Tracking changes in ocean chemistry using thallium isotopes

使用铊同位素追踪海洋化学变化

基本信息

批准号：
NE/F015666/1
负责人：
Sune Nielsen
金额：
$ 31.7万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Fellowship
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%2FF015666%2F1
关键词：
Tracking changes ocean chemistry using

项目摘要

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.

当今科学中最讨论的主题之一是全球气候变化的原因和程度。有一个共识，当前温度的升高与过去50年中大气中二氧化碳浓度的增加息息相关。二氧化碳的上升无疑是由人类活动引起的，但是温度与大气二氧化碳之间的直接联系仍在争论中。全球气候变化发生在许多不同的时间尺度上，从千禧一代到数百万年的100年。为了对未来气候的机制和幅度做出更确定的预测，我们需要确定地球上的气候如何在其整个历史中进化。在地球历史上，我们看上去越多，获得有关气候条件的准确信息越困难。这样做的主要原因是，我们用来推断过去气候的档案中包含在岩石记录中，随着时间的流逝，该档案将逐渐改变和重新工作。例如，在近500-6亿年前的地质学上（约占地球历史的百分之一）被认为是非常温暖的时代，没有任何永久性的冰。但是，尚不清楚导致地球无冰的原因主要是因为在该年龄的某些样本上获得的数据可能已经从其原始组成中改变了。显然，有关这种气候变化的原因和速率以及环境中随附的变化（例如大气中的二氧化碳含量和海洋酸化）的新信息将有助于我们理解地球气候将来可能采取的方向。在这里，我建议在海洋沉积物中应用Thallium（TL）同位素比测量值，该测量显示出了过去10千年的10千年的时间尺度，从而显示出很大的潜力，从而揭示了过去的海洋状况。 thallium是一种带有两个同位素，205TL和203TL的痕量金属，它们均匀地分布在海水中。几年前，人们发现海水和所谓的铁曼加尼（Fe-mn）地壳的TL同位素比相互抵消。我们称此同位素分馏。随后，我证明了您从这些Fe-Mn地壳中获得的值随着时间的推移会变化。 Fe-Mn地壳是直接从海水沉淀的沉积物，因此它们在放置时给我们带来了海洋化学的“快照”。这种样本类型的一个很大的优势是它们保存完好的性质和缓慢的增长率（约2毫米/百万年），这导致一些样品涵盖了8000万年的海水化学。拟议工作的主要目的是确定控制在Fe-MN外壳中观察到的系统TL同位素变化的过程/过程。从本质上讲，这些变化可以有两个可能的解释，这两者都对过去的气候变化有影响。第一个需要随着时间的推移改变海水的TL同位素组成，这意味着Fe-Mn Crusts中的TL同位素只需监测以恒定分馏的海水的监测。转移海水TL同位素组成的唯一方法是改变进出的内容。这仅是由大规模过程（例如全球火山活动或构造过程）引起的，这些过程从根本上影响全球气候。第二个是随着时间的推移，海水和Fe-Mn地壳之间的分馏发生了变化，而海水的TL同位素比保持相对恒定。两种材料（在这种情况下为Fe-Mn沉积物和海水）之间的分馏可能是许多参数的函数，例如温度，pH和化学物种形成，这都是地球气候的必不可少的监测器。我打算执行一系列实验室实验，以研究海水和Fe-Mn地壳之间TL同位素分级的均值。这将使我能够确定这种机制是否负责自然变异性，从而区分上述两个解释。