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Using oceanic Mg and Sr stable isotope records to understand the past Earth

Applicant Shalev Netta
Number 185988
Funding scheme Ambizione
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.02.2020 - 31.01.2024
Approved amount 888'568.00
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Keywords (7)

evaporites; magnesium isotopes; dolomite; stable-strontium isotopes; stable isotopes; seawater; paleoceanography

Lay Summary (German)

Magnesium und Strontium Isotope als Schlüssel zur Vergangenheit der Erde
Lay summary

Veränderungen der chemischen Zusammensetzung der Ozeane in der Vergangenheit werden unter Einbezug verschiedener sedimentärer Archive rekonstruiert. Solche Variationen sind hauptsächlich das Resultat von Veränderungen in kontinentaler Verwitterung, Reaktionen zwischen Meerwasser und der ozeanischen Kruste, mariner Sedimentation, und Lebewesen, die zusammen die ozeanischen In- und Output Flüsse verschiedener Elemente bestimmen. Nur mit Konzentrationen alleine ist es schwer die verschiedenen kontrollierenden Mechanismen auseinander zu halten. Steigende Konzentrationen könnten zum Beispiel das Resultat von steigenden Inputs oder sinkenden Outputs sein. Die stabile isotopische Zusammensetzung (also das Verhältnis verschiedener stabiler Isotope vom selben Element) von Magnesium (Mg) und Strontium (Sr) im Meerwasser kann dabei helfen die kontrollierenden Mechanismen zu identifizieren und quantifizieren. Um diesen Ansatz benutzen zu können, brauchen wir verlässliche Rekonstruktionen der isotopischen Zusammensetzung von Mg und Sr in den Ozeanen der Vergangenheit.

In dieser Studie werden wir die isotopische Zusammensetzung von Mg und Sr im Meerwasser in der Vergangenheit rekonstruieren. Zu diesem Ziel werden wir drei neue Archive etablieren: Gips, Halit and Dolomit, die unabhängig von den üblicherweise benutzten Calciumkarbonat Archiven sind. Die resultierenden Meerwasser Zusammensetzung werden wir dann benutzen um die In- und Outputs von Mg und Sr in der Vergangenheit zu rekonstruieren.

Die gleichen Mechanismen (Verwitterung, Sedimentation, und andere) die die globalen Kreisläufe von Mg und Sr kontrollieren, kontrollieren auch den Kohlenstoffkreislauf und daher das Klima der Erde (wegen des Treibhauseffektes von atmosphärischem Kohlenstoffdioxid). Daher wird die Rekonstruktion von Mg und Sr Kreisläufen auch dabei helfen einige der globalen Veränderungen in der Geschichte der Erde zu verstehen.

Direct link to Lay Summary Last update: 14.01.2020

Responsible applicant and co-applicants



The global cycles of magnesium (Mg) and strontium (Sr), including their oceanic budgets, are controlled by major Earth-system processes, such as weathering of the continental crust, volcanism at mid-ocean ridges and marine sedimentation. Therefore, they are linked to the cycles of many other elements, including that of carbon, and are controlled by the evolution of the Earth’s surface environment. Thus, reconstructions of Mg and Sr oceanic budgets in the past hold fundamental information about Earth’s geologic history including, for example, weathering intensity, sediment burial fluxes and long-term climate change. It has recently been demonstrated that the stable isotope compositions of Mg and Sr (d26Mg and d88/86Sr values, respectively) are effective tools to study the modern oceanic budget of these elements and to quantify the fluxes controlling them. In addition, a few pioneering studies have reconstructed the d26Mg and d88/86Sr records for the past oceans. These records provide important information about the changes in the Mg and Sr cycles in the past, and thus also about changes in the Earth system processes that control them. For d88/86Sr values, however, only a single oceanic record is available, for the Paleozoic and Mesozoic (ca. 540 - 65 Myr). For seawater d26Mg values, three Cenozoic (the last ~65 Myr) records are available but one is in conflict with the other two. Thus, the major limiting factor in this field, and the potential of these isotope systems for the study of Earth evolution, is the availability of robust and comprehensive d26Mg and d88/86Sr records for the past oceans.Here, I propose to establish three new archives for past seawater d26Mg and d88/86Sr values: gypsum/anhydrite (Ca-sulfate minerals), halite (NaCl) and dolomite (CaMg(CO3)2). These archives are all independent from the commonly-used Ca-carbonate archives. I will use these archives to reconstruct past seawater isotopic compositions, their evolution through time, and, thus, the past oceanic budgets of Mg and Sr. Gypsum (anhydrite) and halite are marine evaporites, deposits that are excellent direct recorders of the chemistry of ancient marine-derived brines. Indeed, a significant portion of our knowledge of the history of ocean chemistry is derived from evaporites. Dolomite is a suitable archive for seawater Mg because: 1) seawater is the only abundant source of Mg at the Earth’s surface capable of forming large volumes of sedimentary dolomite; 2) Mg is a major element in dolomite, so that its d26Mg is less vulnerable to diagenetic alteration and; 3) dolomite is an abundant lithology through the entire Phanerozoic Eon (the last ~540 Myr). The proposed study of stable-Sr isotopes in dolomite represents pioneering exploratory work, with the aim of testing the suitability of dolomite as an archive for seawater d88/86Sr.The exploration of these archives will be done in three work packages: (1) a laboratory phase, which will include the establishment of measurement procedures and the derivation of experimental constraints on isotope fractionation upon incorporation from the aqueous to the solid phase; (2) investigation of modern/recent (i.e., Messinian, ~6 Myr) evaporites and dolomites, for which the isotopic composition of seawater is known, in order to evaluate factors that may alter the original signature of seawater and; (3) reconstruction of ancient seawater, in which selected ancient samples will be studied. Then, using a mass balance approach, similar to that recently used by the author to understand the modern Mg isotope cycle, and in combination with literature data, conclusions regarding the implications of long-term variation in the Mg and Sr cycles for the major Earth system processes will be drawn.If funded, this research is expected to provide three new archives that are required to validate and complete the available Ca-carbonate records. In addition, these archives have the potential to be used in the future for other isotopic systems (for example, of Li, Ca and K). As such, in combination with the assessment of implications for the evolution of ocean chemistry and surface Earth conditions, the significance of this research extends well beyond Mg and Sr isotope budgets.