The Eocene epoch provides a close geological analogy for near-future scenarios of greenhouse gas emissions. While geochemical reconstructions and model simulations have illuminated lower thermal latitudinal gradients and seasonal variations during this period, it is debated how the Eocene precipitation regime changed. Lithological data suggest the existence of a broad arid zone centered around 30°N paleo-latitude, while a humid climate was supported by palaeobotanic assemblages in East Asia. Here, we report the occurrence of massive primary lacustrine dolomite and magnesite in central East Asia during the middle Eocene. We provide a novel perspective from magnesium isotopes to link the formation of Mg-carbonates to seasonal dry-wet cycles. Rapid magnesium input during the rainy season and intense evaporation in the dry season likely caused the formation of Mg-carbonates in an enclosed lake. Seasonal dry-wet cycles contributed to the annual thickening of Mg-carbonates, resulting in the apparent homogeneity of Mg isotope compositions rather than displaying Rayleigh fractionation behavior over time. The prevalence of seasonal climate variation in central East Asia during the middle Eocene is further substantiated by a compilation of Eocene lacustrine Mg-carbonates in this region. These findings provide insights into hydroclimatic seasonality during the Eocene, contributing to our understanding of the hydrological cycle response to a greenhouse climate.

Eocene;Magnesium carbonate;Magnesium isotope;Hydroclimate;Seasonality