PETROGRAPHIC, XRD, AND CLUMPED ISOTOPE STUDY OF A CRETACEOUS DOLOMITIC LAKE IN NORTHWEST CHINA

Cretaceous lacustrine strata of northwestern China provide the opportunity to investigate the evolution of continental ecosystems, as well as the Cretaceous Greenhouse from a continental perspective. Samples from an important fossil bird locality located near the town of Changma in the Xiagou Formation were collected and investigated utilizing petrographic observations, powder x-ray diffraction, traditional stable isotopes and clumped isotope paleothermometry.

Petrographic and XRD analyses indicated that the majority of the lacustrine carbonates are fine grained and dolomitic. Carbon isotopic compositions of these samples are enriched, ranging from +0.4‰ to +11.4 ‰, VPDB. Oxygen isotopes range from -8.5 to -1.9‰ VPDB, and clumped isotope values range from 0.652 to 0.707 ‰ compared to the absolute reference frame.

Combining these data, the dolomites are interpreted to fall into three categories. One group includes fine grained dolomites that are likely microbially mediated (high δ¹³C values), under low oxygen environments. These often show a speckled appearance under cathodoluminescence particularly when associated with organic material and likely formed in deeper parts of the lake based on the laminated bedding. A second group were probably also microbially mediated, but occurred in shallower environments based on more massive and bioturbated sedimentary structures. A third group shows some amount of crystal coarsening and zoning that indicates early diagenetic recrystallization. The only calcite found in the samples are pore filling cements.

Clumped isotope temperatures range from 21.1°C to 52.0°C (calcite pore filling cement). Taking into consideration seasonal biases, as well as the shallow formed dolomites, average air temperatures for this area of Asia (~35°N) during the Cretaceous is estimated at 22.3°C. This is consistent for modeled temperatures for this area during greenhouse conditions. In addition, the presence of abundant dolomite suggests that the lake experienced significant evaporative flux. This may also be consistent with recent hypotheses that lakes played an important role in the Cretaceous Greenhouse climate.