsaturated acids showing the most variation between samples. The concentration of n-alkanoic acids and the ratio of saturated to unsaturated acids are plotted against the level of thermal alteration in Figure F7. Initially the proportion of unsaturated acids drops significantly during the early stages of thermal maturation, but then it briefly rises with increased thermal maturation. This second rise in the proportion of unsaturated fatty acids is coincidental with a rise in the concentration of n-alkanoic acids. This sudden influx of n-alkanoic acids likely represents the liberation of covalently bound acids from macromolecular sedimentary organic matter during pyrolysis.