Quantifying carbonate weathering with novel tracers

碳循环、地球气候和地质在一定的时间尺度上紧密相连。在百万年的时间尺度上，大气中的二氧化碳，一种关键的温室气体，保持了地球表面的温度稳定。关键的负面气候反馈阻止了像金星上那样失控的温室。这种长期的地质反馈通常被认为是由有机碳埋藏和硅酸盐化学风化的温度敏感性引起的，但这仍然存在争议。关键的是，通常假设硅酸盐风化作用仅在百万年时间尺度上的气候反馈中起作用，但观察到某些河流系统中的次生碳酸盐沉淀去除了70%最初溶解的Ca，这表明部分硅酸盐风化循环可能在更短的时间尺度上起作用。该项目将使用一套新型同位素示踪剂来量化与碳酸盐风化有关的过程。在一些河流系统中，次生碳酸盐沉淀去除了原来溶液中70%的钙，这一观察结果表明，硅酸盐风化循环的传统观点可能会加速到十年或百年的时间尺度。对次生碳酸盐沉积过程的量化和认识是模拟硅酸盐风化气候反馈的关键。近年来，已经有可能测量一系列稳定的同位素系统，如Ca， Mg和Sr，它们都对碳酸盐降水敏感，但尚未应用于碳酸盐风化系统。在这个项目中，这些将被开发为二次碳酸盐岩形成的敏感示踪剂，在法国南部的一个小集水区，在那里可以很容易地隔离风化过程中的关键过程。该学生将在法国南部进行实地考察，从一个标志性的地点收集一系列样品，那里有丰富的次生碳酸盐沉淀。在剑桥的实验室里，他将使用最先进的质谱法测量河流、泉水和次生碳酸盐中稳定的Ca、Mg和Sr同位素。碳酸盐沉淀的同位素分馏因子和元素分布系数将被确定。Ca、Mg和Sr对碳酸盐沉淀具有不同的元素和同位素敏感性。对碳循环的影响将通过开发碳酸盐风化反应输运模型来评估化学浓度和同位素组成。

The carbon cycle, Earth's climate and geology are intimately coupled on a range of time-scales. On million year time-scales, atmospheric carbon dioxide, a key greenhouse gas, has maintained the surface of the Earth at equable temperatures. Key negative climatic feedbacks have prevented a runaway greenhouse as on Venus. This long-term geological feedback is generally accepted to result from the temperature-sensitivity of organic carbon burial and silicate chemical weathering but this remains controversial. Critically it is generally assumed that silicate weathering only plays a role in the climate feedback on million year time-scales, but the observation that secondary carbonate precipitating in some river systems removes 70% of the Ca that was originally dissolved suggests that part of the silicate weathering cycle may operate with a much shorter timescale. This project will use a suite of novel isotopic tracers to quantify processes associated with carbonate weathering.The observation that secondary carbonate precipitating in some river systems removes 70% of the calcium that was originally in solution suggests that the traditional view of the silicate weathering cycle may be accelerated to decadal or centennial time-scales. Quantification and understanding of the process of secondary carbonate deposition is pivotal to modelling the silicate weathering climate feedback. In recent years it has become possible to measure an array of stable isotope systems such as Ca, Mg, and Sr, all of which are sensitive to carbonate precipitation but have yet to be applied to carbonate weathering systems. In this project, these will be developed as sensitive tracers of secondary carbonate formation, in an small catchment in southern France where it is possible to easily isolate the key processes acting during weathering.The student will conduct fieldwork in southern France collecting an array of samples from an iconic location where abundant secondary carbonate is precipitating. In the labs in Cambridge (s)he will measure stable Ca, Mg, and Sr isotopes using state-of-the-art mass spectrometry in river and spring waters and secondary carbonates. Isotope fractionation factors and elemental distribution coefficients for carbonate precipitation will be determined. Ca, Mg and Sr have contrasting elemental and isotopic sensitivities to carbonate precipitation. The impact on the carbon cycle will be assessed through the development of reactive transport models for carbonate weathering for chemical concentrations and isotopic compositions.