USING SAMPLED PALYNOMORPHS TO INVESTIGATE CHANGES IN δ13C AND δ15N AS A RESULT OF CHEMICAL TREATMENT AND LABORATORY HEATING

The analysis of carbon and nitrogen isotopes of organic matter has become a common method for investigating paleoclimate. As the precision of mass spectrometry develops, instruments become capable of running smaller samples, and it has become possible to analyse specific palynomorphs, rather than bulk organic matter (OM). In most instances bulk OM in rock contains a mixture of organic species, often of marine and terrigenous origin and each with their own individual isotopic composition.

The isotopic composition of samples can be altered by thermal maturation, weathering and chemical treatment which is often used to extract OM from the inorganic rock matrix. Several accounts have been published describing the effects of acid treatment, thermal maturation and weathering of kerogen or bulk OM. However, little research has been completed on the effects of all three of these processes on a single type of palynomorph.

In this study, specimens of the fossil alga Tasmanites were subjected to laboratory heating (simulating thermal maturation) and a range of chemical treatments, to ascertain what effect these procedures would have on its carbon and nitrogen isotope composition. The observed variability of δ13C was of insignificant magnitude when compared with published δ13C excursions attributed to environmental changes. However, oxidation (simulating the oxidative effects of weathering) had a pronounced affect on δ13C (+0.73‰), heating led to slightly less negative δ13C values, and some acid treatments caused more isotopic fractionation (for example hot HF) of carbon than others (for example HCOOH). Nitrogen isotope fractionation due to heating and chemical treatment was much greater than that for carbon, recording significant variation in δ15N compared with published excursions attributed to environmental changes. Heating to 125 °C caused the δ15N of the Tasmanites to become 1.7‰ more positive, relative to the control sample. Most chemical treatments produced similar results, with shifts of between +1.26‰ and -1.97‰ δ15N.