TERRESTRIAL CARBONATE RECORDS OF THE CARBON ISOTOPE EXCURSIONS ASSOCIATED WITH MID-CRETACEOUS (APTIAN-ALBIAN) OCEANIC ANOXIC EVENTS

Coupling between Aptian-Albian marine, atmospheric, and terrestrial carbon reservoirs is indicated by the d¹³C chemostratigraphy of successions of palustrine carbonates in three stratigraphic sections of the Cedar Mountain Fm of Utah; one at Price River (PR; 150 m) and two at Dinosaur National Monument (DNM; 60-80 m). The profiles have well-organized curves that we correlate to Aptian positive d¹³C excursions associated with Oceanic Anoxic Events (OAEs) 1a and 1b, supporting use of palustrine carbonates for continental-marine chemostratigraphic correlations. The 5 ‰ shifts in our continental sections exceed magnitudes reported from coeval marine sections, and duplicate published results on Aptian d¹³C changes in terrestrial C3 plants. Peak d¹³C values of –3 ‰ in the PR section occur in Aptian excursions associated with OAEs 1a and 1b, whereas minimum d¹³C values of –9.3 ‰ occur at DNM in Albian strata. A Late Albian positive excursion of 2 ‰ in the PR section is tentatively correlated with the 400 kyr OAE 1d (Wilson and Norris, 2001, Nature 412:525-429), underscoring the potential for palustrine carbonate successions to capture high-resolution records of global change in the ocean-atmosphere system. Diagenetic studies indicate that chemostratigraphic signals in carbonate beds were encoded by pervasive pedogenic overprinting in vadose and phreatic environments. The terrestrial carbonate chemostratigraphy is interpreted to record stratigraphic changes in soil organic matter derived from C3 paleofloras, driven by temporal change in the d¹³C of atmospheric CO₂. Individual carbonate beds are complex mixtures of micritic, microspar, and vein-filling sparry calcite components, with each showing unique diagenetic trends in C-O isotope space. Microspars have trends conforming to meteoric calcite lines (MCLs), suggesting that they recrystallized in shallow meteoric phreatic groundwaters. Micritic calcites produce trends with covariant ¹³C and ¹⁸O enrichments of 1-2 ‰ from the MCL trends, indicating pedogenic origin in meteoric vadose environments. Standard deviations of d¹³C variations in chemostratigraphic samples (n=5-10) have modes in the 0.1 to 0.2 ‰ range for all three sections, ranging up to 2.4 ‰ in units affected by recrystallization in groundwaters with multiple DIC sources.