CONTINENTAL PALEOTEMPERATURES FROM AN EARLY CRETACEOUS DOLOMITIC LAKE, GANSU PROVINCE, CHINA

摘要

The Lower Cretaceous strata of Asia are renowned for abundant fossil resources. These strata can also offer a wealth of quantitative data regarding the paleoclimate and paleoenvironment of the terrestrial Cretaceous, an important transition in Earth history. Many modern groups of organisms that dominate terrestrial ecosystems today originated or expanded in the Early Cretaceous. In addition, the Cretaceous Period, known to have been characterized by a greenhouse climate, provides opportunities to understand how the climate system functioned during greenhouse climate states. Here we utilize the clumped-isotope paleothermometer to provide paleotemperature records from a lacustrine sequence in northwestern China, previously constrained to the early Aptian. Petrographic and X-ray diffraction analyses indicate that the lake sediments produced primary to early diagenetic dolomites, likely microbially mediated due to high organic-matter input and seasonally warm arid conditions. The majority of the dolomitic carbonates formed in pore waters just below the sediment-water interface or as primary shallow-water precipitates. Clumped-isotope-derived temperatures from all carbonates range from 20.6 degrees C for early diagenetic dolomites to 47.2 degrees C for late-burial calcite cements. Accounting for seasonal biases, mean annual air temperatures are calculated to have averaged 20.2 degrees C. This is broadly consistent with local paleotemperature proxy evidence and climate-model paleotemperature estimates. The data presented provides further quantitative constraints on the Cretaceous greenhouse world, as well as insights into depositional settings that produce low-temperature dolomites.