单一岩性流域内不同水流路径的硅同位素分馏：对长期全球硅循环的启示

Change in silicate weathering rates and organic carbon burial rates may have driven the long term evolution of atmospheric CO2 during the Cenozoic. Silicate weathering not only consumes atmospheric CO2 but also liberates dissolved silicon (DSi), a major nutrient for phytoplankton CO2-consumers and thus may influence the organic carbon burials rates in the ocean. Indeed, continental weathering and diatom productivity may be linked as a positive feedback, i.e. enhanced silicate weathering drives an increase in the silica supply to the oceans and hence diatoms productivity illustrating the tight coupling between the Si and C cycles. Lithology, climate (temperature and water availability), vegetation and geomorphology deeply influence silicate weathering rates and release of dissolved Si. Hence quantifying chemical weathering rates and deciphering the role of these different factors are essential for understanding Earth's long-term climatic evolution. In addition, the hydrology at the catchment scale is complex and multiple water flow paths exist including surface runoff, shallow runoff and deep water circulation. Groundwater discharge might significantly contribute to the solute budget of rivers but its contribution is difficult to assess. The contribution of groundwater to river water discharge is tightly coupled to the water cycle and thus during glacial period, the contribution of groundwater could be more important and could drive the chemical composition of the continental water discharge to the ocean. Additionally, direct subsurface weathering fluxes to the ocean associated to low and temperature chemical reactions are poorly constrained. Water taking different flow paths will acquire different chemical and isotopic signatures that reflect lithology, water/rock interactions and vegetation cycling for example. Si isotopes are fractionated by these processes and thus the different water masses are expected to have different isotopic compositions. Hence, variations of the contribution of these different water bodies to the flux of dissolved Si delivered to the ocean might also impact the Si isotopic composition of the ocean and thus the isotopic composition of diatoms frustules used for paleo-oceanographic reconstructions. We propose to do a systematic of Si isotopes in mono-lithological catchments along full hydrological cycles to capture the seasonal variations of DSi and Si isotopic in rivers. By characterizing the chemical and isotopic signatures of different water masses associated to different flow paths, we hope to decipher their contributions to the continental solutes flux to the ocean and unravel the processes responsible for their Si isotopic compositions.

我们简译了这份申请，但还恳请各位专家仔细阅读附在本申请后CHETELAT先生的英文原稿。硅酸盐风化速率和有机碳埋藏速率的变化可能影响新生代以来大气CO2的演化。硅酸盐岩风化不仅消耗大气中的CO2，还会释放Si，这可能会影响海洋中有机碳的埋藏速率，因为Si是浮游植物的营养盐。因此量化各种因素对化学风化速率的影响有助于我们更好地理解全球气候的长期演化，如植被，岩性，气候，水文路径等。我们计划以整个水文循环为周期对单一岩性的流域进行系统的Si同位素研究，尤其是不同水文路径，地表径流，浅层径流以及深层地下水，通过描述不同流径水体的化学和Si同位素组成特征，从而阐述不同的流经条件，例如岩性，水岩相互作用以及植被对风化的影响。通过揭示河流中DSi和Si同位素的季节性变化规律，结合模型探讨雷琼半岛地区河流对陆地流向海洋的溶解Si通量的贡献，为硅的全球循环提供理论基础。

硅酸盐风化作用消耗二氧化碳，同时释放出溶解硅（DSi）和其他营养物质，这一过程可影响海洋中有机碳的埋藏率。由于硅酸盐风化速率和溶质释放极大地受到岩性、气候（温度和水的可用性）、植被和地貌因素等因素的影响，故量化化学风化率和解释这些因素的作用对于理解地球的长期气候演变就显得至关重要。另外，流域尺度的水文情况复杂，存在多条水流路径（包地表径流，浅径流和深水循环）。水的流动路径不同，其所呈现出的化学特征和同位素特征也不尽相同，而这些化学特征和同位素特征又可以反映岩性，水/岩石相互作用和植被循环。我们对长白山全水文循环小流域的溶质成分进行了系统研究，以观察溪流化学成分与河流化学成分的季节变化。我们分析了不同路径下形成的水团，通过研究其化学特征和C同位素特征来揭示它们对溶质通量的贡献和它们的组成过程。