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

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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.