Unravelling the carbon cycle using silicon isotopes in the oceans

点击翻译按钮获取中文摘要

With rising concerns surrounding the impacts of manmade climate change we need to look not only into the future but also into the past. By understanding how global temperatures and levels of carbon dioxide (CO2) in the atmosphere have naturally fluctuated throughout the earth's history, and the interaction with living organisms, we can take important steps towards predicting the changes that may lie ahead. It is undoubtedly a complex puzzle and there are many ways of trying to solve it. My part of the story involves deep-sea sponges and silicon, the chemical element they use to build their glass-like opal skeletons. Sponge skeletons, or spicules, are helping me to piece together the links between the supply of vital nutrients in different parts of the ocean and the crucial role other marine organisms play in absorbing CO2 from the atmosphere and locking it away in deep sea sediments as organic carbon. Marine sponges are one of the simplest groups of animals, living on the seafloor and feeding by filtering particles from seawater. They themselves don't play a central role in changing climates, but they do share a common need for silicon with another group of marine inhabitants that we think are vitally important to the global climate. Diatoms, a type of microscopic marine algae that live and photosynthesise at the sea surface, are responsible for sequestering nearly half the CO2 that is converted into organic carbon and sinks to the seafloor. By investigating sponges I can learn a lot about the changing availability of silicon that also makes life possible for diatoms. Studies of ice-cores and ocean sediments tell us that over the past million years the earth's climate has cycled every 100 thousand years or so between cold ice ages, with low levels of atmospheric CO2, and warmer periods, with higher CO2 levels. My studies focus on the climate changes that took place since the end of the last ice age, around 15 thousand years ago. I can step back in time by combining analysis of living sponges brought up from the deep during research cruises at sea with fossil sponges taken from seafloor sediment samples. I was amongst the first to show that the chemical fingerprints of sponges, in particular their silicon isotope composition, gives an accurate record of how much silicon was dissolved in the water they grew in. This opens up a unique archive stretching back millennia of the silicon levels in ocean waters down to as much as 4 km beneath the waves - the realm of sea sponges. In general, the more silicon there is supplied to the sea surface, the more diatoms can grow, and the more carbon dioxide they absorb and lock away in the seafloor sediments when they die. Building a picture of past levels of silicon in the oceans means I can test the crucial links between carbon dioxide uptake by diatoms and climate change. Here, I plan to study key geographical areas, which have been sensitive to rapid climate change since the last ice age. My work will provide essential insights into the dynamics of the carbon cycle and hence climate, and point to possible future scenarios and changes in ocean circulation patterns.