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

点击翻译按钮获取中文摘要

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).