The drivers of the Middle Eocene Climatic Optimum (MECO) remain enigmatic. Here we report a high-resolution terrestrial MECO record from the Bohai Bay Basin, eastern China. The record shows episodic Hg enrichments and a volcanogenic Δ199Hg signature during the MECO, and an abrupt chemical weathering enhancement and negative δ13Corg excursion (CIE) during the MECO peak warmth. A high-resolution age model constrains the MECO to ca. 40.59–40.18 Ma with the onset in the first ~320 k.y. Peak warmth and the CIE initiated at 40.27 Ma, corresponding to 405 k.y. and 100 k.y. eccentricity maxima, and lasted for ~90 k.y., reminiscent of the early Eocene hyperthermals. Our findings suggest that episodic volcanism contributed to gradual atmospheric pCO2 rise, leading to the long duration of the MECO onset. Orbital forcing at 40.27 Ma, on top of CO2 forcing, caused the peak warmth and a positive carbon cycle feedback. The regional increase in weathering is inconsistent with evidence for weakened global continental weathering, suggesting strong spatial heterogeneity in the weathering response to MECO warming.

Orbital-scale hydroclimatic variability in terrestrial environments during the middle–late Eocene warmhouse is important to the understanding of climate dynamics of rhythmic cycles. Here, we report two continuous cyclostratigraphic records derived from shallow to deep lake deposits in East Asia to unravel orbital-scale hydroclimatic fluctuations between 43.22 Ma and 37.48 Ma. Our results show that 40 k.y. obliquity cycles paced hydroclimate dominantly during 43.22–41.57 Ma and 40.20–37.48 Ma, with periods of wetter and warmer climate in obliquity maxima. Obliquity may have forced hydroclimatic fluctuations by modulating the intensity of the East Asian Summer Monsoon, possibly through intricate feedback mechanisms involving high-latitude ice sheets within the climate system. Between 41.57 Ma and 40.20 Ma, 405 k.y. and ~100 k.y. eccentricity cycles became prominent. We observed transient warming events at the conjunctions of long and short eccentricity maxima. These findings suggest that the switch in the dominant orbital parameter may have disturbed the global carbon cycle, which subsequently led to continental hydroclimatic change. Our study highlights the role of obliquity and eccentricity cycles in influencing climate dynamics and underscores the importance of considering changes in dominant cycles when investigating continental hydroclimatic change.

Investigating how climatic and hydrological conditions in ecological resource-enriched zones of marginal seas respond to external forcing, particularly during past greenhouse climates, holds considerable significance for understanding current environmental and resource challenges driven by global warming. In marginal seas, climatic hydrological states, including salinity, redox conditions, and productivity, are key environmental parameters controlling organic matter production, preservation, and ultimately the formation of high-quality shale. Herein, high-resolution cyclostratigraphic and multi-proxy geochemical analyses were conducted on a continuous core from the upper part of Member 4 of the Eocene Shahejie Formation (Es4cu) in Well NY1, Dongying Sag, Bohai Bay Basin. Based on these data, a refined astronomical timescale was accordingly established for the studied interval. By integrating sedimentological observations with multiple proxy indicators, including elemental geochemistry (e.g., Sr/Ba and Ca/Al ratios), organic geochemistry, and mineralogical data, the evolution of climate and paleo-water mass conditions during the study period was reconstructed. Spectral analyses revealed prominent astronomical periodicities in paleosalinity, productivity, and redox proxies, indicating that sedimentation was modulated by cyclic changes in eccentricity, obliquity, and precession. It was hereby proposed that orbital forcing governed periodic shifts in basin hydrology by regulating the intensity and seasonality of the East Asian monsoon. Intervals of enhanced summer monsoon associated with high eccentricity and obliquity were typically accompanied by increased sediment supply and intensified chemical weathering. Increased precipitation and runoff raised the lake level while promoting stronger connectivity with the ocean. In contrast, during weak seasonal monsoon intervals linked to eccentricity minima, basin conditions shifted from humid to arid, characterized by reduced precipitation, lower lake level, decreased sediment supply, and a concomitant decline in proxies for water salinity. The present results demonstrated orbital forcing as a primary external driver of cyclical changes in conditions favorable for resource formation in the Eocene lacustrine strata of the Bohai Bay Basin. Overall, this study yields critical paleoclimate evidence and a mechanistic framework for predicting the spatial-temporal distribution of high-quality shale under comparable astronomical-climate boundary conditions.

Details about proxies and time series analysis.

Details about proxies and time series analysis.