ISOTOPIC AND GEOCHEMICAL CHARACTERIZATION OF AN EARLY MESOZOIC PRECESSION CYCLE (NEWARK BASIN, EASTERN NORTH AMERICA): PALEOHYDROLOGIC EVIDENCE FOR MILLENNIAL-SCALE CLIMATE CHANGE

High resolution carbon and oxygen isotopic and major and trace element geochemical characterization of a precession cycle within the Warford member of the Passaic formation, Newark Basin was performed to determine the frequency and variability in paleoclimatic signals associated with paleohydrologic conditions present within the early Mesozoic lake. Lithologically, the Warford member appears to represents a time of perennial lake conditions within an overall drying climate as North America drifted north from the equator. However several geochemical indicators suggest considerable lake level variability within the cycle. δ¹³C (PDB) and δ¹⁸O (PDB) values co-vary suggesting long term, closed basin hydrologic conditions. The isotopic analyses can be divided into two distinct linear trends with different slopes suggesting two modes of paleohydrologic conditions. X-ray diffraction analysis of the carbonate fraction compared with depth rank from Olsen and others, 1996 shows calcite as the dominant mineral at the base and top of the lake cycle when the lake is relatively shallow, dolomite in the deepest section of the cycle, and mixture of dolomite and calcite and/or high Mg-calcite at the transitions between shallow and deep phases. Overprinting the general trend of a rapid deepening and slower shallowing lake trend over the precession cycle seen in several geochemical indicators (V, Mn, Mo, DOP, Al/Ti) is a 2nd order trend of variability. Of particular interest is the variability in total inorganic carbon (TIC), which exhibits 16 significant peaks over the cycle. We interpret these peaks to be millennial-scale events either related to mega-monsoon cycles, lake mixing, and subsequent biological productivity creating basin-wide carbonate-rich sediments or lake evaporation, supersaturation of surface waters, and inorganic carbonate precipitation.