Orbital pacing of environmental perturbations in a Greenhouse World: a 40-Myr record from Upper Devonian-Lower Carboniferous successions on the eastern margin of Gondwana.

内容摘要

Orbital pacing of environmental changes during the Late Devonian-Early Carboniferous (Famennian-Viséan, ~ 372-330 Ma) greenhouse Earth is investigated using high-resolution gamma-ray (GR) logs, chemostratigraphy (ICP-OES, ICP-MS, pXRF), and cyclostratigraphic analysis (multitaper method, wavelet transforms, Fischer plots, Dynamic Noise after Orbital Tuning sea-level modeling) of two petroleum wells from the Jurgurra Terrace, Canning Basin, Western Australia. The sedimentary successions (Nullara, May River, Laurel, Anderson formations) reveal a ubiquitous ~ 5-Myr orbital eccentricity amplitude modulation cycle, along with shorter Milankovitch cycles (405-kyr long-eccentricity, 100-kyr short-eccentricity, 40-50-kyr obliquity, 20-25-kyr precession), and sedimentation rates of 3.5-4.5 cm/kyr derived from evolutionary Time Optimization (eTimeOpt) and orbital tuning. Key findings are: (1) The Hangenberg Event (HE; ~ 3925-4030 m in Rafael 1) and Lower Alum Shale Event (LASE; ~ 3690-3770 m) coincide with 405-kyr eccentricity maxima, imposing monsoonal intensification, nutrient input, and euxinia (recorded by high V/Cr, U/Th, and organic carbon burial). (2) The Late Tournaisian Cooling Event (LTCE; ~ 3500 m) is synchronous with 100-kyr eccentricity-driven glacial-interglacial cycles, with lowered sea level, high Rb/Sr values, and siliciclastic progradation. (3) Wavelet analysis reveals a hierarchical orbital structure in which precession-paced ventilation ended anoxic intervals (e.g., post-HE re-oxygenation), whereas Fischer plots show highstand systems tracts equate to condensed, organic-rich sediment at eccentricity maxima. Geochemical proxies (Sr/Ba, Mg/Ca) also record salinity fluctuations that can be linked to orbital-scale hydrological cycling. DYNOT modeling illustrates that ~ 5-Myr amplitude modulation cycles controlled long-term climate stability, enhancing redox and sea-level extremes. These cycles, supported by global analogs (Ordovician-Silurian, Late Cenozoic), emphasize orbital forcing as the leading cause of greenhouse climate instability and its implications for determining Earth's climatic sensitivity in the absence of ice. The research connects celestial mechanics and Devonian-Carboniferous environmental disasters with improved predictive models for orbital signatures in the sedimentary record.