PALAVAS: Present and pAst weathering fLuxes from tropicAl Volcanic islAndS

在地质时间尺度上，硅酸盐岩石的化学风化作用是大气CO2的最大汇。火山岛的风化已被认为构成了全球大气CO2下降的岩石风化的一个重要部分，因为这种岩石的特点是快速溶解动力学。热带火山岛特别重要，因为它们地势高，在温暖的气候下有持续的地形降水，以及气旋引发的严重物理侵蚀。然而，以前的工作表明，玄武岩风化率可能会强烈下降，随着时间的推移后，新的熔岩侵位。然而，这种以前的工作是基于间接证据，因为它采用了活跃与不活跃火山区的映射，或依赖于溶解测量，可能无法充分捕捉热带岛屿侵蚀和风化的动态，气旋驱动的性质。在这个建议中，我们的目的是测试的假设，在没有景观复兴的构造隆起发现碰撞造山带，风化率（W）的热带火山岛最初非常高的火山场后，侵位，但随后急剧下降，直到达到“到期日”，之后，高初始W不能再持续。在这样一个框架中，侵蚀速率（E）可能与W密切相关：一旦建筑物建造完成，每次火山建造时的高W和E速率都伴随着建筑物浮雕，侵蚀和风化的减弱。我们进一步假设，下降的时间尺度和形状是一个特定的岩石类型（安山岩与玄武岩/中间与镁铁质）的特征，并修改了流行的降水和温度制度。为了验证这一假设，我们采用经典的元素地球化学和新的Be和Li同位素示踪剂的侵蚀，风化率和强度，辅以生物标志物分析和数值模拟去卷积气候控制W，深入研究的热带岛屿瓜德罗普和留尼汪岛（中间与镁铁质）。在这些岛屿上，陆地湖泊档案（<10 ka）将允许记录W和E对短期气候变化的即时响应。海洋档案（<340 ka），直接由河流排水熔岩单位与不同的年龄提供了可能性，以捕捉一个给定的火山领域的镁铁质风化的全部历史。我们将通过分析不同年龄的熔岩单元上流行的现代风化产物（土壤/河流沉积物）来评估我们的代理对镁铁质与中间岩石类型的敏感性，并将我们的研究结果应用到一个数值地表演化模型中，以模拟多个百万年时间尺度上的CO2消耗。PALAVAS将根据对土壤、河流和碎屑沉积档案的分析，跟踪风化和侵蚀速率，从而提供关于热带岛屿风化和侵蚀如何演变以及这种演变如何影响全球二氧化碳消耗的最新观点。

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.