3-2 Molecular and Isotopic Biogeochemistry of Microbial Organic Matter in Eocene Lacustrine Saline Environments

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources

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Abstract:

The carbon isotopic (d¹³C) signatures of organic matter (OM) and biomarkers derived from microbes provide invaluable evidence of the sources of sedimentary OM and help efforts to reconstruct depositional paleoenvironments and paleoclimates. The paucity of studies exploring these characteristics in ancient saline lake settings limits understanding of how such d¹³C signatures reflect the role of microbial activity in these systems. This study examines stratigraphic profiles of the d¹³C signatures of a range of biomarkers in saline lacustrine sediments from the Eocene Qiangjiang Formation of the Jianghan Basin, China, focusing on assessing variations in microbial OM linked to hydrological cycles. It evaluates multiple samples from four discrete rhythmite sequences with distinct differences in their d¹³C_org signatures, reflecting variations in the source of their OM and aspects of the hydroclimate. Statistical analysis of the d¹³C composition of n-alkanes (C₁₉-C₃₅) reveals a consistency in their distributions throughout the stratigraphic sequence that predominantly reflects their origins from aquatic organisms. Paired correlations of d¹³C compositions between a variety of individual biomarkers (acyclic isoprenoid alkanes, steranes, hopanes) and d¹³C_org helps characterize and distinguish OM contributions from Archaea, algae, and bacteria. For example, the close correspondence in the d¹³C compositions between pairs of acyclic isoprenoid alkanes – pristane and phytane (C₁₉, C₂₀), pentamethylicosane and squalane (C₂₅, C₃₀), and norbiphytane and biphytane (C₃₉, C₄₀) – attests to the distinct microbial sources, likely reflecting their utilization of different carbon substrates. The d¹³C compositions of C₂₇ and C₂₈ steranes are markedly less negative than those for their C₂₉ counterparts consistent with their origin from algal and mixed OM sources, respectively. The d¹³C signatures of hopanes align with those of the C₃₉ and C₄₀ acyclic isoprenoid alkanes and b-carotane, reflecting their similar carbon assimilation processes. Thus, evaluation of the d¹³C compositions for source-diagnostic biomarkers helps decipher the varied contributions of OM from microbial sources in a saline lacustrine setting.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10179

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