Hydrogen isotopic composition (delta H-2) of diatom-derived C-20 highly branched isoprenoids from lake sediments tracks lake water delta H-2

摘要

The hydrogen isotopic composition of lake water (delta H-2(lw)) reflects hydrological processes, which can yield information about evaporation and precipitation changes through time when preserved in lake sediment archives. Unfortunately, few proxies exist that record only delta H-2(lw). Instead, most delta H-2(lw) records represent a mix of aquatic and terrestrial material. Highly branched isoprenoids (HBIs), known to be produced by diatoms in marine and lacustrine settings, may be used as a lake water proxy to directly reconstruct hydroclimate, if the hydrogen isotopic composition of HBIs (delta H-2(HBI)) reflects the delta H-2(lw). We test this hypothesis by analyzing 78 sediment samples from 12 lakes in the Adirondack Mountains in New York, for HBI concentrations and delta H-2. delta H-2(HBI) was compared to delta H-2(lw), which showed an average fractionation (epsilon(HBI/lw)) of similar to 127.3 +/- 15.0 parts per thousand (1 sigma) for all samples in all lakes. Consistency in epsilon(HBI/lw) between samples implies that delta H-2(HBI) may be used to reconstruct delta H-2(lw) through time, to help assess how lake systems have changed in the past. Sediment samples collected from deeper (>4 m) zones within the lake had smaller variability in epsilon(HBI/lw) (+/- 11.9 parts per thousand, 1 sigma) than samples from shallower zones, suggesting that epsilon(HBI/lw) may be sensitive to other factors, such as light availability, which may be related to differences in diatom growth habit (e.g., benthic, planktonic). Similarly, the carbon isotopes of HBIs (delta C-13(HBI)) were higher for sediment samples collected in deeper zones in the lake, suggesting that delta C-13(HBI) can be used to further understand differences in HBI synthesis in diatom communities living in different growth habitats. (C) 2020 Elsevier Ltd. All rights reserved.