Magnesium Isotopes Reveal Hydroclimatically Amplified Low-Latitude Weathering During the Early Toarcian

The Early Jurassic Toarcian Stage experienced extreme global warming driven by substantial carbon emissions, triggering intensification of the hydrological cycle, enhanced continental weathering, changes in ocean chemistry, and a biotic crisis. Low-latitude silicate weathering, a key climate regulator, is highly sensitive to climatic shifts, yet its response to Toarcian warming remains poorly understood. This study investigates magnesium isotopes (δ26Mg) from carbonate-hosted silicates in two geographically distinct, low-latitude carbonate platforms to explore the relationship between the hydrological cycle and tropical weathering during the Toarcian. In both records, positive δ26Mg excursions, indicative of intensified continental weathering, are observed shortly after the Pliensbachian–Toarcian boundary, coinciding with magmatic activity in the Karoo Large Igneous Province (LIP). δ26Mg values reach maxima during the onset interval of the T-OAE (Toarcian Oceanic Anoxic Event) negative carbon-isotope excursion, coincident with Ferrar LIP emplacement. The pattern of δ26Mg changes suggests that large-scale carbon emissions (which began close to the Pliensbachian–Toarcian boundary) did not immediately induce an extreme increase in weathering. Rather, cumulative carbon emissions were required to surpass a critical tipping point, thereby triggering the largest-scale impacts at the T-OAE. Our data, coupled with climate simulations using Community Earth System Model, indicate a climate-dependent “production-limited” weathering regime in tectonically inactive regions, where increased rainfall and runoff enhanced denudation and chemical weathering. Our work highlights the essential role of low-latitude continental weathering and carbon sequestration in mediating Earth’s climate during past rapid warming, offering insights into past hyperthermals and serving as analogs for anthropogenic climate change.