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。虽然玄武岩仅占暴露在地球表面的地壳岩石的约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，因为构成玄武岩的含钙硅酸盐矿物似乎很容易溶解，从而以相对较快的速度消耗大气中的二氧化碳（CO2）。有几项研究对冰岛的玄武岩风化进行了研究。一个主要的假设是，所有的河流钙（Ca）和碳酸盐碱度来源于硅酸盐风化。然而，热液方解石遍布冰岛，由于矿物的高溶解度和快速溶解速率，即使是微量水平也会影响河流的地球化学。了解热液方解石风化的作用对量化玄武岩风化反馈的强度以及控制它的因素（如径流、温度和机械侵蚀）有几个影响。该项目将促进与冰岛研究人员和学生建立新的国际伙伴关系。研究员将监督研究生和本科生的研究，他将把项目的发现纳入他的放射性同位素地球化学课程。调查员和他的学生将从事K12代表性不足的少数民族在教育和宣传活动有关的项目奖。该项目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解释为什么地球是一个可居住的星球，从而造福社会。该项目将测试围绕冰岛河流钙同位素组成控制的几种假设，从而可以深入了解河流碱度的来源，并从那里了解玄武岩风化对长期大气CO2水平的影响。该研究还将包括放射性和稳定的锶（Sr）同位素比（87 Sr/86 Sr和#948;88/86 Sr）的分析，这应该提供额外的限制河流钙的来源和循环。在春季、夏季和秋季进行的实地考察将主要集中在冰岛北方的斯卡加峡湾山谷。调查人员将收集河水、泉水（热泉水和非热泉水）、岩石、矿物、土壤和植物。他们还计划从冰岛东部的Berufjördur地区收集岩石和矿物标本。方解石和沸石矿物片沸石形成于深熔岩堆中玄武岩的埋藏变质作用过程中，并且经常一起出现在现在暴露于地表的变质玄武岩中。片沸石钙同位素值低于块状玄武岩，而方解石钙同位素值较高。片沸石的Ca同位素组成可能反映了在热液温度下平衡同位素交换过程中40 Ca的优先掺入，这意味着残留的富含44 Ca的沃茨决定了方解石的高Ca同位素值。为了验证这一假设，拟议的研究还包括实验室规模的实验，旨在量化片沸石钙同位素分馏因子在一系列的物理化学条件下。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式。由于沸石相变质作用和热液循环是玄武岩喷发的普遍特征，冰岛的发现可能普遍适用于其他玄武岩泛滥区。该研究还将推进该小组最近开发的用于测量Ca同位素和Sr同位素值的新型双尖峰TIMS方法。