Lacustrine paleoenvironments from stable isotopes of hydrogen and carbon in lipids

Paleolacustrine sedimentary systems contain economically important natural resources and provide high-resolution records of terrestrial and ecosystem history. This work explores the potential of hydrogen and carbon compound-specific isotope analyses for elucidating the factors that control accumulation of organic matter (OM) in ancient lakes and for providing a paleohydrological record for validating paleoclimatic general circulation models. Isotopic and organic geochemical analyses of Lower Cretaceous lacustrine sections in Gabon, Congo, and Kwanza Basins, West Africa were supplemented by computer simulations of precipitation and wind patterns at 115 and 130 Ma.; GENESIS GCM and hydrogen isotope data indicate that lacustrine environments in Gabon were influenced by significantly higher precipitation in comparison with Congo and Kwanza. The result is in agreement with carbon isotope and biomarker data that suggest more active water mixing in Gabon than in Kwanza, where photic zone anoxia (and thus more water stagnation) was recognized based on the presence of biomarkers from green sulfur bacteria (Chlorobiaceae ). Greater amounts of rainfall in Gabon could be responsible for a decrease in source rock quality in this basin. However, more geochemical and sedimentological data are needed to test the link.; The results of this study confirm earlier reports indicating that compound-specific hydrogen isotope analysis of OM has the potential to become an important tool in isotopic geochemistry. The reconstructed isotopic composition of paleolacustrine water in the Congo Basin provides support for sedimentological data showing hydrological differences during accumulation of Marnes Noire Formation. Hydrogen isotopic values of n-alkanes from the bitumen fraction are most likely to preserve the original biologically derived signal before the onset of oil generation. Interpretation of deltaD values of individual compounds, especially those containing tertiary carbon sets, from units that entered the oil window should be done with caution, because structural changes accompanying thermal maturation may have a large impact on the extent of hydrogen exchange between OM and formation waters.