Early Cretaceous lignites as archive for continental climates of paleo-Asia

The late Early Cretaceous interval (121.4 to 100.5 Ma) was characterized by a gradual warming trend superimposed on an already warm greenhouse climate. Whereas the evolution of ocean temperatures during this time interval is relatively well constrained, information on the response of continental interiors to such climatic extremes is limited. Here we report new data from two continental sections (Shivee Ovoo (SVO) and Tevshin Gobi (TSG); Choir-Nyalga Basin) from central Mongolia, which contain thick, lignite-rich successions (Khukhteeg Fm.) bearing an exceptionally well-preserved fossil flora of various pine and redwood species as well as representatives of extinct seed plant lineages. In order to reconstruct the palaeoenvironmental conditions, a combined approach including brGDGT-based palaeothermometry, coal petrology and palynology is applied, complemented by geochemical measurements (TOC, TS, δ13Corg). The investigated lignites show significant differences in maceral compositions with the TSG samples being rich in mineral detritus and mainly composed of huminite. In contrast, samples from SVO show higher fusinite and liptinite content with generally low mineral detrital contribution. The paleotemperature estimates represent the oldest brGDGT analyses obtained from continental strata so far. The new data indicate that the climatic conditions in mid-latitudes of paleo-Asia during the late Early Cretaceous were characterised by mean annual air temperatures of about 12 ± 5°C. Our brGDGT data show lower continental temperatures for mid-latitude paleo-Asia than previously suggested based on modelling.