Recognition and response of ENSO signals in the middle Eocene Bohai Bay Basin lake (E China)

The behavior of the global climate system on scales from years to centuries is related to several mechanisms, including solar forcing and the El Niño-Southern Oscillation (ENSO). Pre-Quaternary archives are rare, however, due to limited stratigraphic resolution and the accuracy of datings. A middle Eocene lacustrine shale in the Bohai Bay Basin (E China) shows annual laminae that allow establishing its astronomical and varve chronology. Principal component analysis of the sediments in the cored material, their magnetic susceptibility and grayscale scans as well as analysis of the varve thickness in thin sections jointly reveal variations between a warm/dry and a cold/wet climate on the scale of centuries (~200–240 years, ~350 years), probably corresponding with cycles in solar activity. In situ δ13C and δ18O values of the light carbonate laminae show, in combination with varve-thickness data, that algal blooming and carbonate production show ~2.1–8.7 year cycles that could be ascribed to ENSO activity. The data also suggest that solar forcing controlled the ENSO intensity in the area of the Bohai Bay Basin during the middle Eocene.