Seasonal Variation in δ¹³C_{<i>n</i>‐alkane} and δ¹³C/ δ¹⁵N of Bulk Leaves of Four Riparian Plants: Implications for Biomarker Carbon Isotope Records of Past Environments

Abstract Carbon isotopes of long chain n ‐alkanes and plant leaves record biochemical processes and plant responses to environmental factors. We analyzed seasonal variations of δ 13 C n ‐alkane and δ 13 C/δ 15 N 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 δ 13 C. Our data highlight three important results: (a) δ 13 C leaf and δ 13 C 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 δ 13 C n ‐C29 and δ 13 C 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 δ 15 N 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 δ 13 C of intact leaves are distinct from early or mid‐season values, whereas detrital n ‐alkane δ 13 C records the season‐long time‐integrated history of production, loss and replacement of riparian‐produced n ‐alkanes. Thus, although biomarker and bulk leaf δ 13 C may record p CO 2 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.