Environmental influences on the stable carbon isotopic composition of Devonian and Early Carboniferous land plants

Abstract

Systematic analysis of the stable carbon isotopic composition of fossil land plants ($δ^{13}$C$_\textrm{p}$) has the potential to offer new insights regarding paleoclimate variation and plant-environment interactions in early terrestrial ecosystems. $δ^{13}$C$_\textrm{p}$ was measured for 190 fossil plant specimens belonging to 10 genera of Early to Late Devonian age (Archaeopteris, Drepanophycus, Haskinsia, Leclercqia, Pertica, Psilophyton, Rhacophyton, Sawdonia, Tetraxylopteris, and Wattieza) and 2 genera of Early Carboniferous age (Genselia and Rhodeopteridium) collected from sites located mainly in the Appalachian Basin (22–30°S paleolatitude). For the full carbon-isotopic dataset (n = 309), $δ^{13}$C$_\textrm{p}$ ranges from −20.3‰ to −30.5‰ with a mean of −25.5‰, similar to values for modern C$_3$ land plants. In addition to a secular trend, $δ^{13}$C$_\textrm{p}$ exhibits both intra- and intergeneric variation. Intrageneric variation is expressed as a small (mean 0.45‰) $^{13}$C-enrichment of leaves and spines relative to stems that may reflect differential compound-specific compositions. Intergeneric variation is expressed as a much larger (to ~5‰) spread in the mean $δ^{13}$C$_\textrm{p}$ values of coeval plant genera that was probably controlled by taxon-specific habitat preferences and local environmental humidity. Among Early Devonian taxa, Sawdonia yielded the most $^{13}$C-depleted values (−27.1 ± 1.7‰), reflecting lower water-use efficiency that was probably related to growth in wetter habitats, and Leclercqia, Haskinsia, and Psilophyton yielded the most $^{13}$C-enriched values (−23.0 ± 1.6‰, −22.3 ± 1.3‰, and −24.8 ± 1.6‰, respectively), reflecting higher water-use efficiency probably related to growth in drier habitats.