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Oxidative weathering of organic carbon in sedimentary rocks is a major source of CO2 to the atmosphere over geological timescales, but the size of this emission pathway in Earth's past has not been directly quantified due to a lack of available proxy approaches. We have measured the rhenium isotope composition of organic‐rich rocks sampled from unweathered drill cores and weathered outcrops in south Texas, whose stratigraphic successions can be tightly correlated. Oxidative weathering of more than 90% of the organic carbon and ∼85% of the rhenium is accompanied by a shift to lower rhenium isotope compositions in the weathered outcrops. The calculated isotope composition of rhenium weathered from the initial bedrock for individual samples varies systematically by ∼0.7‰ with different fractions of rhenium loss. This variation can be empirically modeled with isotope fractionation factors of α = 1.0002–1.0008. Our results indicate that the isotope composition of rhenium delivered to the oceans can be altered by weathering intensity of rock organic matter and that the rhenium isotope composition of seawater is sensitive to past oxidative weathering and associated CO2 emissions. Plain Language Summary: Carbon dioxide is emitted into the atmosphere when sedimentary rocks containing ancient organic carbon are exposed to oxygen. This process, referred to as oxidative weathering, is an important part of the global carbon cycle that may play a vital role in climate change over geological timescales. In this study, we have investigated whether it is possible to use the isotope composition of rhenium metal as a proxy for this oxidation process since rhenium is liberated from rocks alongside organic carbon. Our findings show that the rhenium removed from rocks during weathering has an isotopic fingerprint that is linked to the degree of weathering—and thus presumably to the extent of carbon oxidation. There are several important features of rhenium isotope behavior that are still underconstrained, not least where Re resides in organic rich rocks, but our findings nonetheless advance an emerging approach to reconstructing the ancient carbon cycle. Key Points: Oxidative weathering removes isotopically heavy rhenium from organic‐rich sedimentary rocksThe isotopic composition of weathered rhenium changes with weathering intensityRhenium isotopes may trace variations in oxidative weathering intensity, and hence, oxidative CO2 emission fluxes in the Earth's ancient past [ABSTRACT FROM AUTHOR]

On greater than million year timescales, carbon in the ocean-atmosphere-biosphere system is controlled by geologic inputs of CO 2 through volcanic and metamorphic degassing. High atmospheric CO 2 and warm climates in the Cretaceous have been attributed to enhanced volcanic emissions of CO 2 through more rapid spreading at mid-ocean ridges and, in particular, to a global flare-up in continental arc volcanism. Here, we show that global flare-ups in continental arc magmatism also enhance the global flux of nutrients into the ocean through production of windblown ash. We show that up to 75% of Si, Fe and P is leached from windblown ash during and shortly after deposition, with soluble Si, Fe and P inputs from ash alone in the Cretaceous being higher than the combined input of dust and rivers today. Ash-derived nutrient inputs may have increased the efficiency of biological productivity and organic carbon preservation in the Cretaceous, possibly explaining why the carbon isotopic signature of Cretaceous seawater was high. Variations in volcanic activity, particularly continental arcs, have the potential of profoundly altering carbon cycling at the Earth's surface by increasing inputs of CO 2 and ash-borne nutrients, which together enhance biological productivity and burial of organic carbon, generating an abundance of hydrocarbon source rocks.