Trace element and isotope partitioning in carbonates in simulated biological environments

点击翻译按钮获取中文摘要

Many marine organisms produce calcium carbonate structures e.g. corals produce skeletons, bivalves produce shells and foraminifera (single-celled organisms) produce tests. These mineral structures provide organisms with tissue support and/or protection from predators and the physical environment. The chemistry of calcium carbonate is affected by environment. In particular seawater temperatures affect how trace elements substitute in place of calcium and the ratio of oxygen isotopes (forms of oxygen with different masses) in the mineral structure. Thus the geochemistry of the calcium carbonate structures provides information on the temperature and chemistry of seawater at the time the organism lived and grew. The mineral structures are preserved after the death of the organism e.g. as coral reefs, and the analysis of fossil specimens offers an excellent route to reconstruct records of past environmental conditions. Such records help us to understand past changes and interactions in global climate and to predict 21st century climate change. Understanding how other factors affect the chemistry of the shells and reefs is key to accurate interpretation of the climate information recorded in fossil specimens. Coral skeletons and foraminifera tests form at specialist calcification sites, either in or adjacent to the organism. The calcification sites contain both soluble and insoluble organic biomolecules (e.g. proteins, lipids), which control and guide the precipitation and growth of the mineral. These biomolecules also affect the chemistry of the mineral. In this research we will analyse modern and fossil corals and foraminifera to determine how the concentrations and compositions of organic biomolecules at the calcification site have varied throughout time. We will then precipitate CaCO3 minerals in vitro under conditions replicating those of past and present calcification sites to determine how variations in biomolecules affect mineral chemistry. In particular we will explore how biomolecules interact with other ions at the calcification site over a range of temperature to control mineral chemistry. We will calculate how relationships between coral skeleton and foraminifera test chemistry and seawater temperature have varied throughout time. By applying these calculations to fossils, we will optimise the accuracy of past seawater temperature estimates. We will also use advanced microscopy techniques to visualise the structure of the mineral precipitated under different conditions and to watch the formation of minerals in real time. These observations will help us to understand how variations in the calcification environment affect the incorporation of trace elements and isotopes in calcium carbonate.