Lignite-bearing strata from Central Mongolia reveals a unique Early Cretaceous paleoenvironment

Early Cretaceous terrestrial environments from inner paleo-Asia are poorly studied. Here we report new data from the Tevshiin Govi opencast lignite mine (TSG; Choir-Nyalga Basin; Aptian-Albian) from central Mongolia. The TSG contains a thick, lignite-rich succession assigned to the Khukhteeg Formation, which bears an exceptionally well-preserved fossil flora composed of various pine and redwood species as well as representatives of extinct seed plants. In order to reconstruct the conditions during peat formation, we used a combined approach including coal petrology and palynology, complemented by geochemical measurements (TOC, TS, δ¹³C_org) and brGDGT-based palaeothermometry. The investigated lignites show maceral compositions with high amounts of mineral detritus, which are dominated by huminite. Calculated maceral indices are indicating a topogenous mire setting. The variability of the GWI_AC indicates regular flooding of the peat. The stratigraphic trend in δ¹³C_org covaries with fusinite content, suggesting that short-lived events such as peat fires had a pronounced effect on the δ¹³C_org signature. Palynological observations show a strong dominance of pollen produced by Cupressaceae and Pinaceae, which is in accordance with the macrofossil findings. No angiosperm pollen was observed. The brGDGT data exhibit mean annual air temperatures of around 10°C (± 3°C), which is slightly cooler than previously thought. Overall, the TSG site reveals a paleoenvironment that was mainly forested with conifers with an understory of pteridophytes and strongly influenced by fluvial processes. During phases of peat formation, the environment was either covered by water or regularly flooded. Overall low inertinite contents reveal varying intensities of peat fires.