Observed seasonal trends of diatom-derived C20 highly branched isoprenoids (HBIs): implications for paleoclimate studies

The hydrogen isotopic composition of lake water (δ²H_lw) contains hydrologic information and can be used as a recorder of lake water hydrology, including the extent of evaporation of the lake system. Initial studies indicate that the hydrogen isotopes of highly branched isoprenoids (δ²H_HBI), synthesized by lake diatoms and preserved in lake sediments are a promising proxy for constraining past δ²H_lw values that are free from terrestrial influences. However, there are many aspects of this proxy, including the seasonality of HBI production, that are unknown and need to be addressed more fully before the proxy can by widely applied. To determine when HBIs are produced throughout the year, and whether there are seasonal biases in δ²H_lw reconstructions, we deployed two sediment traps at Brown’s Lake, in northeastern Ohio. We present HBI concentrations, δ²H_HBI values, HBI carbon isotopes and bulk sediment carbon isotopes from sediment traps collected monthly for 26 months to investigate seasonality of HBIs. We observed HBIs in each of the monthly sediment traps throughout the study interval with an increase in HBI concentration during September and October, suggesting that HBIs are made throughout the year with greater production during fall. We calculated the difference between δ²H_HBI and δ²H_lw values (ε²H_HBI/lw) and observe a range in ε²H_HBI/lw values of up to 64‰, which we speculate is related to changes in the diatom communities that synthesize HBIs throughout the year and between different years. Different diatom communities may have different biosynthetic pathways or metabolisms that result in isotope effects. This study is the first that examines the seasonality of HBIs in lake sediments and provides framework for interpreting the seasonality of hydroclimate records generated from δ²H_HBI values in temperate eutrophic lakes.