PALAVAS: Present and pAst weathering fLuxes from tropicAl Volcanic islAndS

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Over geological time scales the chemical weathering of silicate rocks is the largest sink of atmospheric CO2. Weathering of volcanic islands has been suggested to constitute a substantial fraction of the global atmospheric CO2 drawdown by rock weathering as such rocks are characterized by fast dissolution kinetics. Tropical volcanic islands are of particular importance because of their high relief, sustained orographic precipitation under warm climate, and cyclone-triggered high physical erosion. Yet, previous work suggested that basalt weathering rates may strongly decline with time after emplacement of fresh lavas. Such previous work is however based on indirect evidence, because it employed mapping of active vs. inactive volcanic fields, or relied on dissolved measurements that may not adequately capture the dynamic, cyclone-driven nature of tropical island erosion and weathering. In this proposal, we aim at testing the hypothesis that, in absence of landscape rejuvenation by tectonic uplift found in collisional orogens, weathering rates (W) of tropical volcanic islands are initially very high after a volcanic field is emplaced, but then steeply decline until reaching an "expiry date", after which high initial W can no longer be sustained. In such a framework, erosion rates (E) may be closely related to W: Once edifice building is completed, high rates of W and E each time a volcano is built are followed by waning of edifice relief, erosion, and weathering. We further hypothesize that the timescale and shape of the decline is characteristic for a specific rock type (andesite vs. basalt/ intermediate vs. mafic), and is modified by prevailing precipitation and temperature regimes. To test this hypothesis, we apply classical elemental geochemistry and novel Be and Li isotope tracers of erosion, weathering rates and intensity, complemented by biomarker analysis and numerical modeling to deconvolve climatic controls on W, to the intensely studied tropical islands of Guadeloupe and Réunion (intermediate vs. mafic). On these islands, terrestrial lake archives (<10 ka) will permit to record the immediate responses of W and E to short-term climate changes. Marine archives (<340 ka) that are directly fed by rivers draining lava units with distinct ages offer the possibility to capture the full history of mafic weathering for a given volcanic field. We will assess the sensitivity of our proxies to mafic vs. intermediate rock types by analyzing modern weathering products (soils/ river sediments) that prevail on lava units of distinct ages, and implement our findings into a numerical land surface evolution model to simulate CO2 consumption over multi-Myr timescales. PALAVAS will track weathering and erosion rates through time based on analyses of soils, rivers, and detrital sedimentary archives, and thus will deliver a completely updated view of how weathering and erosion evolve on tropical islands, and how this evolution impacts global CO2 consumption.