Paleoenvironmental evolution during the early Eocene climate optimum in a mid–high-latitude lake–marsh system, NE Asia

The Early Eocene Climatic Optimum (EECO) represents a pinnacle of long-term global warming and is considered an analog for potential ecological impacts in a future high-CO₂ world (atmospheric pCO₂ > 500 ppmv). Recent work from mid- to high-latitude lake–marsh systems in the Northern Hemisphere suggests that the EECO was characterized by a decoupled response in terrestrial and marine deposition, indicating that previous ecological models obtained from Ocean Drilling Program sites may not adequately explain the dynamics of the land–ocean ecological balance during the EECO. Here, new insights involving magnetic mineralogy, high-resolution dynamic sedimentary noise analysis, stable isotope examination, X-ray diffraction, and elemental investigations from the Dalianhe section in NE Asia, which record minimal organic δ¹³C values, sulfate reduction index values, limited C/N ratios, and relatively high dynamic noise orbital tuning lake levels of the EECO in lake–marsh systems at 53.1 ± 9.0°N. Moreover, lake–marsh systems at mid-high latitudes exhibit significantly greater temperature and precipitation shifts than do midlatitude marine systems, with the onset of a warming event (∼52.2 Ma) occurring after marine deposition. These analyses revealed that oceanic ventilation/mixing and East Asian monsoon responses were key drivers of the land–ocean ecological balance during the EECO. These global ecological dynamics demonstrate the sensitivity of terrestrial ecosystems and the lag of warming in response to extreme thermal events, which further constrains potential causal mechanisms for the EECO to multiple systems and highlights the importance of biogeochemical models for understanding global warming.