Astronomical and volcanic modulating organic-rich sediments during Later Ordovician – Early Silurian in South China

Abstract:

The astronomical orbital forcing of climate fluctuations during the Late Ordovician – Early Silurian time span is recorded in organic rich sediments of the marine Wufeng and Longmaxi formations of the Xichang Basin, within a new, key section of Jienietuo, Sichuan Province, China. We report geochemical evidence for reconstructing climate fluctuations and analyzing organic-rich sediments (ORS) accumulation recorded from Upper Kaitain through Lower Rhuddanian, spanning a time interval which witnessed the end Ordovician extinction. We carried out a Gamma Ray (GR, 10 cm intervals) series and we established a high-resolution astronomical time scale (ATS) integrated within the frame of the Geologic Time Scale (GTS). Chemical index of alteration (CIA) and Organic Carbon isotope (δ¹³C_org) data sets indicate a transitional climate between greenhouse and icehouse stages through the Upper Kaitain until the Lower Rhuddanian. Astronomical modulation (from CIA_corr and δ¹³C_org series) of 1.2-myr obliquity and 405-kyr long-eccentricity have great impact on climate changes at different time scales. Meanwhile, Orbital forcing climate changes drives third-order and fourth-order eustatic sequences. Astronomical cycles and volcanic activities play significant roles in ORS accumulation. 1.2 myr Obliquity-induced thermohaline circulation (THC) is the main dynamic mechanism of ORS supply in Yangtze area. THC- induced bottom current transports nutrient-laden water from high latitude regions to the surface water of low-latitude Upper Yangtze area, promoting the marine paleoproductivity. Intense and frequent volcanic activities have promotion effects on paleoproductivity and preservation of ORS. A large amount of volcanic ash produced by intense volcanic activity provide rich nutrients to surface water, causing high productivity in the deep marine environment. The ORS enrichment in shale is also a result of enhanced preservation under extremely anoxic environment caused by volcanism.