Hydrogen isotopes of n-alkanes and n-alkanoic acids as tracers of precipitation in a temperate forest and implications for paleorecords

作者：Freimuth Erika J^*; Diefendorf Aaron F; Lowell Thomas V

来源：Geochimica et Cosmochimica Acta, 2017, 206: 166-183.

DOI：10.1016/j.gca.2017.02.027

• Paleohydrology
• Leaf wax
• Hydrogen isotope
• Apparent fractionation
• Angiosperms

The hydrogen isotopic composition of leaf waxes (delta D-wax) primarily reflects that of plant source water. Therefore, sedimentary delta D(wa)x records are increasingly used to reconstruct the delta D of past precipitation (delta D-p) and to investigate paleohydrologic changes. Such reconstructions rely on estimates of apparent fractionation (epsilon(app)) between delta D-p and the resulting delta D-wax. However, epsilon(app) values are modified by numerous environmental and biological factors during leaf wax production. As a result, epsilon(app) can vary widely among plant species and growth forms. This complicates estimation of accurate epsilon(app) values and presents a central challenge to quantitative leaf wax paleohydrology. During the 2014 growing season, we examined epsilon(app) in the five deciduous angiosperm tree species (Prunus serotina, Acer saccharinum, Quercus rubra, Quercus alba, and Ulmus americana) that dominate the temperate forest at Brown's Lake Bog, Ohio, USA. We sampled individuals of each species at weekly to monthly intervals from March to October and report delta D values of n-C-29 alkanes (delta Dn-C29 (alkane)) and n-C-28 alkanoic acids (delta Dn-C28 (acid)), as well as xylem (delta D-xw) and leaf water (delta D-lw). n-Alkane synthesis was most intense 2-3 weeks after leaf emergence and ceased thereafter, whereas n-alkanoic acid synthesis continued throughout the entire growing season. During bud swell and leaf emergence, delta D-lw was a primary control on delta Dn-C29 (alkane) and delta Dn-C28 (acid) values, which stabilized once leaves became fully expanded. Metabolic shifts between young and mature leaves may be an important secondary driver of delta D-wax changes during leaf development. In mature autumn leaves of all species, the mean epsilon(app) for n-C29 alkane (-107%) was offset by approximately-19% from the mean epsilon(app) for n-C28 alkanoic acid (-88%). These results indicate that in temperate settings n-alkanes and n-alkanoic acids from deciduous trees are distinct with respect to their abundance, timing of synthesis, and epsilon(app) values.

• 出版日期2017-6-1