墨西哥湾白垩系沉积有机质中异常高C_(31)-C_(32)升藿烷的检出及其生物地球化学意义

Abnormally high abundance of C_(31)-C_(32) homohopanes in Cretaceous sedimentary organic matter from the Gulf of Mexico and its biogeochemical implications

地质体中藿烷类化合物以17αβ-藿烷系列为主,且C_(31+)升藿烷相对含量通常呈现出随碳数增加而减小的特征。本文报道了美国墨西哥湾下白垩统Aptian-Albian阶碳酸盐岩可溶有机质中异常高C_(31)-C_(32)升藿烷的检出(17α-C_(31)>C_(30))。与含正常藿烷系列的碳酸盐岩相比,该类低TOC含量碳酸盐岩中可溶有机质表现为低n-C_(17)/n-C_(21)值、5α(H),14α(H),17α(H)-C_(29)规则甾烷优势、C同位素组成偏重、高Pr/Ph和Ts/Tm值等特征。结合碳酸盐岩沉积背景和可溶有机质中甲基藿烷的分布特征,提出该类沉积有机质发育于偏氧化的近岸浅水环境,沉积环境演变导致的生源变化是促使异常高C_(31)-C_(32)升藿烷分布的主要因素,其来源可能与某种异养细菌优势有关。

The 17αβ-hopanes are major hopanoids in sedimentary rocks,generally showing a decreasing trend with increasing carbon numbers from C_(30) to C_(35).This study reports an unusual distribution of 17αβ-hopanes with elevations of C_(31)-C_(32) homohopanes in carbonate rocks from the Early Cretaceous Aptian to Albian in the Gulf of Mexico in the United States.These low-TOC-content carbonate rocks were characterized by low n-C_(17)/n-C_(21) ratios,relatively high concentrations of 5α(H),14α(H),17α(H)-C_(29) regular steranes,heavy stable carbon isotopes,and high Pr/Ph and Ts/Tm ratios.Considering the depositional background and the distributions of methylated hopanes in sedimentary organic matter,the sedimentary organic matter may have been deposited in a nearshore,shallow,and more oxidizing environment.The abnormal increase in C_(31)-C_(32) homohopanes could have resulted from the changes in biogenic sources caused by environmental evolution,and the abnormally high abundance of C_(31)-C_(32) homohopanes is due to an increase in heterotrophic bacteria.