Seasonal Variation in δ13Cn-alkane and δ13C/ δ15N of Bulk Leaves of Four Riparian Plants: Implications for Biomarker Carbon Isotope Records of Past Environments

Carbon isotopes of long chain n-alkanes and plant leaves record biochemical processes and plant responses to environmental factors. We analyzed seasonal variations of δ¹³C_n-alkane and δ¹³C/δ¹⁵N_leaf of two riparian gymnosperm trees (Pinus strobus and Tsuga canadensis) and an angiosperm shrub and grass (Corylus americana and Phalaris arundinacea) to quantify carbon isotope discrimination between atmospheric, bulk leaf, and n-alkane δ¹³C. Our data highlight three important results: (a) δ¹³C leaf and δ¹³C_n-alkane decrease over the growing season for several of the sampled plants and the magnitude of change is largest in the angiosperm C. americana, (b) apparent fractionation between δ¹³C_n-C29 and δ¹³C_leaf (ε_n-C29-leaf) for gymnosperm trees is small (−2 to −3 ‰) and changes little through the growing season, and (c) angiosperm grass and shrubs exhibit variable discrimination (ε) throughout the growing season (−4 to −10‰). ε_n-C29-leaf correlates with leaf C:N and δ¹⁵N_leaf, and differences in ε_n-C29-leaf likely result from differences in stomatal regulation, plant stoichiometry, and the distribution of compounds in leaves and above or below ground biomass. In some of the plants sampled, end of growing season δ¹³C of intact leaves are distinct from early or mid-season values, whereas detrital n-alkane δ¹³C records the season-long time-integrated history of production, loss and replacement of riparian-produced n-alkanes. Thus, although biomarker and bulk leaf δ¹³C may record pCO₂ or water stress, isotopic signatures of sedimentary n-alkanes may also reflect changes in plant resource allocation and the integrated record of isotopic change across a growing season.