CARBONATE CARBON ISOTOPE EXCURSIONS IN THE RUBY RANCH MEMBER OF THE CEDAR MOUNTAIN FORMATION AND THEIR TERRESTRIAL PALEOCLIMATIC IMPACTS

Mid-Cretaceous Oceanic Anoxic Events (OAEs) sharply punctuated the carbon cycle of an ancient greenhouse world. The carbon isotope excursions associated with the OAEs, however, are not perfectly synchronous with the deposition of the marine black shales that define these events, denoting complex feedback processes in the carbon cycle that remain elusive. Several researchers have correlated mid-Cretaceous carbon isotope excursions associated with OAEs into the terrestrial realm through analysis of land plant macrofossils and/or terrestrial organic matter buried in well-dated marine deposits. The burial of allochthonous terrestrial organic carbon in marine strata, however, provides limited information on paleoclimatic impacts in coeval terrestrial environments. Conversely, successions of continental Cretaceous strata preserve autochthonous accumulations of terrestrial carbon substrates (plant macrofossils, dispersed organic matter, pedogenic carbonates) that provide a direct record of terrestrial paleoenvironmental conditions. Autochthonous terrestrial carbon associated with climatically-sensitive paleosols and palustrine carbonates provide direct lines of evidence to interpret paleoclimatic changes during mid-Cretaceous carbon isotope excursions. The development of independent chronostratigraphic data to constrain marine-terrestrial δ¹³C correlations remains challenging. Terrestrial strata of the Ruby Ranch Member of the Cedar Mountain Formation in eastern Utah record the C9-C11 carbon isotope segments of the 116-112 Ma positive carbon isotope excursion straddling the Aptian-Albian stage boundary. Diagenetic and paleohydrologic interpretations of stable isotope data from palustrine carbonates in this interval unequivocally show an aridification event in the North American terrestrial foreland basin during the peak of the positive carbon isotope excursion event, but the need for independent chronostratigraphic verification is restraining the timely publication of these results.