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Data-constrained marine micronutrient biogeochemistry: implementing a new tool to study the marine cycle of Zn and its isotopes

Applicant de Souza Gregory
Number 192116
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.04.2020 - 31.03.2023
Approved amount 216'838.00
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All Disciplines (2)

Discipline
Geochemistry
Oceanography

Keywords (7)

optimisation; zinc isotopes; marine biogeochemistry; ocean modelling; micronutrients; isotope geochemistry; zinc

Lay Summary (German)

Lead
Einzellige Algen im Ozean - das sogenannte «Phytoplankton» - leisten ungeheuer viel im Erdsystem: nicht nur bilden sie die Basis der marinen Nahrungskette, sie tragen durch ihre Photosynthese auch zur Reduktion der atmosphärischen Konzentration des Treibhausgases CO2 bei - und stossen dabei genau so viel lebenswichtigen Sauerstoff aus wie alle Landpflanzen zusammen. Um diese wichtigen Ökosystemdienstleistungen zu erbringen, ist das Phytoplankton auf Nährstoffe angewiesen - auf «übliche» Nährstoffe wie Stickstoff und Phosphor sowie auf Spurenmetalle wie Eisen oder Zink, die wie beim Menschen als Mikronährstoffe dienen.
Lay summary

Ziel dieses Projektes ist es, den marinen Zyklus des Mikronährstoffs Zink durch die Zusammenführung von neuen Beobachtungen mit effizienten Modellen besser zu verstehen. Das globale Meeresbeprobungs-Programm GEOTRACES hat die Verfügbarkeit von Daten über die Konzentration und stabile Isotopie von gelöstem Zink im Meerwasser um ein Vielfaches erhöht; gleichzeit existieren recheneffiziente dreidimensionale Modelle der Ozeanzirkulation, sowie leistungsfähige Algorithmen, die anhand von evolutionären Prinzipien eine Anpassung eines Modells an beobachteten Grössen erlauben, um somit Rückschlüsse über die wichtigsten Prozesse zu ziehen. In diesem Projekt werden wir, einerseits, diese 3 Komponenten in einem flexiblen Tool kombinieren, der es erlaubt, die marinen Zyklen von Mikronährstoffen und ihren Isotopen zu studieren; andererseits werden wir diesen Tool auf den Fall von Zink anwenden, um zwischen unterschiedlichen Hypothesen über die massgebenden Prozesse im marinen Zink-Zyklus zu unterscheiden. Dabei werden wir einen besonderen Fokus auf die stabile Isotopie von Zink legen, die möglicherweise Indizien für eine breitflächige Kontamination des oberen Ozeans mit Zink industriellen Ursprungs enthält.

Direct link to Lay Summary Last update: 30.03.2020

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Abstract

Micronutrient metals are fundamental to marine photosynthesis, which supports the marine food web and modulates the atmospheric inventories of two key gases: oxygen and carbon dioxide. The supply of dissolved micronutrients to photosynthesisers in the sunlit surface ocean is governed by a complex interplay between physical circulation, biological uptake & export, and chemical processes that remove them from, or add them to, seawater. A decade into the international GEOTRACES sampling programme, the body of micronutrient observations has grown to the extent that we can now begin to use modelling tools to begin to unwrap this interwoven set of processes. Here, I propose to develop a computationally-efficient modelling tool that takes advantage of this wealth of data for robust hypothesis-testing, and apply it to assess competing hypotheses regarding the controls on the marine cycle of zinc, a vital micronutrient whose marine cycle remains under debate.Zinc (Zn) plays a variety of structural, regulatory and catalytic roles in cellular biochemistry; most importantly for photosynthesis, it is the metal cofactor in the enzyme carbonic anhydrase that enables the efficient fixation of inorganic carbon. Zinc is thus vitally required and taken up by all phytoplankton species, albeit in widely varying amounts. Recent studies have shown that it is precisely this plasticity in Zn quota that plays a primary role in determining the large-scale distribution of dissolved Zn in the ocean that had puzzled oceanographers for decades: we now know that the general, global-scale correlation of zinc with the macronutrient silicon largely comes about through the coincidence of their elevated uptake by siliceous phytoplankton (diatoms) in the Southern Ocean, without any mechanistic connection between these two elements. What remains the subject of considerable scientific debate is the evidence of the stable isotope composition of dissolved Zn (d66Zn) in seawater. Numerous observational studies have documented the fact that the low-latitude upper ocean contains isotopically-light Zn, contrasting strongly not only with most other biogeochemically-cycled elements, but also with a priori expectations based on the behaviour of Zn isotopes during biological uptake in culture. It has thus been proposed that this unique feature results from the loss of isotopically-heavy zinc from the upper ocean through its adsorption to sinking particles. However, there is little observational evidence, in either the particulate or the dissolved marine pools, that Zn scavenging is quantitatively important in the marine setting; furthermore, very recent observations from the North Atlantic have led to the intriguing hypothesis that the low d66Zn of the upper ocean may in fact reflect the addition of isotopically-light Zn. The exact putative source remains unclear, but both natural and anthropogenic sources of isotopically-light Zn are known to exist.The recently expanded marine Zn elemental and isotopic database has thus revealed key uncertainties and ambiguities in the marine cycle of this important micronutrient element. The work proposed here aims to:-develop a flexible inverse modelling framework using computationally-efficient representations of the three-dimensional ocean circulation coupled to an evolutionary algorithm for parameter optimisation, and document its skill at recovering process information from observational fields; -apply this framework to the marine cycle of Zn and its stable isotopes, in order to assess the explanatory power of hypotheses regarding the processes responsible for the enigmatic marine d66Zn distribution.The proposed research will thus shed light on the processes and fluxes that shape the global marine Zn cycle, helping to resolve a key scientific debate in marine micronutrient biogeochemistry. It will also result in a freely-available modelling tool that should prove valuable in harnessing the process information inherent in other GEOTRACES datasets.
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