From a physiological standpoint changes in carbon isotope composition of plant tissue is linked to changes in water use efficiency of the plants that are ultimately controlled by the opening and closure of the stomatal pores which regulates gas exchange. For carbon isotopes to be used as an accurate and precise method to reconstruct CO2 the major requirement is to demonstrate that changes in CO2 are the main driver of changes in the carbon isotopes. This relationship needs to be independent of other environmental conditions that can affect the way plants use water such as temperature and specifically the amount of water availability.
|Arabidopsis thaliana (common name Thale cress), |
a small flowering weed used as a model plant
in many science fields. Copyright Wikipedia.
We compared the concentration of CO2 in the growing environment to the values predicted by the carbon isotope composition of Thale cress. The data show that there is a wide variation in carbon isotope composition of Thale cress as a function of water availability and actually the amount of CO2 in the atmosphere was less important. In particular there was a strong under prediction of CO2 in experiments designed to simulate very high levels of CO2 in the atmosphere of the Mesozoic and Cenozoic (250 - 2 millions of years ago) eras (≥1500ppm). Our experiment casts doubt on the use of carbon isotopes in plant material as a proxy to reconstruct palaeoatmospheric CO2 and suggests other aspects of the growth environment are probably more important on the carbon isotope composition of plant matter. For now other proxies probably provide more viable data of palaeoCO2, these include changes in stomatal frequency, the Br isotopic composition of foraminifera, the carbon isotopic composition of marine carbon including sedimentary alkenones, dinoflagellate cysts and coccoliths…
Dr Barry Lomax is a lecturer in Environmental Science at the University of Nottingham and his research is focused on quantifying how the Earth's climate has changed over geologic time, how these changes have influenced the Earth's terrestrial biosphere and how in turn the Earth's terrestrial biosphere has influenced climate. Dr Janice Lake is an Independent Research Fellow at the University of Sheffield focussing on plant physiological responses to atmospheric CO2. Dr Phillip Jardine is a researcher at the University of Münster with interests in the fossil pollen and the development of palaeoclimate proxies. Prof Melanie Leng is the Director of the Centre for Environmental Geochemistry at the British Geological Survey and the University of Nottingham and leads a lab group in stable isotope geochemistry. Twitter @MelJLeng