Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation delta H-2 in the Adirondack Mountains, NY (USA)

摘要

The hydrogen isotope values of plant waxes (delta H-2(wax)) primarily reflect plant source water. delta H-2(wax) preserved in lake sediments has therefore been widely used to investigate past hydroclimate. The processes by which plant waxes are integrated at regional and catchment scales are poorly understood and may affect the delta H-2(wax) values recorded in sediments. Here, we assess the variability of sedimentary delta H-2(wax) for two plant wax compound classes (n-alkanes and n-alkanoic acids) across 12 lakes in the Adirondack Mountains that receive similar regional precipitation delta H-2(wax) but vary at the catchment-scale in terms of vegetation structure and basin morphology. Total long-chain (n-C-27 to n-C-35) alkane concentrations were similar across all sites (191 +/- 53 mu g/g TOC) while total long-chain (n-C-28 and n-C-30) alkanoic acid concentrations were more variable (117 +/- 116 mu g/g TOC) and may reflect shoreline vegetation composition. Lakes with shorelines dominated by evergreen gymnosperm plants had significantly higher concentrations of long-chain n-alkanoic acids relative to n-alkanes, consistent with our observations that deciduous angiosperms produced more long-chain n-alkanes than evergreen gymnosperms (471 and 33 mu g/g TOC, respectively). In sediments, the most abundant chain lengths in each compound class were n-C-29 alkane and n-C-28 alkanoic acid, which had mean delta H-2 values of -188 +/- 6 parts per thousand and -164 +/- 9 parts per thousand, respectively. Across sites, the range in sedimentary n-C-29 alkane (22 parts per thousand) and n-C-28 alkanoic acid delta H-2 (35 parts per thousand) was larger than expected based on the total range in modeled mean annual precipitation delta H-2 (4 parts per thousand). We observed larger mean epsilon(app) (based on absolute values) for n-alkanes (-123 parts per thousand) than for n-alkanoic acids (-97 parts per thousand). Across sites, the delta H-2 offset between n-C-29 alkane and the biosynthetic precursor n-C-30 alkanoic acid (epsilon(C29- C30)) ranged from -8 to -58 parts per thousand, which was more variable than expected based on observations in temperate trees (-20 to -30 parts per thousand). Sediments with greater aquatic organic matter contributions (lower C/N ratios) had significantly larger (absolute) epsilon(C29-C30) values, which may reflect long-chain n-alkanoic acids from aquatic sources. Concentration and delta H-2(wax) data in Adirondack lakes suggest that long-chain n-alkanes are more sensitive to regional-scale precipitation signals, while n-alkanoic acids are more sensitive to basin-scale differences in catchment vegetation and wax sourcing. (C) 2019 Published by Elsevier Ltd.