Milankovitch and sub-Milankovitch cycles of the early Triassic Daye Formation, South China and their geochronological and paleoclimatic implications

Abstract

The mass extinction at the end of Permian was followed by a prolonged recovery process with multiple phases of devastation–restoration of marine ecosystems in Early Triassic. The time framework for the Early Triassic geological, biological and geochemical events is traditionally established by conodont biostratigraphy, but the absolute duration of conodont biozones are not well constrained. In this study, a rock magnetic cyclostratigraphy, based on high-resolution analysis (2440 samples) of magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) intensity variations, was developed for the 55.1-m-thick, Early Triassic Lower Daye Formation at the Daxiakou section, Hubei province in South China. The Lower Daye Formation shows exceptionally well-preserved lithological cycles with alternating thinly-bedded mudstone, marls and limestone, which are closely tracked by the MS and ARM variations. Power spectral, wavelet and amplitude modulation (AM) analyses of the ARM and MS series reveal strong evidence for the presence of Milankovitch to sub-Milankovitch frequencies dominated by precession index signal and 4–5ka cycles. Cycles expressed by variations in MS and ARM were likely controlled by changes in the input of fine-grained detrital magnetite, which in turn may have been driven by astronomically induced changes in monsoon intensity in the equatorial eastern Paleotethys during the Early Triassic greenhouse period.On the basis of the 100-ka tuning results, the astronomically constrained duration of the Induan stage is 1.16Ma, with the Griesbachian and Dienerian substages of 490ka and 670ka, respectively. The new astronomical time scale also provides time constraints for the conodont and bivalve biozones and the carbonate carbon isotope (δ13C) records of the Lower Triassic Daye Formation. Time constraints for the conodont biozones include 34ka for Hindeodus parvus, 24ka for Isarcicella staschei–I. isarcica, 366ka for Neogondolella planata–Ng. carinata, 66ka for Neogondolella discreta, 255ka for Neospathodus kummeli and 415ka for Neospathodus dieneri. The duration for the negative δ13C shift from high δ13C values near the Indun/Olenekian boundary to the lowest value in the Early Smithian is estimated as 430ka. Global comparison indicates that Milankovitch and 4–5ka sub-Milankovitch forcing depositional rhythms may have been common in tropical and sub-tropical carbonate platforms during the Early and Middle Triassic time. The ultimate control on the 4–5ka cycles may have been millennial-scale fluctuations in solar insolation.