Quantifying the Strontium Budget of the Oceans, past and present, using coupled Radiogenic and Stable Strontium Isotopes

使用耦合放射源和稳定锶同位素量化过去和现在海洋的锶收支

• 批准号: NE/F018126/1
• 负责人: Kevin Burton
• 金额: $ 19.42万
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2008
• 资助国家: 英国
• 起止时间: 2008 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF018126%2F1
• 关键词: Quantifying; Strontium; Budget; Oceans; past

The chemical evolution of the oceans is controlled by a range of biological and sedimentary processes, many of which are influenced by tectonic and climatic change. Of these, consumption of atmospheric CO2 through chemical weathering of the continents is thought to play a fundamental role in regulating the Earth's temperature. Therefore, records of ancient seawater chemistry potentially provide a means of determining the importance of weathering on the global carbon cycle and its affect on the Earth's climate. Many natural radiogenic isotopes in seawater are sensitive to changes in the balance of input from continental weathering, and sedimentary archives preserve a record of changes in chemical weathering through geological time. For over twenty five years the rubidium-strontium (Rb-Sr) radiogenic isotope system has been amongst the most commonly used and variations in seawater 87Sr/86Sr ratios on both long and short timescales reflect changing continental input through time. However, changes in the 87Sr/86Sr record cannot be used to reconstruct past CO2 consumption rates because it is unlikely that the composition of the continental source (in particular that delivered by rivers) remains constant through time. Indeed, coupled Sr/Ca and 87Sr/86Sr records indicate significant changes in both the chemical weathering flux and composition of weathered material delivered to seawater over the Cenozoic. Remarkably the published 87Sr/86Sr seawater record may itself be compromised because recent data suggests that there are significant variations in the stable isotopes of Sr. This is because the stable 88Sr/86Sr ratio is traditionally considered to be a constant value and used to correct instrumental mass fractionation during measurement of the radiogenic ratio. Consequently, variations in the stable isotope composition may dramatically alter the measured 87Sr/86Sr ratios in marine records relative to their true value. Our own preliminary data for diverse continental rock types and rivers indicates a total variation in the 88Sr/86Sr ratio of at least 0.9 per mil. While such a variation might be considered small compared to other lighter or redox sensitive elements, it results in a variation of 0.45 per mil (450 ppm) in the 87Sr/86Sr ratio (some 50-100 times greater than current analytical uncertainties). If this continental 88Sr/86Sr variation is imparted to seawater through chemical weathering then this may, in turn, significantly alter the 87Sr/86Sr seawater record. For example, some 20% of the change in Cenozoic seawater 87Sr/86Sr could simply be due to variations in 88Sr/86Sr rather than any actual variation in 87Sr/86Sr. Our own preliminary stable Sr isotope data for a 2.3 Ma record obtained from planktonic foraminifera from the Labrador Sea indicates a significant shift (>100 ppm) in the 88Sr/86Sr composition of seawater over this interval, altering both the pattern and magnitude of change seen in the marine 87Sr/86Sr record. Despite the complexities introduced by variations in 88Sr/86Sr, the coupled measurement of both radiogenic and stable isotope ratios offers a means of determining the true 87Sr/86Sr value of seawater. The corrected the corrected 87Sr/86Sr record can then be combined with Sr/Ca data to deconvolve changes in the global average continental flux from changes in global average composition. Where concomitant variations in 88Sr/86Sr may themselves reveal information on the nature of those changes in composition, for example, whether they result from changes in the continental source, hydrothermal exchange or the precipitation of marine carbonate. Overall these results will thus serve to provide a better understanding of the relationship between chemical weathering, its regulation of the atmospheric partial pressure of carbon dioxide, and thus influence on the greenhouse effect and global climate.

海洋的化学演化受到一系列生物和沉积过程的控制，其中许多过程受到构造和气候变化的影响。其中，通过大陆化学风化消耗大气二氧化碳被认为在调节地球温度方面发挥着重要作用。因此，古代海水化学记录可能提供一种确定风化对全球碳循环的重要性及其对地球气候影响的方法。海水中的许多天然放射性同位素对大陆风化输入平衡的变化敏感，沉积档案保存了整个地质时期化学风化变化的记录。二十五年来，铷-锶 (Rb-Sr) 放射性同位素系统一直是最常用的系统之一，海水 87Sr/86Sr 比率在长和短时间尺度上的变化反映了大陆输入随时间的变化。然而，87Sr/86Sr 记录的变化不能用于重建过去的二氧化碳消耗率，因为大陆来源（特别是河流输送的来源）的成分不太可能随着时间的推移保持不变。事实上，Sr/Ca 和 87Sr/86Sr 的耦合记录表明，化学风化通量和新生代流入海水的风化物质成分均发生了显着变化。值得注意的是，已发表的 87Sr/86Sr 海水记录本身可能受到损害，因为最近的数据表明 Sr 的稳定同位素存在显着变化。这是因为稳定的 88Sr/86Sr 比率传统上被认为是恒定值，并在放射成因比测量期间用于校正仪器质量分馏。因此，稳定同位素组成的变化可能会显着改变海洋记录中测量的 87Sr/86Sr 比值相对于其真实值的变化。我们对不同大陆岩石类型和河流的初步数据表明 88Sr/86Sr 比率的总变化至少为 0.9/mil。虽然与其他较轻或氧化还原敏感元素相比，这种变化可能被认为很小，但它会导致 87Sr/86Sr 比率出现 0.45 每密耳 (450 ppm) 的变化（比当前分析不确定度大约 50-100 倍）。如果这种大陆 88Sr/86Sr 变化通过化学风化作用传递给海水，那么这可能反过来显着改变 87Sr/86Sr 海水记录。例如，新生代海水 87Sr/86Sr 约 20% 的变化可能只是由于 88Sr/86Sr 的变化，而不是 87Sr/86Sr 的任何实际变化。我们自己从拉布拉多海浮游有孔虫获得的 2.3 Ma 记录的初步稳定 Sr 同位素数据表明，在此期间海水 88Sr/86Sr 成分发生了显着变化 (>100 ppm)，改变了海洋 87Sr/86Sr 记录中看到的变化模式和幅度。尽管 88Sr/86Sr 的变化带来了复杂性，但放射性同位素比和稳定同位素比的耦合测量提供了一种确定海水真实 87Sr/86Sr 值的方法。然后可以将校正后的 87Sr/86Sr 记录与 Sr/Ca 数据组合，以将全球平均大陆通量的变化与全球平均成分的变化去卷积。 88Sr/86Sr 的伴随变化本身可能揭示有关这些成分变化性质的信息，例如，它们是否是由大陆来源、热液交换或海洋碳酸盐沉淀的变化引起的。总的来说，这些结果将有助于更好地理解化学风化之间的关系，其对大气二氧化碳分压的调节，从而对温室效应和全球气候的影响。

项目成果

Characterising the stable (d 88/86 Sr) and radiogenic ( 87 Sr/ 86 Sr) isotopic composition of strontium in rainwater

表征雨水中锶的稳定（d 88/86 Sr）和放射性（ 87 Sr/ 86 Sr）同位素组成

• DOI: 10.1016/j.chemgeo.2015.05.010
• 发表时间: 2015
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: Pearce C

Controls on stable strontium isotope fractionation in coccolithophores with implications for the marine Sr cycle

• DOI: 10.1016/j.gca.2013.11.043
• 发表时间: 2014-03
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: E. Stevenson;M. Hermoso;R. Rickaby;J. Tyler;F. Minoletti;I. Parkinson;F. Mokadem;K. Burton