Ground-truthing the diatom-bound nitrogen isotope proxy: Experimental and field studies in the marine and lacustrine environments (NISOPROX)

Bioavailable nitrogen (N) controls marine biological productivity and thus the capacity of the global ocean to sequester atmospheric CO 2 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 15 N/ 14 N ratio, or d 15 N) of organic matter in the sedimentary record. Bulk sedimentary d 15 N 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 d 15 N 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 d 15 N signature of diatom frustule-bound N is acquired (i.e., how well it tracks the nitrate source) by determining the relationships among the d 15 N values of the nitrate source to the diatoms, the d 15 N of the bulk diatom biomass, and the d 15 N 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 d 15 N 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 d 15 N 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 d 15 N and diatom-bound d 15 N in the downcore records w ill shed light on the effects of early diagenesis on bulk sedimentary organic matter, and will allow us to reevaluate the use of bulk sediment d 15 N as a proxy for reconstructing the past N cycle in the oceans and lakes.