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

English title Ground-truthing the diatom-bound nitrogen isotope proxy: Experimental and field studies in the marine and lacustrine environments (NISOPROX)
Applicant Studer Anja
Number 200766
Funding scheme Project funding
Research institution Departement Umweltwissenschaften Universität Basel
Institution of higher education University of Basel - BS
Main discipline Other disciplines of Earth Sciences
Start/End 01.08.2021 - 31.07.2025
Approved amount 560'447.00
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All Disciplines (4)

Discipline
Other disciplines of Earth Sciences
Hydrology, Limnology, Glaciology
Oceanography
Geochemistry

Keywords (13)

Lake Nylandssjön; diatom biomass; sediment trap; isotope effect; Lake Lugano; nitrate; diatom-bound nitrogen isotopes; diatom frustule-bound nitrogen; Lake Sempach; nitrogen cycle in the ocean and lakes; lake sediments; diatom cultures; diagenetic alteration

Lay Summary (German)

Lead
Die Stickstoff (N) Isotopenzusammensetzung von Sedimentablagerungen am See- und Meeresgrund gibt Auskunft über den Nährstoffkreislauf von Ozeanen und Seen in der Vergangenheit. Dies wiederum liefert Informationen über frühere Klimabedingungen und Umweltveränderungen. Das Ziel dieses Projekts ist, systematisch zu untersuchen, inwiefern die N Isotopenzusammensetzung fossiler Kieselalgen (Diatomeen) in Sedimentablagerungen das ursprüngliche, unverfälschte Nährstoffsignal des Oberflächenwassers widerspiegelt und somit als Indikator für die Rekonstruktion vergangener Klima- und Umweltbedingungen genutzt werden kann.
Lay summary

Inhalt und Ziel des Forschungsprojekts

Das übergeordnete Ziel dieses Projekts ist die Isotopenfraktionierung von N in Kieselalgen in Laborexperimenten und in Feldstudien zu untersuchen. Anhand von Algenkulturen im Labor wird die Aufnahme von N aus dem Wasser in die Biomasse der Kieselalge bis hin zur Einbindung in deren Schale verfolgt und es wird untersucht, ob sich die Isotopenzusammensetzung nach deren Tod verändert. Des Weiteren wird die N Isotopenzusammensetzung von Sedimentfallen und Oberflächensedimenten in Seen miteinander verglichen und die Einlagerung in tiefer gelegene Sedimentschichten verfolgt, um den Einfluss von Diagenese auf die N Isotopensignatur von Kieselalgen über längere Zeiträume zu untersuchen.  

Wissenschaftlicher und gesellschaftlicher Kontext

Dieses Forschungsprojekt erlaubt das systematische Testen einer neuen Messgrösse (Proxy) für die Klima- und Umweltforschung der Vergangenheit. Es bildet die Grundlage für zukünftige Studien die zum Ziel haben, den Stickstoffkreislauf der polaren Ozeanregionen sowie der Seen zu untersuchen um frühere Klimabedingungen und Umweltveränderungen auf verschiedenen Zeitskalen zu rekonstruieren. 
Direct link to Lay Summary Last update: 19.04.2021

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Associated projects

Number Title Start Funding scheme
145695 Late Pleistocene evolution of nutrient cycling in the subarctic Pacific and Southern Ocean inferred from diatom-bound nitrogen isotopes 01.06.2013 Fellowships for prospective researchers

Abstract

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.
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