Unravelling the carbon cycle using silicon isotopes in the oceans

随着人们对人为气候变化影响的担忧日益加剧，我们不仅需要展望未来，也需要回顾过去。通过了解全球气温和大气中二氧化碳（CO2）的水平在整个地球历史上是如何自然波动的，以及与生物的相互作用，我们可以在预测未来可能发生的变化方面迈出重要的一步。毫无疑问，这是一个复杂的难题，有很多方法可以解决它。我的故事部分涉及深海海绵和硅，它们用来构建玻璃状蛋白石骨架的化学元素。海绵骨架，或针状体，正在帮助我拼凑出海洋不同地区重要营养物质供应与其他海洋生物从大气中吸收二氧化碳并将其作为有机碳封存在深海沉积物中的关键作用之间的联系。海洋海绵是最简单的动物群体之一，生活在海底，通过过滤海水中的颗粒为生。它们本身并没有在气候变化中发挥核心作用，但它们确实与另一群海洋居民有着共同的硅需求，我们认为它们对全球气候至关重要。硅藻是一种生活在海面上并进行光合作用的微小海洋藻类，它负责将转化为有机碳并沉入海底的近一半的二氧化碳隔离。通过研究海绵，我可以了解到很多关于硅的可用性的变化，硅也使硅藻的生命成为可能。对冰芯和海洋沉积物的研究告诉我们，在过去的100万年里，地球的气候每10万年左右就会在大气中二氧化碳含量低的寒冷冰期和二氧化碳含量较高的温暖期之间循环一次。我的研究重点是自上一个冰河时代结束以来，大约1.5万年前发生的气候变化。我可以通过分析海上研究游船从深海中打捞上来的活海绵和从海底沉积物样本中采集的海绵化石，来追溯过去。我是第一个证明海绵的化学指纹，特别是它们的硅同位素组成，可以准确记录它们生长的水中溶解了多少硅的人之一。这打开了一个独特的档案，可以追溯到几千年前海水中的硅含量，一直到海浪以下4公里处——海绵的领域。一般来说，提供给海面的硅越多，硅藻生长的就越多，它们死后吸收并锁在海底沉积物中的二氧化碳也就越多。绘制出过去海洋中硅含量的图像意味着我可以测试硅藻吸收二氧化碳和气候变化之间的关键联系。在这里，我计划研究自上一个冰河时代以来对快速气候变化敏感的关键地理区域。我的工作将为碳循环和气候的动态提供重要的见解，并指出未来可能的情景和海洋环流模式的变化。

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.