Astronomically forced changes in chemical weathering and redox during the Anisian (Middle Triassic): Implications for marine ecosystem recovery following the end-Permian mass extinction

Abstract

The timing and pattern of ecosystem recovery following the end-Permian mass extinction are still highly controversial, and a potential influence from orbital climate forcing is often not considered. To investigate this issue, a cyclostratigraphic analysis of an Anisian (Middle Triassic) marine platform succession (Guanling Formation, Yongning section, South China) using elemental proxies and lithofacies has been carried out. Spectral analysis of proxies for terrestrial flux (Fe/Al and Zr/Al) indicate significant cycles. Based on available age constraints and sedimentation rate modelling, the recognized cycles include 405-kyr long eccentricity, 100-kyr short eccentricity and ~ 20-kyr precession cycles. Anchoring these signals to a dated and widespread “green bean” volcanic ash layer (246.6 Ma) and tuning to stable 405-kyr long-eccentricity cycles yields an astronomical time scale spanning ~4.2 + 0.3/−1.4 Myr (246.7 to 242.5 + 0.3/−1.4 Ma) in the Anisian. This time scale constrains the ages of the Luoping biota (which marks the earliest complex Mesozoic marine ecosystem in South China) and the Panxian marine vertebrate biota to 243.8 + 0.3/−1.4 and 243.4 + 0.3/−1.4 Ma, respectively. The early Anisian is characterized by intercalated claystone and marls with high values in terrestrial flux proxies, indicating an intensified hydrological cycle due to a humid climate in the study area. This was followed by a long-term cooling trend into the late Anisian, as indicated by declining terrestrial flux proxy values. Seawater redox conditions were assessed using V/Al, Mo/Al and lithofacies analyses, and these data reveal possibly two putative deoxygenation events during the Anisian. Our new high-resolution redox data can also be correlated with results on Anisian ocean redox changes elsewhere. Our new astronomical timescale and high-resolution geochemical data provide key constraints on the timing of marine ecosystem recovery after the end-Permian mass extinction. Orbital forcing seems to have played an important role in modulating chemical weathering and seawater chemistry dynamics that impacted the timing of ecosystem recovery.