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

Andrew C Smith; Christopher P Kendrick; Vicky L Moss-Hayes; Christopher H Vane; Melanie J Leng

doi:10.1002/rcm.7755

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

Rapid Commun Mass Spectrom. 2017 Jan 15;31(1):21-26. doi: 10.1002/rcm.7755.

Authors

Andrew C Smith¹, Christopher P Kendrick¹, Vicky L Moss-Hayes², Christopher H Vane², Melanie J Leng^{1

3}

Affiliations

¹ NERC Isotope Geosciences Facilities, British Geological Survey, Nottingham, NG12 5GG, UK.
² British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK.
³ Centre for Environmental Geochemistry, University of Nottingham, Nottingham, NG7 2RD, UK.

PMID: 27699888
DOI: 10.1002/rcm.7755

Abstract

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.