Bioavailable nitrogen (N) controls marine biological productivity and thus the capacity of the global ocean to sequester atmospheric CO₂ in the abyss through the production and remineralization of sinking algal organic matter. In lakes, high concentrations of bioavailable N cause eutrophication, increased algal growth, and in turn oxygen loss. Past changes in the input/output and internal cycling of fixed N (e.g., nitrate) in the marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (the ¹⁵N/¹⁴N ratio, or d¹⁵N) of organic matter in the sedimentary record. Bulk sedimentary d¹⁵N signatures, however, can be biased by secondary alteration and external (e.g., terrestrial) N inputs, so that recently, the focus has shifted to measuring the d¹⁵N of organic N that is trapped and protected in the mineral structure of (micro-)fossils, such as diatoms, foraminifera and corals, which is thought to record the pristine N isotope signature of nitrate in the surface water. Yet, the validity of these new N isotope proxies is still under scrutiny, as the exact modulating controls during microfossil-bound N isotope signature generation remain uncertain.

The overarching goal of the proposed study is to ground-truth the diatom-bound N isotope paleo-proxy in the marine and lacustrine environments through a combination of experimental and field studies. In a first work package, we want to investigate how the d¹⁵N signature of diatom frustule-bound N is acquired (i.e., how well it tracks the nitrate source) by determining the relationships among the d¹⁵N values of the nitrate source to the diatoms, the d¹⁵N of the bulk diatom biomass, and the d¹⁵N of diatom-bound N in laboratory diatom culture experiments, as well as in the modern ocean water column and in lakes. Thereby, we will also attempt to assess the effects of changing environmental conditions and diatom assemblages. In a second work package, focusing on lacustrine sediments, we will examine whether fractional decomposition in the water column and/or diagenetic (i.e., altering) effects in the sediment during early burial alters the pristine N content and the d¹⁵N signature of diatom-bound N over time. Towards this goal, we propose combined N isotope analyses of sediment trap, surface sediment, and downcore sediment material from a time-series of varved sediment cores from a lake in Sweden, as well as degradation experiments of diatom cultures. Finally, in a third work package, we want to explore, for the first time, the application of the diatom-bound d¹⁵N proxy in lacustrine sediments of Swiss lakes as a recorder of the eutrophication history over the past century.

The proposed research will assess the integrity of diatom-bound N as a proxy for paleoenvironmental change in marine and lacustrine sediments. Furthermore, the combined analyses of bulk sediment d¹⁵N and diatom-bound d¹⁵N in the downcore records will shed light on the effects of early diagenesis on bulk sedimentary organic matter, and will allow us to reevaluate the use of bulk sediment d¹⁵N as a proxy for reconstructing the past N cycle in the oceans and lakes.