Fingerprints of El Niño-Southern Oscillation and solar activity cycles recorded in the middle Eocene Bohai Bay Basin (East China)

Abstract 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). However, due to limited stratigraphic resolution and the accuracy of dating methods, pre-Quaternary archives are rare. A middle Eocene lacustrine shale in the Bohai Bay Basin of East China contains annual laminae which provides a site to study the astronomical and varve chronology of the basin. 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 cold/wet climate on the scale of centuries (~200–240 years and ~350 years, respectively), probably corresponding with cycles in solar activity. In situ δ13C and δ18O values of the light carbonate laminae indicate, in combination with varve-thickness data, that algal blooming and carbonate production occurred at ~2.1–8.7-year cycles, which could be ascribed to ENSO activity. Our finding of the ENSO variability during this notably warm interval indicates that evident interannual variability will likely continue to exist in our future greenhouse planet.