Solar forcing on elemental and nanomechanical variations in Late Cretaceous lacustrine deposits.

Decennial to annual-scale paleoclimate proxy variations are often attributed to solar forcing and/or unforced internal climate oscillations, but recognition of such variations in nanomechanical properties is extremely rare. Here we present a well-preserved, millimeter-scale laminated shale-carbonate couplet from Late Cretaceous petroliferous Qingshankou Formation in the Songliao Basin, NE China (ca. 90.53 Ma). Cyclicity analyses of elemental and nanomechanical data reveal periods of 47.7 ± 5.2–20.3 ± 2.2 year and 15.1 ± 1.6–7.4 ± 0.8 year attributed to Hale and Schwabe solar cycles, respectively; a 5.3 ± 0.6–2.5 ± 0.3 year period probably reflecting ENSO-like climatic fluctuations. Solar activities govern climatic changes that reshape aquatic environment, which further controls the laminated sedimentary composition and mechanical properties. The mechanically weak interfaces between solar-forced biogenic soft carbonates and terrigenous hard detritus are susceptible to crack propagation, affecting horizontal hydraulic fracturing in shale oil exploitation. This study introduces an integrated science-engineering research paradigm in shale oil field.