Palynofloral and carbon isotopic patterns of an Early Cretaceous peat mire in Laurasia: Implications for regional short-versus long-term climate variability

Abstract

The Early Cretaceous #14 and #15 coal seams of the Yimin Formation (Hailar Basin, Inner Mongolia, China) have been characterised using palynological and stable carbon isotopic analyses. These seams represent the stratigraphically highest (and thus youngest) Albian-aged coal seams studied in the region. The palynoflora is dominated by spore-producing pteridophytes (mainly ferns) alongside a community of mainly pinacean and cupressacean gymnosperms. Notably, no angiosperms were recorded. The lower part of the seam contains diverse pteridophyte assemblages reflecting an environment subject to periodic flooding, as evidenced by ash yield values. The middle section marks a shift from forested to a fern-dominated vegetation, forming a transitional to possibly domed mire under relatively stable tectonic and sedimentological conditions that favoured thick peat accumulation. In the uppermost part, floral diversity increases compared to the lower sections. The presence of lycophytes and bryophytes in the upper parts of the coal seam suggest wetter conditions prior to drowning of the palaeomire, which terminated peat deposition. These environmental shifts are further supported by both the ash yields as well as highly fluctuating stable carbon isotopic values in the lower part of the seam, with consistently lighter values occurring during wetter conditions. Although there are clearly shorter-term climatic conditions occurring over the span of a single palaeomire, data across a number of other studies in the Yimin Sag, and broadly in the Hailar Basin (and elsewhere), indicate that the climate has remained warm and predominantly humid for the previous 40 Ma.