Quantifying cation exchange: Re-assessing the weathering signature of continental waters

量化阳离子交换：重新评估大陆水域的风化特征

• 批准号: NE/K000705/1
• 负责人: Edward Tipper
• 金额: $ 7.73万
• 依托单位: University of St Andrews
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FK000705%2F1
• 关键词: Quantifying; cation; exchange; Re; assessing

Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid. This solute Ca and Mg gets subsequently buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction has provided the climatic feedback that has maintained Earth's climate equable and inhabitable over the entire history of the Earth. To understand how Earth's climate functions, it is therefore vital to understand silicate weathering and to estimate the flux carbon dioxide associated with modern chemical weathering. Modern day silicate weathering fluxes are estimated from the chemistry of rivers or natural waters. Natural waters contain positively charged elements or cations such as Ca, Mg, Na and K, and it has been understood for decades that the relative and absolute concentrations of these elements depend of the type of rocks that are drained. For example, Ca is mainly derived from the weathering of limestones, whereas Na and K are mainly derived from the weathering of silicate minerals such as feldspar. This distinction is important because only the Ca derived from silicate weathering is important for carbon dioxide consumption. Therefore, the Ca flux from silicate weathering is usually estimated based on Na, which has been thought to a more reliable estimate of silicate weathering. However, chemical weathering is more complex than simple mineral dissolution and a series of other chemical reactions also occur such as cation exchange. This is a process whereby the positively charged cations in solution are attracted to negatively charged mineral surfaces on clays, a process known to buffer groundwaters. One of the key chemical exchanges is Ca for Na, meaning that Na may not provide a true estimate of silicate weathering at all. Recent isotopic data suggests that cation exchange might be more significant that previously thought, which until now has been very hard to fingerprint. One method is to use naturally occurring tracers or isotopes, that allow chemical reactions to be tracked. In this work, it is proposed to examine the naturally occurring isotopes of the elements Li and Mg to examine the role of cation exchange in global budgets. However, to be able to do this successfully, a series of experimental work is proposed to examine the behaviour of the isotopes of Mg and Li during cation exchange. Once we understand how our tracers work we can use them to re-evaluate our understanding of natural waters, and better estimate fluxes of carbon dioxide associated with chemical weathering, with the ultimate aim of better understanding Earth's climate.

化学风化作用调节地球的碳循环，从而调节地质时间尺度上的全球气候。硅酸盐矿物中的钙和镁在用碳酸溶解的过程中释放到溶质相中。这种溶质钙和镁随后以钙和镁碳酸盐的形式埋藏在海洋盆地中，将碳从大气转移到碳酸盐岩储层中。这个简单的反应提供了气候反馈，使地球的气候在整个地球历史上保持平稳和适宜居住。因此，为了了解地球气候如何运作，了解硅酸盐风化并估计与现代化学风化相关的二氧化碳通量至关重要。现代硅酸盐风化通量是根据河流或天然水域的化学成分估算的。天然水中含有带正电的元素或阳离子，例如钙、镁、钠和钾，几十年来人们一直认为这些元素的相对和绝对浓度取决于排出的岩石类型。例如，Ca主要来源于石灰石的风化，而Na和K则主要来源于长石等硅酸盐矿物的风化。这种区别很重要，因为只有来自硅酸盐风化的钙对于二氧化碳的消耗很重要。因此，硅酸盐风化产生的Ca通量通常是根据Na来估计的，这被认为是硅酸盐风化的更可靠的估计。然而，化学风化比简单的矿物溶解更为复杂，还会发生阳离子交换等一系列其他化学反应。这是溶液中带正电的阳离子被吸引到粘土上带负电的矿物表面的过程，该过程已知可以缓冲地下水。关键的化学交换之一是 Ca 与 Na 的交换，这意味着 Na 可能根本无法提供硅酸盐风化的真实估计。最近的同位素数据表明，阳离子交换可能比以前认为的更重要，而到目前为止，这还很难确定。一种方法是使用天然存在的示踪剂或同位素，以跟踪化学反应。在这项工作中，建议检查元素 Li 和 Mg 的天然同位素，以检查阳离子交换在全球预算中的作用。然而，为了能够成功地做到这一点，提出了一系列实验工作来检查阳离子交换过程中镁和锂同位素的行为。一旦我们了解了示踪剂的工作原理，我们就可以利用它们重新评估我们对天然水域的理解，并更好地估计与化学风化相关的二氧化碳通量，最终目标是更好地了解地球气候。

项目成果

Experimental constraints on Li isotope fractionation during clay formation

• DOI: 10.1016/j.gca.2019.02.015
• 发表时间: 2019-04
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: R. Hindshaw;Rebecca Tosca;Thomas L. Goût;I. Farnan;N. Tosca;E. Tipper

Mineral dissolution and precipitation in carbonate dominated terranes assessed using Mg isotopes

使用镁同位素评估以碳酸盐为主的地体中的矿物溶解和沉淀

• 发表时间: 2013
• 作者: Tipper, ET

Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy

• DOI: 10.1016/j.earscirev.2013.10.004
• 发表时间: 2014-02-01
• 期刊: EARTH-SCIENCE REVIEWS
• 影响因子: 12.1
• 作者: Fantle, Matthew S.;Tipper, Edward T.
• 通讯作者: Tipper, Edward T.

Mg and Ca isotope constraints on biogeochemical cycles: from the small catchment to the global scale

镁和钙同位素对生物地球化学循环的限制：从小流域到全球规模

• 作者: Tipper, E.T.

Contrasting Li and Mg isotope behaviour in small Alpine rivers: tracing seasonal changes in weathering processes (Invited)

对比高山小河流中的锂和镁同位素行为：追踪风化过程中的季节变化（特邀）

• 发表时间: 2013
• 作者: Tipper, ET