Geochemistry of High-Molecular Weight Dimethylalkanes

Abstract

—The homologous series of high-molecular weight dimethylalkanes (HMWDMAs) with either odd- or even-numbered carbon chains in the range from C19–20 to C30–31 have been identified in organic matter from recent and partially lithified deposits of Siberia and the Russian Platform by chromatography–mass spectrometry. The first homologous series is represented by even-numbered 3,4-HMWDMAs followed by the alternation of odd-numbered 3,5-HMWDMAs, even-numbered 3,6-HMWDMAs, and odd-numbered 3,7-HMWDMAs. The most abundant are 3,7-dimethylalkanes. The microbial origin of high-molecular weight dimethylalkanes is the most likely explanation for their presence in the fossil organic matter. The precursors of HMWDMAs might have been tetra- and diether lipids of archaea and bacteria. It is assumed that HMWDMAs and other immature hydrocarbons from great depths (SV-27 and SG-6 superdeep boreholes) result from the decomposition of asphaltenes, which occluded the related compounds inside their structure during the early stages of generation and carried them unchanged throughout the “oil window”.

Molecular Indicators of Sources and Biodegradation of Organic Matter in Sediments of Fluid Discharge Zones of Lake Baikal

“…Thus, acetate-ion was not detected in the pore water of sediment cores from MV Malenky (up to 139 cm) and Kedr (152 cm; [49]). The isotopic composition of siderites from zones of the focus fluid discharge [120] points to The received data on the distribution of 3,6-DMA (only even homologues) and 3,7-DMA (only odd homologues) agree with the previous research [63], but the series of 3,5-DMA in the OM of bottom sediments of Lake Baikal contains both odd and evenchained homologues that were not detected previously. The ubiquitous distribution of these compounds in the sediments of the lake (even their low content) allows us to define them as a background geochemical characteristic.…”

“…Besides the traditional set of molecular markers, a series of methylated alkanes (3,5-, 3,6-, 3,7-dimethylalkanes -DMA, m/z 99, 113, 127; Figure 8) were also identified in bottom sediments of Lake Baikal. These structures have been previously detected in OM of recent and partially lithified deposits of Siberia and the Russian Platform [63,117,118] and in sediments of the Academician Ridge of Lake Baikal [111]. Precursors of these compounds with a high probability are di-and tetraether lipids of archaea and bacteria [63,119].…”

“…These structures have been previously detected in OM of recent and partially lithified deposits of Siberia and the Russian Platform [63,117,118] and in sediments of the Academician Ridge of Lake Baikal [111]. Precursors of these compounds with a high probability are di-and tetraether lipids of archaea and bacteria [63,119].…”