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Distribution and carbon isotope composition of pregnane in carbonate-evaporitic rocks from the Bonan Sag, Bohai Bay Basin, Eastern China: Insights into sources and associated lake environments
Organic Geochemistry ( IF 2.6 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.orggeochem.2020.104127 Yongdong Zhang , Yongge Sun , Qin Liu
Organic Geochemistry ( IF 2.6 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.orggeochem.2020.104127 Yongdong Zhang , Yongge Sun , Qin Liu
Abstract The distribution and carbon isotope composition of pregnane was determined in eight samples of carbonate-evaporitic rock collected from depths of 2938–3012 m in Well Luojia 14, Bonan Sag of Bohai Bay Basin, Eastern China, as a means to identify sources, and draw conclusions about formation processes and associated lake environments. The results in four samples from the 2989–3006 m depth range indicated abnormally high abundances of pregnane relative to long-chain (≥ C27) homologues, with ratios of pregnane to C27 ααα20R sterane between 5.24 and 21.01 and averaging 13.91. Meanwhile, values of gammacerane index (gammacerane/C30 αβ hopane) and n-C21-/n-C22+ ratios were as high as 3.41–9.29 (av. 5.01) and 21.68–60.76 (av. 33.42) respectively, indicating the presence of stable oxic-anoxic stratification in the water column and blooms of heterotrophic bacteria. The δ13C values for pregnane were similar to those of primary producer-derived lipids, such as pristane and phytane, but lower by ∼ 8‰ than for 4-methyl pregnane. This is consistent with the reports of 13C depletion in steranes (≥ C27) relative to their 4-methyl analogues. Taken together these results suggest the abnormal enrichment of pregnane might result from severe bacterial alteration of algal derived lipids in response to the stabilization of water column stratification forced by a salinity gradient. In four samples from within the 2938–2948 m band and at 3012 m, similar δ13C values for pregnane, pristane and phytane suggest an algal origin for pregnane. However, pregnane/C27 ααα20R sterane ratios, gammacerane index values and n-C21-/n-C22+ ratios were all relatively low, with averages at 3.20, 0.43 and 4.60, respectively, suggesting much less bacterial influence on algal lipids and perhaps indicating relatively unstable stratification of the water column, possibly caused by a reduced input of freshwater.
更新日期：2021-01-01
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