LAKE SEDIMENT INSIGHTS INTO BIOGEOCHEMICAL CYCLING UNDER DIFFERENT REDOX ENVIRONMENTS

With rising surface water temperatures due to climate change, lakes are expected to experience widespread, prolonged periods of seasonal stratification and low-oxygen conditions. Changes to hypolimnetic oxygen levels will affect the composition and preservation of organic matter. The organic carbon (δ¹³C_org) and bulk nitrogen (δ¹⁵N) isotope composition of sedimentary organic matter has been widely used to trace biogeochemical cycling in lacustrine environments. This study aims to determine associations between water column oxygenation and lake sediment isotope composition by compiling a global dataset of 217 lakes with 43 anoxic and 174 oxic lakes. Anoxic and oxic lake sediments form a dataspace ranging from δ¹⁵N: +10 to -10‰ and δ¹³C_org: -31 to -20‰. Anoxic lakes contain more positive and negative values for δ¹³C_org likely reflective of a diversity of microbial metabolisms under anaerobic waters. Oxic lakes contain more negative values for δ¹⁵N potentially indicating atmospheric or anthropogenic influences. Based on a subset of anoxic (n = 13) and oxic (n = 21) lakes containing a wider array of physiographic parameters, anoxic lakes show a negative correlation between lake elevation and δ¹³C_org as well as δ¹⁵N, meanwhile, oxic lakes exhibit a positive correlation between δ¹⁵N and lake elevation. Therefore, while C and N cycles are typically considered to be tightly coupled, we find important differences related to oxygenation that have implications for understanding future biogeochemical cycling as well as for interpreting the rock record. Paleolake formations (n = 11) often plot within the modern data array, but lakes reconstructed as alkaline (pH > 9) can have more extreme δ¹⁵N and δ¹³C_org values than any modern lakes, and typically have δ¹⁵N values values in excess of 10‰, supporting previous studies that have suggested this threshold for alkaline paleolake deposits. Future work will focus on using this dataset to identify geochemical and lake morphological variables that drive isotopic trends under different lake oxygen conditions.