Tracking changes in ocean chemistry using thallium isotopes

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.