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，通过大陆的化学风化作用消耗大气中的二氧化碳被认为在调节地球温度方面起着重要作用。因此，古代海水化学的记录可能为确定风化对全球碳循环的重要性及其对地球气候的影响提供了一种手段。海水中的许多天然放射性成因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。25年来，铷锶（Rb-Sr）放射性成因同位素系统一直是最常用的同位素系统之一，海水87Sr/86Sr比值在长、短时间尺度上的变化反映了随时间变化的大陆输入。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，因为大陆源的成分（特别是河流输送的成分）不太可能随时间保持不变。事实上，Sr/Ca和87Sr/86Sr的耦合记录表明，新生代输送到海水中的风化物质的化学风化通量和组成都发生了显著变化。值得注意的是，已公布的87Sr/86Sr海水记录本身可能会受到损害，因为最近的数据表明sr的稳定同位素存在显著变化。这是因为稳定的88Sr/86Sr比率传统上被认为是一个恒定值，用于校正测量放射源比时的仪器质量分馏。因此，稳定同位素组成的变化可能会显著改变海洋记录中87Sr/86Sr比值的真实值。我们自己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.尽管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，但它导致87Sr/86Sr比率的变化为0.45 / mil (450 ppm)（比目前的分析不确定性大50-100倍）。如果大陆88Sr/86Sr的变化是通过化学风化作用传递给海水的，那么这可能会显著改变87Sr/86Sr的海水记录。例如，新生代海水87Sr/86Sr中约20%的变化可能仅仅是由于88Sr/86Sr的变化，而不是87Sr/86Sr的实际变化。Labrador海浮游有孔虫2.3 Ma记录的初步稳定Sr同位素数据表明，在这段时间内，海水的88Sr/86Sr组成发生了显著变化（> - 100ppm），改变了海洋87Sr/86Sr记录的变化模式和幅度。尽管88Sr/86Sr的变化带来了复杂性，但放射性成因和稳定同位素比值的耦合测量提供了一种确定海水87Sr/86Sr真实值的方法。校正后的87Sr/86Sr记录可以与Sr/Ca数据相结合，对全球平均成分变化对全球平均大陆通量变化进行反卷积。88Sr/86Sr的伴随变化本身可能揭示有关这些组成变化性质的信息，例如，它们是由大陆来源的变化、热液交换还是海相碳酸盐的沉淀引起的。总的来说，这些结果将有助于更好地理解化学风化作用及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之间的关系。

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.