EARLY CRETACEOUS BASELINE ATMOSPHERIC pCO2 AND BUILDUP ESTIMATED FROM A CALCRETE SUCCESSION, CEDAR MOUNTAIN FORMATION, UTAH, USA

We calculate a baseline Aptian-Albian atmospheric pCO₂ of 1,000 ppm by applying the equation of Ekart et al. (1999, AJS 299:805-827) to isotopic analyses of pedogenic and palustrine carbonates from eight correlated chemostratigraphic positions. The positions are from the Ap7 C-isotope feature of Herrle et al. (2004, EPSL 218:149-161) to the C15 C-isotope feature of Bralower et al. (1999, JFR 29:418-437) at two sections in the Cedar Mountain Formation (Ruby Ranch Road and Muddy Creek, eastern Utah). These sections encompass the Aptian-Albian Ap7-C15 (~125-100 Ma) interval (Ludvigson et al., 2010, JSR 80:955-974). Significantly, our results indicate a buildup in atmospheric pCO₂ of ~350 ppm above baseline values during the C9-C11 positive carbon isotope excursion. Furthermore, this excursion corresponds with our reported shifts in δ¹³C values determined from coordinated carbonate and sedimentary organic carbon. We realize that S(z), the soil-derived component of total soil CO₂ at depth z, significantly varies with season, depth, soil type, and paleolatitude. Thus, unlike other studies that use a constant S(z), we report a range of pCO₂ values using several S(z) estimates. Direct comparison of our pCO₂ measurements to compiled estimates from the same interval determined on pedogenic carbonates, stomata, and liverworts recently published (Royer, 2010, PNAS 107:517-518) emphasizes the need for a non-uniform S(z) approach. Finally, we correlate our pCO₂ with high-resolution marine chemostratigraphic records (δ¹³C(carb), δ¹³C(org), ⁸⁷Sr/⁸⁶Sr), and magma flux output from the Kergulean Plateau (Indian Ocean). The 116 to 111 Ma rise and fall in pCO₂ during the C9-C11 carbon isotope excursion, for which we have identified a pCO₂ buildup, coincides with the well-documented mid-Cretaceous strontium isotope low, and with a peak in magma production at the Kerguelan Large Igneous Province. Therefore, we interpret a tectonic driver for this global change event.