The land plant δ¹³C record and plant evolution in the Late Palaeozoic

Abstract

Based on the evaluation of 1323 carbon isotope values for Silurian to Permian terrestrial organic carbon, measured on plant fossils, cuticules, humic coals and bulk terrestrial organic matter (TOM), we conclude that the temporal trend in δ¹³C_TOM records variations in the global carbon cycle, notably an increase in the fractional burial of light (terrestrial) organic matter in Late Palaeozoic sediments. δ¹³C_TOM values suggest that the Late Palaeozoic pO₂ peak could have been restricted to a time frame of ∼ 40 Ma. Carbon isotope data from four taxonomic groups reveal small differences that could be a consequence of habitat conditions. No significant differences in organic carbon isotopic composition in relation to variable climatic conditions are discernible. The carbon isotopic composition solely reflects C₃ plant metabolism.

Introduction

The last decade has witnessed an increasing number of studies related to early land plant evolution, its effect on the cycling of carbon and hence on the climatic history of the earth during Palaeozoic time (Gensel and Edwards, 2001, Hemsley and Poole, 2004). Scholle and Arthur (1980); cited in Robinson and Hesselbo (2004) were among the first to address that shifts in the carbon isotopic composition of marine carbonate carbon should be detectable in land plant organic matter due to the geochemical link of the marine and terrestrial realms via atmospheric carbon dioxide. Since then, the ¹³C/¹²C ratio of fossil plant remains and bulk terrestrial organic matter (TOM) has been used as a proxy for reconstructing carbon cycle dynamics, palaeoecological and palaeoclimatological conditions (Bocherens et al., 1993, Jones, 1994, Faure and Cole, 1999, Nguyen Tu et al., 2002, Gröcke, 2002, Beerling and Royer, 2002, Hesselbo et al., 2003, Berner, 2004) or as a tool for chemostratigraphic correlation (Gorter et al., 1994, Stott et al., 1996, Hansen et al., 2000, Sephton et al., 2002, Ando et al., 2002, Hasegawa, 2003, Heimhofer et al., 2003, Robinson and Hesselbo, 2004).

The aim of our present study was to investigate whether long term shifts and possibly short term variations can be found in land plant organic matter. Additionally we checked isotope variations due to gross climatic conditions and differences related to plant groups.

The Late Palaeozoic was a time of profound geological changes, notably the rapid colonization of terrestrial environments by land plants since the Silurian, a change from greenhouse conditions in the Silurian–Devonian to icehouse conditions in the Permo-Carboniferous, and the final assembly of the Supercontinent Pangea in the Permian. The rise of land plants (as a new carbon sink) removed the greenhouse gas CO₂ from the atmosphere and potentially ushered in the Carboniferous-Permian glacial episode (e.g. Kump et al., 2000, Beerling and Berner, 2005). The evolutionary innovations that caused the increased storage of CO₂ did not appear prior to the Devonian, although the first significant evidence of embryophytes occupying land is from the Middle Ordovician, and the earliest occurrence of vascular plants is Middle Silurian (Gensel and Edwards, 2001, Wellman et al., 2003). These innovations include the production of lignin since the Early Devonian (Boyce et al., 2003a) and the evolution of wood since the Middle Devonian (Rowe, 2000). Wood containing the biopolymer lignin is resistant to biodegradation. Reduced decomposition contributed to increased carbon storage in terrestrial sinks.

The evolution of the carbon isotopic composition of terrestrial organic matter in context with the coeval spread of plant ecosystems is reviewed in this study.