Plants as a proxy for monsoon reconstruction – a case study from the Eocene of East Asia

Eocene precipitation patterns of eastern Eurasia are studied in focus on potential imprints of monsoonal circulation. Based on the application of the Coexistence Approach (CA) on 239 palaeofloras, three precipitation variables are reconstructed and six derived monsoon indices are tested for the early, middle and late Eocene. Under the Eocene greenhouse and diverging palaeogeography, atmospheric circulation and precipitation patterns fundamentally differed from modern. Our reconstruction reveals mainly zonal patterns and significantly higher-than-present precipitation at high and mid-latitudes, with means being highest in the early Eocene. The monsoon indices calculated from CA data yield ambiguous evidences for monsoon circulation during the Eocene. For East Asia, most data suggest a weak or absent summer monsoon, with the medians of the Eocene index values being at the lower ends or below typical modern values for monsoonal conditions. Moreover, they consistently indicate the absence of a winter monsoon, probably related to a weak or absent Siberian High throughout the Eocene. For sites located on the Indian Plate, the indices considered here yield contradictory results. The monsoon indices have their strengths and flaws and hence complemental application of various different indices is proven useful. • Eocene precipitation patterns are fundamentally different from the modern ones. • Eocene precipitations were significantly higher at high and mid-latitudes. • Our data suggest a weak or absent summer monsoon. • Our data indicate the absence of winter monsoon and weak or absent Siberian High. • Hadley and Ferrell cells were weaker than the present, polar cell – absent or very weak.