Evolving Eocene–Miocene seasonality decoded from aragonitic laminae deposited within the Qaidam Basin, North Tibetan Plateau

Abstract

A thorough understanding of sub-decadal paleoclimate variability typically depends on ultra-precise geochronological constraints that are rarely obtained. While the seasonally resolved nature of varved successions enlarges the opportunity, their identification beyond Quaternary ages is difficult; thus is frequently met with skepticism. The Cenozoic strata in the Qaidam Basin, for instance, feature expanded rhythmite successions, whereas the periodicity of these laminae remains to be explored. Targeting Eocene- (VE; ∼42.0 Ma), Oligocene- (VO; ∼26.9 Ma) and Miocene-aged (VM; ∼19.5 Ma) intervals, herein we report layer-based petrographic observations coupled with mineralogical and phase-specific geochemical analyses of millimeter-scale rhythmic alterations between yellowish- and grayish-colored aragonitic layers. Noting a constant δ13Ccarb offset (∼ 2.8‰) between yellowish- and grayish-colored laminae, these isotope data imply seasonally paced alternations between microbial induction and inorganic carbonate precipitation. In addition, covariations between inorganic oxygen isotope (δ18Ocarb) and elemental ratios (e.g., Mn/Fecarb, Sr/Basilic and Chemical Index of Alteration, CIAsilic) reveal evolving seasonality, capturing Eocene–Oligocene aridification followed by an early Miocene reversal. Therefore, the varved fill of the Qaidam Basin serves as an annually resolved, and as yet under-explored, archive of evolving Cenozoic paleoseasonality in the aftermath of the Indo–Asia collision.