The Calcium and Strontium (radiogenic and stable) isotope geochemistry of weathering in Iceland

冰岛风化作用的钙和锶（放射性和稳定）同位素地球化学

• 批准号: 1613359
• 负责人: Andrew Jacobson
• 金额: $ 37.5万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-10-01 至 2021-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1613359&HistoricalAwards=false
• 关键词: Calcium; Strontium; radiogenic; stable; isotope

This project aims to elucidate the role of basalt weathering in Earth's long-term carbon cycle. While basalt represents only ~5% of the crustal rocks exposed at the Earth's surface, basalt weathering may have a disproportionately large effect on long-term climate change because Calcium-bearing silicate minerals composing basalt appear to dissolve easily, thereby consuming atmospheric carbon dioxide (CO2) at a relatively rapid rate. Several studies have examined basalt weathering in Iceland. A major assumption is that all of the riverine Calcium (Ca)and carbonate alkalinity originates from silicate weathering. However, hydrothermal calcite occurs throughout Iceland, and even trace levels are expected to impact river geochemistry owing to the mineral's high solubility and fast dissolution rate. Understanding the role of hydrothermal calcite weathering has several implications for quantifying the strength of the basalt weathering feedback and the factors that appear to control it, such as runoff, temperature, and mechanical erosion. The project will foster a new international partnership with Icelandic researchers and students. The investigator will supervise research by graduate and undergraduate students, and he will incorporate project findings into his radiogenic isotope geochemistry course. The investigator and his students will engage K12 underrepresented minorities in education and outreach activities related to the project award. The project will benefit society by providing new insight into the long-term carbon cycle and related geochemical phenomena that explain why the Earth is a habitable planet.This project will test several hypotheses surrounding controls on the Ca isotope composition of Icelandic rivers, which in turn can provide insight into sources of riverine alkalinity and from there, the impact of basalt weathering on long-term atmospheric CO2 levels. The research will also include analyses of radiogenic and stable Strontium (Sr) isotope ratios (87Sr/86Sr and δ88/86Sr), which should provide additional constraints on the sources and cycling of riverine Ca. Fieldwork conducted during the spring, summer, and fall will mostly concentrate on the Skagafjördur Valley in northern Iceland. Investigators will collect river water, spring water (thermal and non-thermal), rocks, minerals, soils, and plants. They also plan to collect rock and mineral specimens from the Berufjördur region in eastern Iceland. Calcite and the zeolite mineral heulandite form during burial metamorphism of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Heulandite Ca isotope values are lower than those for bulk basalt, while calcite Ca isotope values are higher. The Ca isotope composition of heulandite likely reflects the preferential incorporation of 40Ca during equilibrium isotope exchange at hydrothermal temperatures, implying that the residual 44Ca-enriched waters determine the high Ca isotope values of calcite. To test this hypothesis, the proposed study also includes bench-scale experiments designed to quantify the heulandite Ca isotope fractionation factor under a range of physicochemical conditions. The project is potentially transformative because it will test a long-standing paradigm surrounding the role of basalt weathering in long-term climate regulation. Because zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions, findings for Iceland may apply generally to other flood basalt provinces. The research will also advance novel double-spike TIMS methods for measuring Ca isotopes and Sr isotope values recently developed by this group.

该项目旨在阐明玄武岩风化在地球长期碳循环中的作用，虽然玄武岩仅占地球表面暴露的地壳岩石的 5%，但玄武岩风化可能对长期气候变化产生不成比例的巨大影响。因为组成玄武岩的含钙硅酸盐矿物似乎很容易溶解，以相对较快的速度消耗大气中的二氧化碳（CO2），这是冰岛玄武岩风化的一个主要假设。河流中的钙 (Ca) 和碳酸盐碱度均来自硅酸盐风化作用，但热液方解石遍布冰岛，由于该矿物的高溶解度和快速溶解速率，即使是痕量水平也可能会影响河流地球化学。热液方解石风化的研究对于量化玄武岩风化反馈的强度以及似乎控制它的因素（例如径流、温度和机械侵蚀）具有多种意义。该研究人员将与冰岛研究人员和学生建立新的国际合作伙伴关系，该研究人员将监督研究生和本科生的研究，并将项目研究成果纳入他的放射性同位素地球化学课程中。与该项目奖项相关的外展活动将通过提供对长期碳循环和相关地球化学现象的新见解来造福社会，从而解释为什么地球是一个宜居星球。该项目将测试围绕钙控制的几个假设。冰岛河流的同位素组成，这反过来可以深入了解河流碱度的来源，并由此了解玄武岩风化对长期大气二氧化碳水平的影响。该研究还将包括对放射性和稳定锶（Sr）同位素比率的分析。 （87Sr/86Sr 和 δ88/86Sr），这应该对河流 Ca 的来源和循环提供额外的限制。春季、夏季和秋季将主要集中在冰岛北部的 Skagafjördur 山谷，调查人员将收集河水、泉水（热水和非热水）、岩石、矿物、土壤和植物。他们还计划收集岩石和矿物。来自冰岛东部 Berufjördur 地区的标本和沸石矿物片沸石是在深部熔岩堆中玄武岩的埋藏变质过程中形成的，并且经常一起出现在现在暴露于的变玄武岩中。片沸石的 Ca 同位素值低于基础块体，而方解石的 Ca 同位素值较高，片沸石的 Ca 同位素组成可能反映了 40Ca 在热液温度下的平衡同位素交换过程中的优先结合。残留的富含 44Ca 的水决定了方解石的高 Ca 同位素值。为了检验这一假设，拟议的研究还包括旨在量化 44Ca 的实验室规模实验。该项目具有潜在的变革性，因为它将测试围绕玄武岩风化在长期气候调节中的作用的长期范例，因为沸石相变质作用和热液循环是普遍存在的特征。玄武岩喷发的研究结果可能普遍适用于其他洪水玄武岩省份。该研究还将推进用于测量钙同位素和钙同位素的新型双尖峰 TIMS 方法。该小组最近开发了 Sr 同位素值。