Tracking changes in ocean chemistry using thallium isotopes

当今科学界最具争议的话题之一是全球气候变化的原因和程度。人们一致认为，当前的气温上升与过去50年来大气中二氧化碳浓度的增加密切相关。二氧化碳的增加无疑是由人类活动引起的，但是温度和大气二氧化碳之间的直接联系仍然存在争议。全球气候变化发生在许多不同的时间尺度上，从一千年到几亿年。为了对未来气候的机制和规模做出更确定的预测，我们需要确定地球上的气候在整个历史中是如何演变的。我们研究的地球历史越久远，就越难以获得关于气候条件的准确信息。其主要原因是，我们用来推断过去气候的档案包含在岩石记录中，随着时间的推移，这些记录会逐渐改变和重新加工。例如，在最近的5000万至6000万年前（约占地球历史的1%）的地质时期，人们认为两极没有任何永久的冰，非常温暖。然而，目前尚不清楚是什么原因导致地球无冰，主要是因为从这个时代的一些样本中获得的数据可能已经改变了它们的原始成分。显然，关于这种气候变化的原因和速率以及随之而来的环境变化（如大气中二氧化碳含量和海洋酸化）的新信息将有助于我们了解地球气候未来可能走向的方向。在这里，我建议在海洋沉积物中应用铊（Tl）同位素比例测量，它显示出巨大的潜力，可以揭示过去的海洋状况，从而也可以揭示地球气候，时间尺度为10年或数百万年。铊是一种微量金属，具有205Tl和203Tl两种同位素，在海水中均匀分布。几年前，人们发现海水和所谓的锰铁（Fe-Mn）结壳的Tl同位素比率是相互抵消的。我们称之为同位素分馏。随后我展示了，你从这些铁锰结壳中得到的值会随着时间系统地变化。铁锰结壳是直接从海水中沉淀下来的沉积物，因此当它们沉积下来时，它们给我们提供了海洋化学的“快照”。这种样品类型的一个很大的优点是它们保存良好的性质和缓慢的生长速度（大约毫米/百万年），这导致一些样品覆盖了8000万年的海水化学。所提出的工作的主要目的是确定控制在铁锰地壳中观测到的系统Tl同位素变化的过程。从本质上讲，这些变化有两种可能的解释，这两种解释都与过去的气候变化有关。第一种方法需要随着时间的推移改变海水的Tl同位素组成，这意味着铁锰地壳中的Tl同位素只是以恒定的分馏来监测海水的Tl同位素。改变海水中Tl同位素组成的唯一方法是改变流入或流出的海水。这只可能是由大规模的过程造成的，比如全球火山活动或构造过程，它们将从根本上影响全球气候。二是海水与Fe-Mn结壳的分馏随时间变化，而海水的Tl同位素比值保持相对恒定。两种物质（在这个例子中是铁锰沉积物和海水）之间的分异可能是一系列参数的函数，如温度、pH值和化学形态，这些都是地球气候的重要监测指标。我打算进行一系列实验室实验，研究海水和铁锰结壳之间Tl同位素分馏的方法。这将使我能够确定这种机制是否对自然变率负责，从而区分上述两种解释。

One of the most debated topics in science today is the cause and extent of global climate change. There is a consensus that the current rise in temperatures correlates well with an increase in atmospheric CO2 concentrations over the last ~50 years. The rise in CO2 is undoubtedly caused by human activity, but the direct link between temperature and atmospheric CO2 is still debated. Global climate change occurs on many different time scales from millennial to 100's of millions of years. In order to make more certain predictions about the mechanisms and magnitude of future climate we need to determine how the climate on Earth evolved throughout its history. The further back in Earth's history we look the more difficult it is to obtain accurate information about climatic conditions. The prime reason for this is that the archives we use to infer past climate are contained within the rock record, which becomes progressively altered and reworked over time. For example, times as geologically recent as 50-60 million years ago (about one percent of Earth history) are thought to be very warm without any permanent ice at the poles. It is, however, unclear what caused the Earth to be ice-free mainly because data obtained on some samples of this age may have been altered from their original composition. Clearly, new information on the causes and rates of such climate shifts and the accompanying changes in the environment such as CO2 contents of the atmosphere and ocean acidification would help us in understanding the direction Earth's climate might take in the future. Here I propose the application of thallium (Tl) isotope ratio measurements in marine sediments, which shows great potential to uncover past ocean conditions, and thereby also Earth's climate, over time scales of 10's of millions of years. Thallium is a trace metal with two isotopes, 205Tl and 203Tl, which are homogenously distributed in seawater. A few years ago it was discovered that the Tl isotope ratios of seawater and so-called ferro-manganese (Fe-Mn) crusts are offset from each other. We call this isotopic fractionation. Subsequently I have shown that the values you get from these Fe-Mn crusts vary systematically over time. Fe-Mn crusts are sediments that precipitate directly from seawater and therefore they give us a 'snapshot' of ocean chemistry when they are laid down. A great advantage of this sample type is their well preserved nature and slow growth rate (around 2mm/million years), which results in some samples covering 80 million years of seawater chemistry. The main aim of the proposed work is to determine the process/processes that control the systematic Tl isotope variations observed in Fe-Mn crusts. Essentially these variations can have two possible explanations, both of which have implications for past climate change. The first entails changing the Tl isotope composition of seawater over time, implying that Tl isotopes in Fe-Mn crusts simply monitor that of seawater with a constant fractionation. The only way of shifting the Tl isotope composition of seawater is by altering either what goes in or out. This is only likely to be caused by large scale processes, such as global volcanic activity or tectonic processes, which will fundamentally affect global climate. The second is that the fractionation between seawater and Fe-Mn crusts changed over time, while the Tl isotope ratio of seawater remained relatively constant. Fractionation between two materials (in this case Fe-Mn sediments and seawater) can be a function of a host of parameters such as temperature, pH and chemical speciation, which are all essential monitors of Earth's climate. I intend to perform a series of laboratory experiments that investigate the means by which Tl isotopes fractionate between seawater and Fe-Mn crusts. This will enable me to determine if this mechanism is responsible for the natural variability and thereby distinguish between the two explanations proposed above.