PROGRESS AND PROSPECTS FOR CARBON ISOTOPE CHEMOSTRATIGRAPHY OF EARLY CRETACEOUS TERRESTRIAL STRATA

Aptian-Albian terrigenous clastic strata of the Cedar Mountain Formation (CMF) in the North American Cretaceous foreland basin of Utah, and lacustrine marls of the Xiagou Formation (XF) of Changma basin in Gansu Province, P.R. China are continental deposits that capture highly-resolved records of the global Early Cretaceous C-isotope stratigraphy. These observations indicate a tight temporal coupling between marine, atmospheric, and terrestrial carbon pools during the carbon isotope perturbations attending Early Cretaceous global change. The calcrete succession in the CMF captures the early Aptian Ap7 to late Albian C15 C-isotope features (Ludvigson et al., 2010, JSR 80:955-974; newer work in review). The long-ranging C9-C11 positive carbon isotope excursion (CIE) spanned from about 115 to 112 Ma, and coincided with a late Aptian peak in Cretaceous volcanism. Baseline carbonate δ¹³C values in chemostratigraphic profiles are about -6‰ VPDB, with peak values of about -3‰ VPDB in the C10 interval. Coordinated carbonate-organic δ¹³C data from calcretes yield paleobarometric pCO₂ estimates of about 1,000 ppm in early Aptian baseline positions, with a buildup up to about 1,200 ppm at the peak of the C10 positive CIE, and a subsequent long-term fall to about 600 ppm in the late Albian C15 feature. The C10 stratigraphic interval also provides sedimentary evidence for an aridification event in the leeward rain shadow of the Sevier orogenic highlands. Diagenetic studies of calcitic precursors in dolocretes from the C10 interval suggest that syndepositional pore fluids experienced ¹⁸O enrichments through evaporative losses to the atmosphere (up to 35% by volume). This C10 interval is also marked by a prominent reddening of strata in comparison to bounding gleyed strata. Laminated lacustrine marls of the XF produce organic matter δ¹³C values ranging from about -32 up to -22‰ VPDB in coherent stratigraphic patterns that we correlate to the 122 Ma early Aptian Selli event (new work in review). The C3, C4, C5, and C6 C-isotope segments are present in correlated sections, and help clarify stratigraphic relationships between two fossil bird quarries (You et al., 2005, Historical Biology 17:7-14).