AN INVESTIGATION INTO THE GREEN RIVER FORMATION AND EOCENE HYDROSPHERE

Abstract

The Wilkins Peak Member of the Green River Formation in southwest Wyoming is composed largely of lacustrine deposits dating back to the Eocene Climatic Optimum around 50 million years ago. The deposits show evidence of recurring lake high stands that are paced by orbital cycles. One of the ongoing research questions in this area is how orbital-scale changes in rainfall source and intensity contributed to this cyclical deposition. One hypothesis is that rainfall was delivered from the Pacific Ocean, paced by orbital forcing. Another hypothesis is that rainfall arrived from the Gulf of Mexico, via a “proto-monsoon” system. Terrestrial plants in the region continuously record a chemical signal of precipitation, which is well preserved in these lacustrine deposits. Using samples of a rock core obtained by the USGS, we were able to extract the organic compounds present in the formation. Gas chromatography and stable isotope mass spectrometry allowed us to further analyze deuterium and hydrogen ratios in plant-derived fatty acids in concurrent layers of rock. Present-day rainfall deriving from the Pacific Ocean and Gulf of Mexico basins have very different D/H compositions, so we were able to compare obtained data with these modern-day values for a proxy estimate ofthe origins ofEocene precipitationover the proto American West. Completed analysis of half of the original samples suggests evidence of orbital forcing at work in shifts of D/Hin lithified organic sedimentsand thus shifts in precipitation origins. Further testing is needed to conclusively link observed trends topatterns of repetition we expect to see in the data based on known sedimentation rates in the Green River basin.