A molecular and isotopic geochemical study of Meso- to Neoproterozoic (1.73-0.85 Ga) sediments from the Jixian section, Yanshan Basin, North China

This paper systematically describes the molecular and isotopic characteristics of Meso- to Neoproterozoic (1.73-0.85 Ga) sediments from the Jixian section, Yanshan Basin, North China. A correlation is made between molecular fingerprints and paleoenvironmental and paleontologic data acquired from these strata. The agreement between these parameters suggest that the biomarkers preserved in these Proterozoic sediments are good indicators of the evolution of the paleoenvironment and early life in the Yanshan Basin. The fact that extended regular isoprenoids (>C-20) were detected in most of the studied samples suggest a sustained lipid input to sedimentary organic matter from halophiles and other possible archaebacteria through Mesoto Neoproterozoic. Two dehydroabietin isomers (i.e. 18- and 19-norabieta-8,11,13-trienes) were unexpectedly present in these samples, which provides evidence suggesting that the compounds with abieta structure could be derived from primitive organisms such as algae. The stable carbon isotope characteristics of kerogen and coexisting carbonate show a rapid decline in atmospheric CO, concentration from 1.73 to 1.58 Ga, and a stable concentration afterwards. The difference (Deltac) in isotopic composition between carbonate and associated kerogen lies in zonal variations along the stratigraphic sequence, which can be attributed to the alternations of marine transgressions and regressions that occurred in the Yanshan Basin during the Proterozoic. The carbon isotope relationships between kerogen and alkanes from these strata are consistent with the hypothesis that before 1.4 Ga both n-alkanes and isoprenoids had been derived mainly from heterotrophic halophiles and other archaebacteria, but after that only isoprenoids originated from archaebacteria whereas n-alkanes were possibly derived from photosynthetic organisms. Changes in the source of n-alkanes around 1.4 Ga may suggest a decline of heterotrophic reworking to primary lipids, which can be related to the formation of a quiet, shallow-water environment of an epicontinental sea at this stage in the Yanshan Basin. (C) 2003 Elsevier Science B.V. All rights reserved.