Application of stable isotopes to terrestrial paleoclimate: tracing interactions among the soil-plant-atmosphere continuum

为了了解人类活动对当前全球气候变化趋势的影响，我们必须首先认识到在整个地球近代历史中自然气候波动的幅度。科学家可以使用过去气候的间接测量来延长相对较短的历史气候记录。在化石材料中发现的特定元素的数量可以提供有关它们过去形成时的气候条件的信息。矿物质、水和有机物含有常见的元素，如氧、碳和铁，它们由不止一种稳定的同位素组成。同位素是同一元素的原子，它们的区别只是质量不同，通常是化学反应速率不同。同一元素的两种同位素之间反应速率差异的大小对温度和与光合作用等生物功能相关的动力学效应很敏感，而光合作用对气候也很敏感。例如，植物中重碳同位素与轻碳同位素的比例在温暖季节的草中比在寒冷气候中茁壮成长的树木要大。因此，土壤中保存的有机物的碳同位素比率可以用来确定过去在这些土壤上生长的暖季与冷季植被的存在。同样，植物中作为溶质吸收的副产品而形成的矿物质，如果生长在温暖的气候而不是寒冷的气候中，其重氧同位素与轻氧同位素的比例会更高。我的研究项目是评估气候和植物生物学对通过土壤-植物-水系统循环的自然物质的同位素变化的影响。本研究将集中研究保存完好的植物残留物，如草细胞中沉淀的矿物质、纤维素、土壤中的腐殖质和植物腐烂产生的富铁矿物。我们的目标是开发和完善将古代植物材料的同位素组成与古代大陆气候联系起来的模型。除了古气候应用之外，土壤中保存的生物物质的稳定同位素分析为了解现在和过去的生物地球化学循环提供了一个独特的机会。

In order to understand the impact of human activity on the current trends in global climate change, we must first recognize the amplitude of natural climatic fluctuations throughout earth's recent history.  Scientists can use indirect measures of past climate to extend the relatively short historical climatic record.  The amounts of specific elements found in fossilized materials can provide information regarding the climatic conditions during their formation in the past.  Minerals, water and organic matter contain common elements, such as oxygen, carbon and iron that are composed of more than one stable isotope.  Isotopes are atoms of the same element that differ only in their mass, and commonly in their rate of chemical reaction.  The magnitude of the difference in reaction rates between two isotopes of the same element is sensitive to temperature and kinetic effects associated with biological functions such as photosynthesis, which are also sensitive to climate.  For example the ratio of heavy to light carbon-isotopes in plants is greater for warm-season grasses than trees that thrive in cooler climates.  It follows then that the carbon-isotope ratio of organic matter preserved in soils may be used to identify the presence of warm- versus cool-season vegetation growing on those soils in the past.  Likewise, minerals formed in plants as a by-product of solute-uptake, will have a higher ratio of heavier to light oxygen isotopes if grown in warm climates rather than cool climates.   My research program assesses the effects of climate and plant biology on the isotopic variations of natural materials cycled through the soil-plant-water system.  This study will focus on well-preserved plant residues in soils, such as minerals precipitated in the cells of grasses, cellulose, humic substances in soils and iron-rich minerals produced by plant decay.  Our goal is to develop and refine models that relate the isotopic composition of ancient plant materials to ancient continental climates.  In addition to paleoclimate applications, the stable isotopic analysis of biogenic materials preserved in soils provides a unique opportunity to learn about present and past biogeochemical cycles.