Atmospheric carbon isotope signatures in phytolith-occluded carbon

A new method of deriving an atmospheric δ13CO2 record using phytolith-occluded carbon as a proxy is described here. The method is designed to measure carbon-13 in ancient phytolith-occluded carbon and convert this signal into an atmospheric carbon record of the atmosphere. Phytoliths are very small particles of silica (between 5 and 100 μm) that form distinctive and repeatable shapes in most plants. When phytoliths form within a plant, some of the host organic matter is trapped inside the phytolith. Phytoliths have been shown to contain occluded carbon, and are present in most terrestrial sedimentary deposits. Moreover, because they survive well in most soils and sediments, the trapped carbon remains intact, and preserved from contamination and alteration. When plants assimilate atmospheric CO2 they fractionate against 13C, leaving the atmosphere relatively enriched in 13C. Therefore any record of isotopic change in plant tissue can be used as a proxy for change in atmospheric 13CO2. Experiments were conducted to characterise and measure the natural variability of modern phytolith-occluded carbon. These included measurement of carbon isotopic fractionation effects between the atmosphere and whole plant material, measurement of carbon isotope fractionation between whole plant matter and phytolith-occluded carbon, and a determination of carbon compounds present in phytolith-occluded carbon. A formula was developed and applied to separate the plant physiological factors and the embedded atmospheric 13CO2 from the phytolith-occluded carbon. Thereby, demonstrating that it is possible to derive modern atmospheric 13CO2 values from phytolith carbon.