Astronomical cycles recorded in the sedimentary rhythms of deep-sea bedded chert and its significance on biogeochemical cycle

Abstract

Quasi-periodic changes in Earth's orbital parameters (i.e., Milankovitch cycles) are widely recorded in rhythmic sediments. Astrochronology, which uses such sedimentary cycles, has been used to improve the geologic timescale and advance our understanding of Earth system dynamics, principally during the Cenozoic Era. Paleozoic to Mesozoic radiolarian bedded chert deposits consist of rhythmic alternations of chert and shale that are potentially related to Milankovitch-scale changes in the flux of biogenic silica . Here we review recent progress in astrochronology and its applications to Permian to Cretaceous bedded chert deposits. The sedimentary rhythms of bedded chert display a full range of climatic precession- and eccentricity-related cycles: a 20 kyr cycle is preserved as a chert-shale couplet and ~100, 405, 2000-4000, and 10,000 kyr cycles are recorded as variations in the thicknesses of individual beds. Using an anchor at the end-Triassic extinction level at 201.5 ± 0.2 Ma, an astronomical timescale for the Triassic-Jurassic Inuyama bedded chert (Inuyama-ATS) is established. Estimates of the burial flux of biogenic silica show fluctuations of 20%-50% over 100 kyr to 30 Myr cycles, suggesting that orbital-scale chemical weathering on Pangea would have controlled the sedimentary rhythms of bedded chert through the biogeochemical Si cycle.