Carbon isotope alteration during the thermal maturation of non-flowering plant species representative of those found within the geological record.

RATIONALE: The carbon isotope (δ13 C value) composition of fossil plant material is routinely used as a proxy of past climate and environment change. However, palaeoclimate interpretation requires assumptions about the stability of δ13 C values in plant material during its decomposition and incorporation into sediments. Previous work on modern angiosperm species shows δ13 C changes of several per mille during simulated decomposition experiments. However, no such tests have been undertaken on non-flowering plants, which are found extensively within the geological record. These plants have distinctly different cellulose-to-lignin ratios from those of their angiosperm counterparts, potentially creating hitherto unknown variations in the original to fossil δ13 C signatures. METHODS: To test the extent of δ13 C change during decomposition we have subjected a number of plants, representing more basal, non-flowering plant lineages (cycads, ferns, horsetails and dawn redwood), to artificial decay using a hydrothermal maturation technique at two temperatures over periods of up to 273 hours. Subsamples were extracted every 12-16 hours and analysed for their δ13 C and %C values using a Carlo Erba 1500 elemental analyser, and VG TripleTrap and Optima mass spectrometers. RESULTS: The %C values increased for all samples through the maturation process at both temperatures with the largest increases observed within the first 24 hours. Decreases in δ13 C values were observed for all plants at 300°C and for two of the species at the lower temperature (200°C). The maximum shift in the δ13 C value related to experimental decomposition was -0.90‰ (horsetail), indicating a preferential loss of 13 C during thermal maturation. CONCLUSIONS: The reduction in the δ13 C value potentially suggests a preferential loss of isotopically heavier cellulose in relation to the isotopically lighter lignin component during maturation. The isotopic offset observed here (<0.9‰) means that palaeoclimatic interpretation of δ13 C values from non-flowering plant material within the geological record remains robust, but only where interpretations are based on variations in δ13 C values greater than 1‰. Copyright © 2016 John Wiley & Sons, Ltd.