Hydrogen isotopic composition (δ2H) of diatom-derived C20 highly branched isoprenoids from lake sediments tracks lake water δ2H
2020
Corcoran, Megan C. | Diefendorf, Aaron F. | Lowell, Thomas V. | Freimuth, Erika J. | Schartman, Anna K. | Bates, Benjamin R. | Stewart, Alexander K. | Bird, Broxton W.
The hydrogen isotopic composition of lake water (δ²Hₗw) 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 δ²Hₗw. Instead, most δ²Hₗw 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 (δ²HHBI) reflects the δ²Hₗw. We test this hypothesis by analyzing 78 sediment samples from 12 lakes in the Adirondack Mountains in New York, for HBI concentrations and δ²H. δ²HHBI was compared to δ²Hₗw, which showed an average fractionation (εHBI/ₗw) of −127.3 ± 15.0‰ (1σ) for all samples in all lakes. Consistency in εHBI/ₗw between samples implies that δ²HHBI may be used to reconstruct δ²Hₗw 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 εHBI/ₗw (±11.9‰, 1σ) than samples from shallower zones, suggesting that εHBI/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 (δ¹³CHBI) were higher for sediment samples collected in deeper zones in the lake, suggesting that δ¹³CHBI can be used to further understand differences in HBI synthesis in diatom communities living in different growth habitats.
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