Testing the veracity of the Mg/Ca palaeotemperature proxy in cultured marine molluscan calcite

气候变化对整个人类社会来说是一个重要而紧迫的问题。科学家需要了解地球气候在不同时间尺度上的变化，以便他们能够向政界人士和公众通报未来气候变化可能产生的影响的规模和程度。为了了解气候，有必要收集有关关键的海洋学和气候参数，例如海水温度在最近和过去的地质上如何变化的信息。然后，这些数据可以用来模拟并最终预测气候在不同时间尺度上的变化，即几年、几十年和几个世纪。然而，不幸的是，这些关键参数的仪器记录只能追溯到100年前左右，但许多关键信息可以从气候处于不同状态的早期获得。由于在较早的时间间隔内无法直接测量所需的参数，因此必须采取间接方法，测量的不是实际的气候或海洋参数，而是以相同方式作出反应并仍保存在最近或地质记录中的其他东西。这种方法被称为基于代理的重建，代理是可以测量的物理或化学信号，并且还显示出与需要确定的气候或海洋参数的关系。除了物理或化学替代材料外，还需要一份通过现代和地质时间保存信息的档案材料，这些档案包括树木年轮、珊瑚和海洋软体动物。所有这些生物都表现出递增的生长模式，例如年生长带，这样就可以重建代用生物的高时间分辨率记录，并最终重建气候和海洋参数。珊瑚和软体动物分泌包括方解石在内的碳酸钙骨架，已有研究表明，锶和镁等元素可以通过一种被称为替代的过程来取代这些矿物中的一些钙，而且替代量可以与它们生活的海水温度有关。因此，珊瑚和软体动物骨骼是档案材料，矿物中锶和镁钙比的测量是替代测量，可以用来重建这种情况下的温度。然而，这种基于代理的方法的成功使用有赖于代理与感兴趣的气候或海洋参数之间存在敏感和精确的关系。我们最近观察到在恒温培养的软体动物贝壳中镁和钙的比例发生了显着的变化，因此我们打算在本研究中更详细地研究这种变化的程度。有必要对镁与钙比例的变化程度进行详细的评估，以便我们能够提高在这些生物中使用这一指标进行温度重建的精确度。为了实现这一目标，我们将在实验室水族箱中，在受控温度条件下，培育两种贝壳结构截然不同的软体动物。然后，将使用几种分析技术来研究新外壳材料中镁与钙比例的变化程度，其中一种技术可以每隔百分之一毫米测量一次外壳结构中的镁和钙浓度。通过了解贝壳中镁与钙的比例如何变化，我们应该能够调整我们的采样策略，以提供更精确的校准，从而更精确地重建海水温度，从而提高对过去和未来气候变化的理解。

Climate change is an important and pressing issue for all of human society. Scientists need to understand how Earth's climate varies over different timescales so that they can inform politicians and the public about the likely scale and extent of the impacts of any future climate change. To understand climate it is necessary to gather information about how key oceanographic and climatic parameters, e.g. seawater temperature, changed in the recent and geological past. These data then can be used to model, and ultimately predict, how climate varies over different timescales of interest, i.e. years, decades and centuries. Unfortunately, however, instrumental records of these key parameters only extent back 100 years or so, but much critical information can be obtained from earlier times when the climate was in different states. Because direct measurements of the required parameters are not available for the earlier time intervals an indirect approach then must be taken, whereby measurements are made not of the actual climatic or oceanographic parameter but something else that responded in the same way and still is preserved in the recent or geological record. This approach is known as a proxy-based reconstruction, with the proxy being either a physical or chemical signal that can be measured and that also exhibits a relationship with the climatic or oceanographic parameter that needs to be determined. In addition to the physical or chemical proxy an archive material that preserves the information through recent and geological time also is required and these archives include tree rings, corals and marine molluscs. All of these organisms exhibit an incremental growth pattern, e.g. yearly growth bands, such that high temporal resolution records of the proxies and ultimately climatic and oceanographic parameters can be reconstructed. Corals and molluscs secrete calcium carbonate skeletons that include the mineral calcite and it has been shown that elements such as strontium and magnesium can replace, by a process termed substitution, some of the calcium in these minerals and that the amount of substitution can be related to the temperature of the seawater in which they lived. Therefore, the coral and mollusc skeletons are the archive materials and the measurement of the ratios of strontium and magnesium to calcium in the minerals are the proxy measurements, which can be used to reconstruct temperature in this case. Successful use of such a proxy-based approach is reliant, however, on there being sensitive and precise relationships between the proxy and the climatic or oceanographic parameter of interest. We have recently observed significant variation in magnesium to calcium ratios in mollusc shells cultured at constant temperature and therefore we intend to investigate the degree of such variability in more detail in this study. Detailed assessment of the level of variation in magnesium to calcium ratios is necessary so that we can improve the precision of temperature reconstructions made using measurement of this proxy in these organisms. To meet this objective we will grow two mollusc species, with contrasting shell structures, in laboratory aquaria under controlled temperature conditions. The degree of variability of the magnesium to calcium ratio within the new shell material then will be investigated using several analytical techniques, including one that can make the measurements of magnesium and calcium concentrations every one hundredth of a millimetre across and through the shell structure. By understanding how the magnesium to calcium ratio varies within the shells we should be able to adjust our sampling strategy to provide a more precise calibration and hence more precise seawater temperature reconstructions, thereby improving understanding of past and future climate change.