Magnesium isotopic evidence for intensified continental chemical weathering during the Smithian-Spathian transition

Abstract

The Smithian-Spathian (S-S) transition (∼249.7 to 248.7 Ma) of the Early Triassic was marked by profound paleoenvironmental upheavals, including a pronounced temperature decline following the hyperthermal event in the late Smithian, a major positive excursion in carbonate carbon isotope composition (δ¹³C_carb), and a significant biotic crisis. Yet, the mechanisms and causal linkages underlying these coeval phenomena remain elusive. In this study, we present high-temporal-resolution magnesium isotope data from siliciclastic residues within carbonate rocks (δ²⁶Mg_residue), a well-established proxy for continental chemical weathering intensity, across the S-S boundary in the Waili and Jiarong sections in South China. Our results document a distinct positive δ²⁶Mg_residue shift in the lowermost Spathian in both sections, signifying intensified continental chemical weathering. This isotopic shift broadly coincides with the positive δ¹³C_carb excursion and the recovery of terrestrial ecosystems but postdates the late Smithian thermal maximum. We interpret this temporal sequence to suggest that the recovery of terrestrial ecosystems, particularly the expansion of vascular land plants, acted as a primary driver of enhanced continental chemical weathering. The resulting increase in continental chemical weathering likely amplified nutrient flux to the ocean, potentially promoting local marine anoxia and climate cooling observed in the earliest Spathian. These findings underscore the critical role of terrestrial-marine coupling in modulating Early Triassic biogeochemical feedbacks and global climate evolution.