Sediment Organic Matter in Areas of Intense Methane Release in the Laptev Sea: Characteristics of Molecular Composition

Abstract

—We present results of study of the molecular composition of organic matter (OM) in the bottom sediments of the Laptev Sea by gas chromatography–mass spectrometry, isotope gas chromatography–mass spectrometry, and Rock-Eval pyrolytic analysis. The OM of all collected sediment samples shows a significant terrigenous contribution. Compounds that are biomarkers of methanotrophic microorganisms are also found. A positive correlation between the contents of the studied biomarkers and the contents of pelite and total organic carbon is observed at the sites with documented intense methane bubbling. For example, the average content of C30 hopenes at the “methane” stations is twice higher than that at the “background” ones. The average content of C32 αβ-hopanes in sediment samples from the methane seepage area is 1.5 times higher than that at the background stations. We suggest that the increased C30 αβ-hopane content (~1.5 times higher within the methane seepage area) and the decreased moretane index relative to the C31 hopane index are due to the inflow of OM of petroleum origin. The presence of biphenyl in sediments indicates its petroleum origin, which supports our assumption of the migratory nature of petroleum hydrocarbons in the methane seepage area. Triterpenoids found in the sediment OM indicate diagenetic bacterial transformation of OM in the methane seepage areas, which shows that methane has been released for a long time. We assume the intense activity of the consortium of methanotrophs and sulfate reducers in the methane seepage areas.

Source apportionment of methane escaping the subsea permafrost system in the outer Eurasian Arctic Shelf

“…The existence of pathways to transport methane from these deep sources to the water column in our study area is also consistent with recent seismic data ( 25 ), which show ∼500-m wide gas conduits in the sediment, correlating with a fault zone and cuts through the Neogene succession to the basement. Further support for a migratory inflow of petroleum hydrocarbons from below is also given by a recent biomarker study in the studied seep area ( 26 ): Excess amounts of two molecular markers typical of a petrogenic source have been found in the surface sediment of the studied seep area, with significant differences between the seepage area and the “background areas” without apparent seeps. Taken together, the triple isotopes and these other ancillary data are consistent with a deep thermogenic source of methane.…”

The Formation of Authigenic Carbonates at a Methane Seep Site in the Northern Part of the Laptev Sea

“…The source of thermogenic methane is likely to be gas condensate or oil reservoir in underlying sedimentary sequences. The molecular composition of sediment organic matter in the study area suggests hydrocarbon migration from underlying deposits [70]. Faults of the Laptev Sea Rift System can serve as pathways for upward migration of methane.…”

Characterization of Organic Matter of the Laptev Sea Eroded Coastal Sediments: A Case Study from the Cape Muostakh, Bykovsky Peninsula

“…Interestingly, the distribution of terpenoids in the m/z 191 chromatogram (Figure 10) in all sediments of the Cape Muostakh do not contain the Ts, moretane, hop-17( 21)-ene, hop-21( 22)-ene, diplopterol, but it is enriched by unsaturated terpenoids (Table 3), as compared to the bottom sediments of the Laptev Sea, collected in the expedition in 2011 (Figure S7) [48]. However, at the same time, unknown hopenes Uh1, Uh2, Uh3 and hopane UH, identified in the sediments of the Muostakh cliff, were not found in the bottom sediments of the Laptev Sea [48]. Here it should also be noted that as well unlike the bottom sediments of the Laptev Sea, there is no relationship between the A/B and C/D parameters (Figure S8), which is evidence of the uneven accumulation of organic matter and strong different degrees of bacterial activity at the different cliff levels.…”

“…Conversely, OEP index increase over the cliff section, while Eh decrease (Figure 8c). The Σ(C17-C19)/Σ(C27-C31) ratio and C/D index, which characterize the relative content of low molecular weight n-alkanes that are markers of hydrobionts and bacterial activity [46,47], have low values compared with those for the bottom sediments of the Laptev Sea and other areas of the Eastern Arctic [45,46,48,49], and reach maximums in samples "11" and "21.5" (Table 2). Alongside, there is a positive correlation between these coefficients and the Ki index, which may indicate some contribution of marine OM (sample "5", "11") or intense bacterial activity in sediments (sample "21.5").…”

“…2 A/B is the ratio of the sum of the peak areas of n-alkanes over the even envelope of the molecular mass distribution (C14, C15, C16, C17, C18, C19, C20, C21, C22, C24, C26, C28, C30, C32, C34) to the sum of the peak areas of high-molecular n-alkanes (the markers of the terrestrial vegetation -C23, C25, C27, C29, C31, C33, C35) by m/z 57 (see [46], and Figure S6). 3 C/D is the ratio of the sum of the peak areas of low molecular weight n-alkanes (C14, C15, C16, C17, C18) to the sum of the peak areas of high molecular weight n-alkanes (C19, C20, C21, C22, C24, C26, C28, C30, C32, C34) over the even envelope of the molecular mass distribution by m/z 57 (see [48], and Figure S6). 4 E/O is the index of the ratio of the even n-alkanes to odd n-alkanes.…”