TERRESTRIAL CARBONATE RECORDS OF CRETACEOUS (APTIAN-ALBIAN) CARBON ISOTOPE EXCURSIONS FROM DEPOSITS IN NORTH AMERICA AND CHINA

Recent publication of Aptian-Albian δ¹³C chemostratigraphic records from calcrete successions in the Cedar Mountain Formation (CMF) of Utah (Ludvigson et al., 2010, JSR 80:955-974) and China (Li et al., 2013, Palaeo-3 385:171-189), and lacustrine marls in China (Suarez et al., 2013, GSL-SP382:143-154) are opening a window into the continental paleoclimate dynamics of the Cretaceous greenhouse world. Pedogenic carbonate records of pCO₂ changes associated with Aptian-Albian carbon isotope excursions (CIEs) from China (Li et al., 2014, Geol. Mag. 151:830-849) and North America (Ludvigson et al., 2015, Cret. Res. 56:1-24) show important similarities. Despite differing methodological approaches to construction of the time series and boundary conditions for pCO₂ estimates, both published records indicate (1) long-term decline in pCO₂ levels through the Aptian-Albian interval; (2) abrupt changes in pCO₂ levels on time scales (< 10⁶ yrs) that are temporally-related to the CIEs, and (3) peak pCO₂ levels near the Aptian-Albian boundary at the position of the C10 C-isotope feature of Bralower et al. (1999, J. Foram. Res 29:418-437). Paleosols from the C10 interval in the CMF are prominently reddened in comparison to underlying and overlying paleosols characterized by lower chroma colors. Dolomitized calcretes from the C10 interval in the CMF also contain diagenetic evidence for an aridification event near the Aptian-Albian boundary, demonstrating that carbon cycle perturbations during this time interval had profound paleoclimatic impacts on terrestrial ecosystems. Given the evidence for abrupt continental climate change during the Aptian-Albian interval, there is a pressing need for more and better geochronologic constraints. We advocate the use of LA-ICP-MS U-Pb dating of populations of zircons (n=300) separated from mudstone paleosols as an alternative approach to dating strata in continental basins that lack fortuitous preservation of intercalated volcanic deposits.