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Evaluating the contribution of Marine Aerosols to the Mo Surface Water Cycle

English title Evaluating the contribution of Marine Aerosols to the Mo Surface Water Cycle
Applicant Nägler Thomas
Number 188461
Funding scheme Project funding (Div. I-III)
Research institution Gruppe Isotopengeologie Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geochemistry
Start/End 01.11.2019 - 30.04.2022
Approved amount 343'520.00
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All Disciplines (2)

Discipline
Geochemistry
Climatology. Atmospherical Chemistry, Aeronomy

Keywords (5)

geochemical proxy; paleo-oceanography; Aerosols; Mo-Isotopes; anthropogenic input

Lay Summary (German)

Lead
Die Molybdän-Konzentration [Mo] und Mo-Isotopenzusammensetzung (d98Mo) in Aerosolen werden erstmals eingehend untersucht. Dazu soll Schnee in unterschiedlichem Abstand zur Küste und aus unterschiedlichen Höhenlagen beprobt werden. Die erwarteten Ergebnisse sind von grosser Bedeutung für die Unterscheidung zwischen natürlichem und anthropogenem Mo in der Umwelt und zur Rekonstruktion Umweltbedingungen in der geologischen Vergangenheit.
Lay summary

Inhalt und Ziel des Forschungsprojekts

d98Mo Werte in Flusswasser unterscheiden sich deutlich von denen in Gesteinen. Die gängige  Erklärung durch Fraktionierungsprozesse in Böden ist im Widerspruch zu Massenbilanzrechnungen. Eine bisher nicht untersuchte Mo Quelle ist der Eintrag durch marine Aerosole, deren Ursprung (Ozeanwasser) die nötige Isotopen-Zusammensetzung hat. Die Kombination mit 87Sr - und Elementdaten ist ein neuer Weg um Mo aus marinen Aerosolen von Mo aus anderen Quellen (z.B. industriellen) zu unterscheiden und zu quantifizieren.

Um effizient die zur d98Mo Analyse erforderliche Menge von 10 - 20 kg Wasser Mo zu erhalten werden Schneeproben genommen. Drei  Lokalitäten wurden ausgewählt: a) Jungfraujoch – aufgrund seiner Höhe kaum durch anthropogene Quellen belastet -> natürliches Mo-Signal in Zentraleuropa, b) Region Strengbach (Vogesen, Fr) –> gut charakterisierter zentraleuropäischer Standort mit Niederschlagsanalysen aus über 30 Jahren c) Ostküste Kanadas –> viel Niederschlag, starker mariner Eintrag.

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Die Charakterisierung von marinen Aerosolen kann verwendet werden, um anthropogene Mo Quellen (= Verschmutzung) zu identifizieren. Daraus ergeben sich auch Rückschlüsse auf andere Schwermetalle in der Umwelt.

Zudem kann diese Charakterisierung benutzt werden um die Redoxverhältnisse in vorzeitlichen Ozeanen zu rekonstruieren, die mit der Entwicklung des Lebens im Zusammenhang stehen.

Direct link to Lay Summary Last update: 04.10.2019

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
160034 Turning Points in Earth History 01.07.2015 Project funding (Div. I-III)
126759 Investigation on the control of the Mo isotope signature of river water: approaching a refined global Mo model for paleredox reconstructions 01.10.2009 Project funding (Div. I-III)

Abstract

Evaluating the contribution of Marine Aerosols to the Mo Surface Water CycleHere, I seek funding for a project that will provide the first in-depth assessment of the Mo concentration [Mo] and Mo isotopic composition (MoIC) contributing to the hydrosphere through precipitation. Numerous publications over the last 12 years show that the MoIC of river waters are impossible to relate to a unique, single source or process. Gener-ally, the MoICs of river water dissolved loads are significantly enriched in heavier Mo isotopes relative to average continental crust values (allegedly the ultimate Mo source). This isotopic difference is almost unanimously ex¬plained in past literature through complex processes in soils that result in preferential retention of the lighter isotopes of Mo. However, a hitherto unconstrained potential source of elevated Mo isotopic compositions in rivers is precipita¬tion of marine aerosols, since ocean water is enriched in heavy Mo isotopes. The results of this study will be crucial in the robust evaluation the isotope budgets of different Mo sources in continental environments. This is important not only to studies of the modern Mo surface cycle (e.g. for discrimination between natural and anthropogenic airborne Mo), but also as a key input parameter in models reconstructing paleo-oceanic environmental conditions. All considerations of the ancient Mo cycle throughout the entire geological record depend on constraints from modern Mo fluxes. Be-sides MoIC, the main tools will be Sr isotopes (87Sr/86Sr), trace element, and oxygen isotope data on precipitation, which are all necessary for contextual and supporting information on Mo sources and evaporation-precipitation pro-cesses. Three airborne Mo sources are considered: a) Marine source: The MoIC of modern seawater is homogenous, as is the Sr isotope ratio. Thus, mass balance models combining both systems can constrain a unique end-member point source for seawater in 87Sr/86Sr vs. d98/95Mo space. Moreover, d98/95Mo is highest in seawater compared to other surface water reservoirs, and [Mo] is enriched. These fingerprints are ideal tools to constrain the contributions of marine aerosols to continental environments. These data will be complemented by trace metal ratios (e.g. Co/Mo or Ni/Mo) that are indicative of a marine source. b) Continental source: Another important airborne source is fine-grained particulates, mainly comprising sili¬cates and potentially iron oxyhydroxides. The minerals are characterised by MoICs significantly below ocean water, and in the case of the oxyhydroxides, significantly different due to their scavenging of light Mo isotopes. Thus, this airborne contribution is readily discernible based on MoIC and element tracers (e.g., Fe and Al) from marine aerosols. c) Anthropogenic sources: Airborne Mo is characteristically associated with aerosols from oil combustion, and this can be a possible Mo source in industrial- or traffic loaded areas. These aerosols, however, are also enriched in other metals far beyond the levels in crustal rocks and marine waters, such that this source should also be discernible with trace metal to Mo ratios, which are orders of magnitude different of those of the aforementioned sources. The combination of different isotopic and elemental proxies is the unique approach put forward here to more rigor-ously constrain marine aerosol-derived Mo. The need for this assessment is evident through there being only one published result of MoIC from precipitation to date. The lack of data is primarily re¬lated to the analytical challenge to accurately determine the isotopic ratio for such extremely low concentration samples. To be able to collect enough sample (estimated masses of 15 to 20 kg), we will resort to snow samples, follow¬ing the approach of studies published in the 1980s. Due to the intrinsic complexities of the logistics and analyti¬cal procedures, this study is limited to three locations: a) The small (0.8 km2) Strengbach catchment (France, http://ohge.unistra.fr), which has been monitored over 33 years. The acidity of its soils has been demonstrated to result in part from anthropogenic causes and the Mo surface cycle and mass balance has context from previous studies (Nägler et al. in press). b) The eastern coast of Newfoundland & Labrador, Canada, which receives regular and large amounts of snowfall and has obvious potential for constraining airborne Mo fluxes dominated by seaspray based on previously published S-isotope results. Also the new data would link east coast Canadian results to an inland catchment (Ottawa River Valley, sampled 2018). c) The High Altitude Research Station Jungfraujoch offers an ideal environment for precipitation studies. It is far above most sources of air pollution (3500 m a.s.l) and thus is ideal for measuring the Mo composition of the natural European background. At Jungfraujoch, aerosol measurements have been recorded continuously since the 1990s.
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