Hydrogen isotopes (δ2H) of diatom-derived C20 highly branched isoprenoids from lake sediments track lake water δ2H

Abstract:

The hydrogen isotopic composition of lake water (δ²H_lw) contains hydrological information, which can yield information about precipitation and evaporation. When applied to sediment archives, records of these processes can be generated to better understand past hydroclimate variability. Unfortunately, few proxies exist that record only δ²H_lw. Instead most of these proxies (e.g. leaf waxes, cellulose) represent a mix of aquatic and terrestrial material, thereby limiting our ability to track δ²H_lw on paleoclimate timescales. Highly branched isoprenoids (HBIs), known to be synthesized by diatoms, offer exciting potential as a proxy for δ²H_lw if the δ²H of the HBIs (δ²H_HBI) can be shown to reflect δ²H_lw. We present preliminary results from 12 lakes in the Adirondacks region in NY that demonstrate δ²H_HBI tracks δ²H_lw with a calculated fractionation (ε) between the HBIs and lake water of –127.3 ± 15.0‰ for all 12 lakes. There is greater consistency (–123.0 ± 11.9‰) between ε in samples taken at deeper (> 4 m) water column zones in the lake whereas samples collected from shallower zones have more negative ε suggesting that other factors (e.g. lake water pH, salinity, diatom growth habitat, and the seasonality of HBI synthesis) may influence fractionation of HBIs. We plan on expanding this initial dataset by including additional δ²H_HBI and δ²H_lw measurements from lakes in the northeastern and midwestern regions of the US in order to explore this variation observed in ε and further validate the utility of HBIs as a sensitive recorder of lake water. We are optimistic that δ²H_HBI reflects δ²H_lw and may provide a new way to reconstruct past hydrological processes in lake systems.