Multiproxy strategy for determining palaeoclimate parameters in the Ruby Ranch Member of the Cedar Mountain Formation

Abstract

AbstractA multiproxy approach for evaluating palaeoclimate parameters in deep-time can result in improvements to inter-related factors affecting palaeohydrology. Here we utilize diverse geochemical tools to improve palaeoclimate estimates for the Cedar Mountain Formation (CMF). Prior research utilized stable carbon and oxygen isotopes to develop chemostratigraphic correlations to the late Aptian–early Albian, hypothesized aridity during a positive carbon isotope excursion (CIE) and estimated pCO2 through this event. This study refines estimates using petrographical analyses, bulk geochemical proxies for mean annual precipitation (MAP) and clumped isotope palaeothermometry. MAP rates range from 736 to 1042 mm a−1 with a slight decrease during the hypothesized aridity event. We interpret warm-biased temperatures (with an average of 32.9°C) that do not vary significantly through the study section. Carbonate nodules are likely to have precipitated in highly evaporative conditions as indicated by the presence of dolomite. Utilizing a simple Rayleigh fractionation model and two estimates of δ18O of water, we suggest that evaporation of 2–57% is necessary to result in an enriched end member δ18Ow. These data suggest that an increase in aridity is a result of lower MAP rates and greater evaporation during seasonal extremes. Lastly, revised pCO2 calculations suggest overestimates but indicate a shift towards greater concentrations during the positive CIE.