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

腕足动物方解石的微量金属地球化学：了解过去的新窗口

• 批准号: NE/H018018/1
• 负责人: Caroline Lear
• 金额: $ 9.26万
• 依托单位: CARDIFF UNIVERSITY
• 依托单位国家: 英国
• 项目类别: Training Grant
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH018018%2F1
• 关键词: Trace; metal; geochemistry; brachiopod; calcite

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).

这项研究的首要目标是开发一种新的“古温度计”——一种测定过去海水温度的方法，并利用这种新工具研究地球气候历史上重要的和有争议的时期。古温度计是根据腕足动物方解石壳的第二层中镁与钙的比例计算的。在这个广泛的目标中，有三个具体的目标。目的1是量化腕足动物方解石中Mg/ ca -温度的关系。先前的研究表明：(1)生活在温暖水域的腕足动物的方解石壳中含有更多的镁，(2)腕足动物的方解石是跨季节沉积的，并且在Mg/Ca中表现出季节性变化。因此，学生将通过两种方式实现第一个目标；通过比较第二层的Mg/Ca比值与年平均温度，以及比较Mg/Ca的壳内循环与温度的壳内循环（由记录的温度范围和壳内delta18O确定）。后一种方法已成功地应用于其他海洋生物，例如腹足类动物的Sr/Ca比值。这个目标的最终目的是，学生将确定每一度温度变化腕足方解石Mg/Ca的百分比变化。目的2是将Mg/Ca古温计应用于志留纪（~ 428 Ma）的一个显著时间间隔，该时间间隔以全球碳循环的巨大扰动为标志。据推测，这种扰动与全球变暖事件有关，但尚未发现温度变化的直接证据。该学生将构建一个高分辨率的腕足动物Mg/Ca记录，这样即使不知道海水Mg/Ca的绝对值，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这一引人注目的事件中海水温度的第一个直接记录，并提高我们对碳循环与深时间气候变化之间联系的理解。目的3利用Mg/Ca古温计研究始新世-渐新世边界34 Ma的地球温室-冰窖过渡。最近的研究表明，南极冰盖的形成是由大气中二氧化碳水平的下降引发的，二氧化碳水平通过了气候系统固有的一个重要“阈值”。始新世-渐新世气候转变是地球气候史上的一个转折点，它包含了关于南极冰盖在二氧化碳水平变化下的稳定性的宝贵信息，这与气候变暖时冰盖行为的未来预测直接相关。有人提出（例如根据花粉记录），气候转变的一个重要特征是季节性的增加（即冬季较冷）。学生将从新西兰的经典区域研究跨越气候转变的腕足类动物。他们将构建气候转变前后的Mg/Ca壳内剖面，以检验冰盖增长与季节性增加有关的假设。季节性的增加会产生更大的壳Mg/Ca谱的振幅变化。平均壳Mg/Ca将用于估计这些古纬度（南纬~40度）的总体冷却。