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Pyrogenic carbon abundance and isotope composition can be used to infer past change in tropical vegetation structure and function

Wednesday, 31st July 2019 | Presented by Prof Michael Bird|Australian Laureate Fellow, Distinguished Professor

Pyrogenic carbon (PyC; soot, char, black carbon) is produced by the incomplete combustion of organic matter accompanying biomass burning and fossil fuel consumption. It is pervasive in the environment, distributed throughout the atmosphere as well as soils, sediments and water in both the marine and terrestrial environment. As PyC is derived ultimately from plant material it retains information on the vegetation that was burnt, encoded in its stable carbon isotope composition. PyC preserves well in sedimentary archives because it is relatively resistant to degradation, and microcharcoal particle counting has long been used to generate proxy records of fire incidence in the past. In some circumstances, PyC is relatively easy to isolate but in many others, PyC is very small, ancient, and dispersed in a matrix (e.g. soil or sediment). Hydrogen pyrolysis (HyPy) is a technique that we have optimized for the quantification and isolation of PyC from a variety of matrices for determination of radiocarbon age and stable isotope composition.

Ecosystem d13C values vary widely across the tropics as a result of changes in the balance of vegetation using C4 versus C3 photosynthesis and information on changes in ecosystem C3:C4 balance can be obtained from the development of d13C time series from PyC in sedimentary archives. Stable isotope analysis of PyC in sedimentary archives by HyPy therefore offers the possibility of developing a more nuanced understanding of the interplay between fire regime and vegetation structure/function (and climate) in the tropics, in the past. This talk will provide an introduction to HyPy as an analytical tool and results from modern ground-truthing studies aimed at underpinning the interpretation of ancient PyC d13C time series. It will also present case studies where the technique has been used to develop proxy records of biomass burning and vegetation dynamics, where the results can be compared against particle counting approaches and palynological information

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