Trace metal geochemistry of brachiopod calcite: a new window to the past

这项研究的首要目标是开发一种新的“古温度计”--一种过去确定海水温度的方法，并使用这种新工具来研究地球气候历史上重要而有争议的事件。古温度计是基于腕足类方解石壳第二层中的镁和钙的比率。在这个大目标中，有三个具体目标。目的1是定量测定腕足类方解石中的镁钙温度关系。以前的工作表明：(I)生活在较温暖水域的腕足类动物的方解石外壳中含有更多的镁，(Ii)腕足类方解石是跨季节沉积的，并显示出镁/钙的季节性变化。因此，学生将通过两种方式实现第一个目标：通过比较贝壳第二层的镁/钙比与平均年温度，以及通过比较壳内镁/钙循环和壳内温度循环(根据记录的温度范围和壳内增量18O确定)。后一种方法已经成功地应用于其他海洋生物，例如腹足类中的锶/钙比率。这个目标的最终目标是学生将确定腕足类方解石的镁/钙每度温度变化的百分比。目标2是将镁/钙古温度计应用于志留纪(~428 Ma)的一段戏剧性的时间间隔，这是以全球碳循环受到巨大扰动为标志的。有人假设，这种扰动与全球变暖事件有关，但还没有找到温度变化的直接证据。学生将构建整个事件的高分辨率腕足动物镁/钙记录，这样即使在不知道海水镁/钙绝对值的情况下，也可以计算出温度变化的幅度和方向。这一目标将提供地球历史上首次直接记录这一备受瞩目的事件的海水温度，并提高我们对碳循环和深层气候变化之间的联系的理解。目的3利用镁钙古温度计研究34 Ma始新世-渐新世界线上地球温室-冰库的转变。最近的研究表明，南极冰盖的形成是由大气二氧化碳水平下降引发的，穿过了气候系统固有的一个重要的“门槛”。始新世-渐新世气候过渡是地球气候历史上的一个关键点，其中包含有关南极冰盖在二氧化碳水平变化下的稳定性的宝贵信息，这与未来对气候变暖时冰盖行为的预测有直接关系。有人提出(例如，根据花粉记录)，气候过渡的一个重要特征是季节性增加(即较凉爽的冬季)。这名学生将从新西兰的经典部分研究跨越这种气候过渡的腕足动物。他们将构建气候转变前后的镁/钙贝壳内剖面，以检验冰盖增长与季节性增加有关的假设。季节性的增加会使壳体的镁/钙剖面产生更大的幅度变化。壳层的平均镁钙比将被用来估算这些古纬度(南纬约40度)的总体降温。

The overarching goal of the research is to develop a new 'palaeothermometer' - a means of determining seawater temperature in the past, and use this new tool to study important and controversial episodes in Earth's climate history. The palaeothermometer is based on the ratio of magnesium to calcium within the secondary layer of brachiopod calcite shells. Within this broad goal, there are three specific objectives. Objective 1 is to quantify the Mg/Ca-temperature relationship in brachiopod calcite. Previous work has shown that (i) brachiopods living in warmer waters contain more magnesium in their calcite shells, and (ii) brachiopod calcite is deposited across seasons, and displays seasonal variations in Mg/Ca. Therefore, the student will achieve this first objective in two ways; by comparing Mg/Ca ratios from the secondary layer of shells with mean annual temperatures, and by comparing intra-shell cycles in Mg/Ca with intra-shell cycles in temperature (determined from recorded temperature ranges and intra-shell delta18O). The latter approach has been successfully applied to other marine organisms, e.g. Sr/Ca ratios in gastropods. The ultimate aim of this objective is that the student will determine the % change in brachiopod calcite Mg/Ca per degree temperature change. Objective 2 is to apply the Mg/Ca palaeothermometer to a dramatic interval of time in the Silurian (~ 428 Ma), which is marked by an enormous perturbation to the global carbon cycle. It has been hypothesised that this perturbation was associated with a global warming event, but direct evidence for a temperature change has not yet been found. The student will construct a high-resolution brachiopod Mg/Ca record across the event, such that even without knowing absolute values of seawater Mg/Ca, the magnitude and direction of temperature change can be calculated. This objective will provide the first direct records of seawater temperature across this high profile event in Earth's history, and improve our understanding of the links between carbon cycling and deep time climate change. Objective 3 is to use the Mg/Ca palaeothermometer to study Earth's greenhouse-icehouse transition at the Eocene-Oligocene boundary, 34 Ma. It has recently been shown that the formation of the Antarctic ice sheet was triggered by decreasing levels of atmospheric CO2, passing through an important 'threshold' intrinsic to the climate system. The Eocene-Oligocene climate transition represents a pivotal point in Earth's climate history, and contains valuable information regarding the stability of the Antarctic ice sheet under changing levels of CO2, which is of direct relevance to future projections of the behaviour of the ice sheet as climate warms. It has been suggested (for example on the basis of pollen records) that an important feature of the climate transition is an increase in seasonality (i.e. cooler winters). The student will study brachiopods spanning this climate transition from classic sections in New Zealand. They will construct intra-shell profiles of Mg/Ca before and after the climate transition to test the hypothesis that the ice sheet growth was linked to an increase in seasonality. An increase in seasonality would produce larger amplitude variations in the shell Mg/Ca profiles. Mean shell Mg/Ca will be used to estimate the overall cooling at these palaeolatitudes (~40 degrees south).