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

海洋的化学演化受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，通过大陆的化学风化消耗大气中的二氧化碳被认为在调节地球温度方面起着基础性的作用。因此，古代海水化学的记录可能会提供一种手段，来确定风化对全球碳循环的重要性及其对地球气候的影响。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很敏感，沉积档案保存了地质时期化学风化变化的记录。二十五年来，Rb-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比值在长时间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记录的变化不能用来重建过去的二氧化碳消耗率，因为大陆来源的成分(特别是河流输送的)不太可能随时间保持不变。事实上，组合的锶/钙和87锶/86锶记录表明，在新生代期间，输送到海水中的风化物质的化学风化通量和成分都发生了重大变化。值得注意的是，已公布的~(87)Sr/~(86)Sr海水记录本身可能会受到损害，因为最近的数据表明，锶的稳定同位素有很大的变化。这是因为稳定的~(88)Sr/~(86)Sr比值传统上被认为是一个常数值，并被用来在测量放射性成因比时校正仪器质量分馏。因此，稳定同位素组成的变化可能会极大地改变海洋记录中所测得的87Sr/86Sr比值与其真实值的关系。我们自己对不同陆相岩石类型和河流的初步数据表明，~(88)Sr/~(86)Sr比值的总变化至少为每密尔0.9。虽然与其他较轻或氧化还原敏感的元素相比，这种变化可能被认为是很小的，但它导致87Sr/86Sr比值的变化为每毫升0.45(450ppm)(比目前分析不确定度大约50-100倍)。如果这种大陆变化是通过化学风化作用传递给海水的，那么这反过来又可能显著改变87Sr/86Sr的海水记录。例如，新生代海水中约20%的变化可能仅仅是由于88Sr/86Sr的变化，而不是87Sr/86Sr的任何实际变化。我们从拉布拉多海浮游有孔虫获得的2.3 Ma记录的初步稳定的锶同位素数据表明，在这段时间内，海水的~(88)Sr/~(86)Sr组分发生了显著的变化，改变了海相~(87)Sr/~(86)Sr记录中的变化模式和变化幅度。尽管~(88)Sr/~(86)Sr的变化带来了复杂性，但放射性成因和稳定同位素比值的耦合测量提供了一种确定海水的真实~(87)Sr/~(86)Sr值的方法。然后，校正后的~(87)Sr/~(86)Sr记录可与锶/钙数据相结合，从全球平均成分的变化中反卷积全球平均大陆通量的变化。其中，伴随的~(88)Sr/~(86)Sr变化本身可能揭示这些成分变化的性质的信息，例如，它们是否源于陆源的变化、热液交换或海相碳酸盐的沉淀。总体而言，这些结果将有助于更好地理解化学风化及其对大气二氧化碳分压的调节之间的关系，从而对温室效应和全球气候产生影响。

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.