PALAVAS: Present and pAst weathering fLuxes from tropicAl Volcanic islAndS

在地质年代尺度上，硅酸盐岩石的化学风化作用是大气CO2最大的汇。火山岛的风化作用被认为构成了岩石风化作用造成的全球大气CO2减少的很大一部分，因为这类岩石具有快速溶解动力学的特点。热带火山岛特别重要，因为它们地势高，温暖气候下持续的地形降水，以及气旋引发的高度物理侵蚀。然而，先前的研究表明，在新熔岩就位后，玄武岩的风化速率可能随着时间的推移而急剧下降。然而，这些先前的工作是基于间接证据，因为它使用了活火山与非活火山的地图，或者依赖于可能无法充分捕捉热带岛屿侵蚀和风化的动态、气旋驱动的性质的溶解测量。在本提案中，我们旨在验证这样一个假设，即在没有碰撞造山带构造隆升的情况下，热带火山岛的风化速率(W)在火山场嵌入后最初非常高，但随后急剧下降，直到达到“失效日期”，之后高初始W不再能够持续。在这样的框架下，侵蚀速率(E)可能与W密切相关：一旦建筑物建成，每次火山建成时，W和E的高速率都伴随着建筑物浮雕的减弱、侵蚀和风化。我们进一步假设，下降的时间尺度和形状是特定岩石类型（安山岩vs玄武岩/中质vs基性）的特征，并受到当时的降水和温度制度的影响。为了验证这一假设，我们将经典的元素地球化学和新型的Be和Li同位素示踪剂应用于侵蚀、风化速率和强度，辅以生物标志物分析和数值模拟，对热带瓜德罗普岛和rmacic岛进行了深入研究，对W的气候控制进行了反褶解。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）直接由河流注入，这些河流排出的熔岩单元具有不同的年龄，为捕捉给定火山场的完整的基性风化历史提供了可能。我们将通过分析不同时代的熔岩单元上普遍存在的现代风化产物（土壤/河流沉积物）来评估我们的代理对基性与中间岩石类型的敏感性，并将我们的发现应用到陆地表面数值演化模型中，以模拟多个myr时间尺度上的二氧化碳消耗。PALAVAS将根据对土壤、河流和碎屑沉积档案的分析，追踪风化和侵蚀速率，从而提供热带岛屿上风化和侵蚀如何演变的完整最新观点，以及这种演变如何影响全球二氧化碳消耗。

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.