Present and pAst weathering fLuxes from tropicAl Volcanic islAndS – PALAVAS

Over geological time scales chemical weathering of silicate rocks is the largest sink of global atmospheric CO2. Weathering of volcanic islands has been suggested to constitute a substantial fraction of this drawdown as volcanic rocks are characterized by high concentrations of Ca and Mg that locks up CO2 in ocean sediment. 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 relies on dissolved load measurements that does capture the highly dynamic tropical island erosion and weathering in soils, nor its temporal evolution.
In this proposal, we aim at testing the hypothesis that, in absence of landscape rejuvenation by tectonic uplift commonly encountered 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. As such, tropical volcanic weathering and erosion may be closely related: Once edifice building is completed, high erosion rates (E) cannot be sustained, and pulses of W and E each time a volcano is built are followed by waning of edifice relief, E, and W. We further hypothesize that the timescale and shape of the decline is characteristic for a specific rock type (intermediate vs. mafic), and is modified by prevailing precipitation and temperature regimes.
To test this hypothesis, we employ elemental weathering budgets and novel Be and Li isotope tracers of weathering rate and intensity, complemented by biomarker analysis and numerical modeling to deconvolve climate controls on W. We apply these in a time-resolved sampling scheme on the intensely studied tropical islands of Guadeloupe and Réunion (intermediate vs. mafic). Terrestrial lake archives (< 10 ka) will permit to reconstruct the immediate responses of W and E to short-term climate changes. Marine archives (<800 ka) offshore Réunion capture the full history of mafic weathering. We will assess the sensitivity of our proxies to mafic vs. intermediate rock types by analyzing modern weathering products (soils/ river sediments) and implement our findings into a numerical land surface evolution model to simulate CO2 consumption over multi-million year timescales.
PALAVAS will track weathering and erosion rates through time based on analyses of soils, rivers, and sedimentary archives, and thus will deliver a completely updated view of how weathering and erosion evolve on tropical islands, and how this evolution impacts CO2 consumption.