Séminaire de Stefan Lalonde

Mardi 11 Avril à 14h dans la salle des séminaires (1.10) : Stefan Lalonde (Laboratoire Géosciences Océan, Brest)

The chemical evolution of seawater through geological time: insights from Precambrian iron formations

Iron oxyhydroxide minerals have precipitated directly from seawater throughout geological time. While such occurrences today are generally restricted to settings strongly influenced by hydrothermalism, during much of the Precambrian iron oxyhydroxide muds accumulated over large areas of the ocean floor to be preserved as iron formation (IF). The Precambrian IF record is extensive, spanning every continent and dating as far back as Earth’s oldest known sedimentary deposits. As nearly pure chemical sediments, the precipitates that formed IF captured elemental and isotopic signatures of evolving ancient seawater by sorption and co-precipitation reactions, rendering IF a rich record of ancient Earth’s evolving marine geochemistry. That record is explored here using a database of >3000 chemical and isotopic analyses of IF compiled from literatures sources as well as new analyses. Using phosphorus as an example, partitioning of trace elements between seawater and precursor IF minerals will be presented in terms of how modern observation, laboratory experiment, and the IF rock record itself can each be employed to relate trace element concentrations in ancient iron oxides to the waters from which they formed. Secular trends (or lack thereof) in the IF record of selected micronutrients will then be presented with particular regard to the hypothesis that shifting marine trace element availability guided the evolution of biological demand. Finally, emerging isotope proxies for the redox evolution of the ocean-atmosphere system (Cr, Mo) will be examined in the context of the IF record and recent studies suggesting free oxygen on the early Earth well before the ca. 2.5 Ga Great Oxidation Event.