HYDROGEN ISOTOPES OF DIATOM-DERIVED C20 HIGHLY BRANCHED ISOPRENOIDS FROM LAKE SEDIMENTS TRACK LAKE WATER HYDROGEN ISOTOPES

The hydrogen isotopic composition of lake water (δ²H_lw) contains hydrologic information, which can yield records of past evaporation and precipitation changes when studied from lake sediment archives. Unfortunately, few proxies exist that record only δ²H_lw and instead represent a mix of aquatic and terrestrial material. Diatoms are single-celled microalgae that produce highly branched isoprenoids (HBIs) in marine and lacustrine settings. We present a pilot dataset that indicates that the hydrogen isotopic composition of HBIs (δ²H_HBI) is a novel way to track δ²H_lw. A total of 42 sediment samples from 10 lakes in the Adirondacks in NY were analyzed for δ²H_HBI and compared to δ²H_lw to determine that the average fractionation (ε_HBI/lw) between HBIs and lake water is -127+16‰. There is a large consistency between ε_HBI/lw in samples taken at deeper (>4m) lake water depths whereas samples collected from shallow depths have a larger ε_HBI/lw suggesting that other factors may influence HBI fractionation. Further work needs to be done to determine what factors such as salinity, seasonality, pH and diatom community compositions, might also influence fractionation. We are optimistic that the δ²H of HBIs can be used to reconstruct lake water δ²H through time to in order to better assess how lake systems have changed in the past.