Compound-specific δD– δ13C analyses of n-alkanes extracted from terrestrial and aquatic plants

Abstract

Stable hydrogen and carbon isotopic compositions of individual n-alkanes were determined for various terrestrial plants (33 samples including 27 species) and aquatic plants (six species) in natural environments from Japan and Thailand. In C3 plants, n-alkanes extracted from angiosperms have a δD value of −152±26‰ (relative to Standard Mean Ocean Water [SMOW]) and δ 13C value of −36.1±2.7‰ (relative to Peedde Belemnite [PDB]), and those from gymnosperms have a δD value of −149±16‰ and δ 13C value of −31.6±1.7‰. Angiosperms have n-alkanes depleted in 13C relative to gymnosperms. n-Alkanes from C4 plants have a δD value of −171±12‰ and δ 13C value of −20.5±2.1‰, being a little depleted in D and much enriched in 13C compared to C3 plants. n-Alkanes of CAM plants are a little depleted in D and vary widely in δ 13C relative to those of C3 and C4 plants. In aquatic plants, n-alkanes from freshwater plants have a δD value of −187±16‰ and δ 13C value of −25.3±1.9‰, and those from seaweeds have a δD value of −155±34‰ and δ 13C value of −22.8±1.0‰. All n-alkanes from various plant classes are more depleted in D and 13C relative to environmental water and bulk tissue, respectively. In addition, the hydrogen and carbon isotopic fractionations during n-alkane synthesis are distinctive for these various plant classes. While C3 plants have smaller isotopic fractionations in both D and 13C, seaweed has larger isotopic fractionations.