Can we use carbon isotopes to tell us about past levels of CO2 in the atmosphere?...by Barry Lomax and Melanie Leng

This carbon cycle diagram shows the storage and annual exchange of carbon
between the atmosphere, hydrosphere and geosphere in gigatons - or billions of
tons - of carbon (GtC). Carbon isotopes of plant materials from the geological
record have been used in the past to predict past CO₂. Our research suggests
more work needs to be done to understand this proxy.
This is a public domain image from Wikipedia.

Dr Barry Lomax and Prof Melanie Leng are isotope geochemists who work on understanding how the isotopic composition of environmental materials can tell us about past environments. Here they blog about their new paper, available via open access in the premiere geochemistry journal (Geochimica et Cosmochimica Acta), co-authored by Dr Janice Lake and Dr Phillip Jardine on the use of carbon isotopes in plant materials to predict atmospheric CO₂. The paper sets out to test this relationship to determine if it could be used as a tool for estimating changes in atmospheric CO₂ concentrations through geological time.
  
Understanding both the long term carbon cycle and rapid perturbations in atmospheric CO₂ observed through the geological record has become increasingly important as we enter a period of human induced carbon cycle variations especially the rapidly increasing atmospheric CO₂. A major limiting step in understanding the climate system sensitivity to changes in atmospheric CO₂ over geological time has been the variability in modelled solutions of “palaeo-CO₂” concentration which vary considerably between different types of modelling solutions. To validate estimates of past CO₂ we need CO₂ proxies, those currently most commonly used include the study of the numbers of stomata in fossil leaves, or the carbon isotope chemistry of fossil organisms. The carbon isotope composition of fossil plant material is thought to provide a direct measure of the past atmospheric CO₂ concentrations through geological time, however there is extensive scientific controversy and debate in the literature. We set about to validate this approach which if successful would allow us to assess changes in amount of CO₂ in the atmosphere from the dawn of vascular plants in the Lower Palaeozoic which would help us predict the responses of modern plants to changing CO₂ concentrations.

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.

In our experiment we used Arabidopsis thaliana (common name Thale cress), a small flowering plant native to Eurasia and Africa, where it is considered a weed as it readily colonises roadsides and disturbed land and has a short life cycle. Thale cress has a relatively small genome, and it was the first plant to have its genome sequenced. It is used to understand molecular biology of many plant traits, including flower development and light sensing. Thale cress is an ideal experimental plant as its short life span allows for many experiments to be carried out within a short period of time. In our experiments, the Thale cress was exposed to different watering regimes (low, medium and saturated) and grown over a wide range of CO₂ concentrations (from 380 to 3000ppm; the current amount of CO₂ in the atmosphere is over 400ppm and climbing rapidly) relevant to conditions what plants would have been subjected to through plant evolution.

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