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

摘要

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 of identification of the sources and to draw conclusions about possible formation processes and associated lake environments. The results in four samples from the 29893006 m depth range indicated abnormally high abundances of pregnane relative to long-chain (>C-27) homologues, with ratios of pregnane to C-27 alpha alpha alpha 20R sterane between 5.24 and 21.01 and averaging 13.91. Meanwhile, values of the gammacerane index (gammacerane/C-30 alpha beta hopane) and n-C-21 similar to/n-C22+ ratios were as high as 3.41-9.29 (mean 5.01) and 21.68-60.76 (mean 33.42) respectively, indicating the presence of stable oxic-anoxic stratification in the water column and blooms of heterotrophic bacteria. The delta C-13 values for pregnane were similar to those of primary producer-derived lipids, such as pristane and phytane, but lower by similar to 8% than that of 4-methyl pregnane. This is consistent with the reports of C-13 depletion in steranes (>= C-27) 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 delta C-13 values for pregnane, pristane and phytane suggest an algal origin for pregnane. However, pregnane/C-27 alpha alpha alpha 20R sterane ratios, gammacerane index values and n-C-21 similar to/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. (C) 2020 Elsevier Ltd. All rights reserved.