Intensified hydrological cycle during the Early Eocene Climatic Optimum (EECO) recorded in the Xining Basin, NE Tibet

The evolution of Asian climate during the Cenozoic is traditionally linked to shifts in paleogeography such as the proto-Paratethys Sea incursions and uplift of the Tibetan Plateau driving monsoonal circulation and affecting the mid-latitude westerlies in Central Asia. In contrast, the role of global climate in the Asian hydrological cycle remains unclear. Here, we present a new stratigraphic record from the terrestrial Xining Basin in central China, which covers the Early Eocene Climatic Optimum (EECO), a period characterized by long-term global warmth and elevated atmospheric CO₂ levels. The record is dated using magnetostratigraphy and extends the previously studied Paleogene strata down to 50.9 Ma (chron C23n). We use a variety of paleoclimate proxies, to derive the hydroclimatic evolution of the basin at this time. The lithostratigraphy is characterized by organic-rich mudrocks and gypsum beds (reaching TOC contents of up to 1.7%) interpreted as an alluvial mudflat to saline lake. The higher organic content of the strata indicates either increased organic productivity or preservation, both of which suggest a wetter depositional environment during the EECO. This is corroborated by palynological records showing a large increase in the abundance and diversity of trilete spores, indicating a wetter biome at this time. In addition, the d¹³C values of the bulk organic matter and leaf waxes (both C₂₉ and C₃₁), suggest a reduction in water stress on plants and a wetter environment as well. These observations are in stark contrast to the arid red beds, evaporites and xerophytic pollen observed in the underlying Cretaceous-Paleocene strata and overlying middle-late Eocene deposits. The peak global warmth of the EECO is thus clearly linked to an intensified Asian hydrological cycle suggesting a major driving role for global climate.