Climatic response to solar activity recorded in the Eocene varves from Qaidam Basin, northern Tibetan Plateau

The Earth’s climate has been proved to be consistently paced by the quasi-periodic sunspot activity since the Paleoproterozoic. However, climatic response to the sunspot cycles in high-altitude areas under greenhouse condition is still unclear, largely due to the lack of high-resolution palaeoclimatic data. Here we present a continuous, 30-cm-long core sample from the Eocene upper Xiaganchaigou Formation in the southwestern Qaidam Basin in the northern Tibetan Plateau. This core is marked by well-preserved annually chemogenic varves, composed of couplets of light micrite and dark clastic laminae. Varve thickness mainly ranges from 0.1 to 0.5 mm. Power spectrum of the bed number series of varve thickness shows a distinct cycle with a period of ∼20 year, which can be related to the 22-year Hale sunspot cycle. In addition, we use XRF (X-ray fluorescence) Ca concentration and K/Ti ratio data and μ-XRF (micro-X-ray fluorescence) Ca and K intensity data as paleoclimate proxies to conduct detailed cyclostratigraphic analyses. Power spectra of these proxies show sedimentary cycles at wavelengths of ∼40–12 mm, ∼8–3 mm, 3–1 mm and 0.2–0.1 mm, which are most likely caused by the ∼90-year Gleissberg sunspot cycle, 22-year Hale sunspot cycle, 11-year Schwabe sunspot cycle and annually seasonal cycle, respectively. The consistent results from the above five independent paleoclimate proxies indicate that climate in the high-altitude Qaidam Basin in the Eocene greenhouse period was paced by multiple-scale sunspot cycles. Periodic variances in total solar irradiance (TSI) and galactic cosmic ray (GCR)-related clouds paced by sunspot cycles may play a key role on modulating regional climatic changes in the Qaidam Basin.