STABLE ISOTOPE PALEORECORDS, EARLY CRETACEOUS YIXIAN FM (BARREMIAN), LIAONING PROVINCE, P.R. CHINA: PALEOHYDROLOGIC AND PALEOCLIMATIC INTERPRETATIONS

Lacustrine deposits of the Yixian Formation (YF) are widely renowned for exceptional preservation of soft-bodied tissues from an Early Cretaceous terrestrial biota, including feathered dinosaurs and birds. The YF was deposited at ~ 39 °N paleolatitude, corresponding with a Cretaceous zonal MAT of 17.4 °C. Early diagenetic carbonate nodules are common in the YF. While most lacustrine deposits consist of volaniclastic debris, there are some primary lacustrine marls. Carbon & oxygen isotopic studies of these authigenic carbonates provide insights into the paleohydrology of the catchment basin of the YF, and internal dynamics of the ancient lake system. All authigenic carbonates in the YF have δ¹³C values greater than 0 ‰ ranging up to > +13 ‰ VPDB, suggesting that anaerobic microbial methanogenesis exerted a major control on dissolved carbon cycling in the lake system. Laminated marls, and early diagenetic calcite spars filling ostracode shells in laminites, produce a meteoric calcite line (MCL) trend with δ¹⁸O of -8.5 ‰ VPDB. Data from calcite layers in marls are arrayed in C & O isotope space along a positive linear covariant trend (PLCT) with δ¹⁸O ranging up to -5 ‰. Calcite nodules have isotopically lighter end-member values that array along a MCL trend with δ¹⁸O values of -5 ‰, with PLCT trends unique to each nodule, with δ¹⁸O values ranging up to -3 ‰. We suggest that the two MCL trends are related to lake-level highstands (MCL δ¹⁸O = -8.5) and lowstands (MCL δ¹⁸O = -5) controlled by long-term changes in precipitation-evaporation balances in the basin. Evaporation pan experiments suggest that at MAT of 17.4 °C, evaporative losses of lake volume from 15-22 % could explain offsets in MCL values. Nodules with “lowstand” MCL values produce the highest δ¹³C values, suggesting that methane outgassing from the lake was most pronounced during lowstand episodes. Lakewater δ¹⁸O values estimated from the two MCL values in the YF (-8.1 and -4.6 ‰ SMOW) are compatible with isotopic estimates of Cretaceous paleoprecipitation from the same paleolatitudes in North America (Ufnar et al., 2004, GSA Bull 116:471), suggesting that Cretaceous atmospheric moisture transport over Asia was grossly similar to that over North America.