Háskóli Íslands

On Wednesday the 9th of August, Rebecca Anna Neely will defend her Ph.D. thesis in geology. The thesis title is “Molybdenum isotope behaviour in aqueous systems”

Opponents are Dr. Thomas F. Nägler, Associate Professor of Isotope Geology at the Institute of Geology at the University of Bern, Switzerland and Dr. Caroline L. Peacock, Associate Professor of Biogeochemistry at the School of Earth & Environment at the University of Leeds, United Kingdom.

Adviser is Dr. Sigurður Reynir Gíslason, Research Professor of Geochemistry at the Institute of Earth Sciences at the University of Iceland. The assessment committee included Dr. Kevin Burton, Professor of Geochemistry Durham University UK and Professor of Paleoceanography at Université Paul Sabatier in Toulouse, France and Dr. Eric H. Oelkers, Professor of Aqueous Geochemistry at the University College London, Director of Research CNRS, Université Paul Sabatier, Toulouse, France and Visiting Adjunct Professor at the Institute of Earth Sciences, University of Iceland

Dr. Magnús Tumi Guðmundsson, professor and head of Faculty of Earth Sciences at the University of Iceland, will chair the ceremony.

Abstract

Molybdenum isotopes are used to quantify changes in Earth’s surface paleoredox conditions but their application relies upon a simplified model in which rivers dominate the ocean input with minor contributions from hydrothermal fluids. The effect of groundwater discharge is rarely considered. This study finds that cold groundwaters (δ98MoGROUNDWATER -0.1‰) are compositionally similar to their host rocks (δ98MoBASALT -0.15‰) whilst hydrothermal waters are enriched in heavy isotopes (δ98MoHYDROTHERMAL +0.2‰ to +1.8‰). Using flux estimates from the literature, the inclusion of these data results in the revaluation of the Mo ocean input from +0.5‰ (just rivers) to +0.35‰ (combined), in the modern day.

As a bioessential element, Mo is important in many biogeochemical cycles: especially, as a cofactor in nitrogenase, the most common nitrogen fixing enzyme. Biological fractionations of some 1.5‰ are observed, with light Mo removed from Lake Mývatn by cyanobacterial uptake during an algal bloom. If preserved, these biological fractionations may need to be considered in the interpretation of the sedimentary record.

Despite the growing evidence that the vapour-phase - formed through magma degassing and fluid boiling - can selectively concentrate and transport metals, the effects on metal stable isotopes remain poorly understood. For example, Mo isotopes show great variability in ore deposits, some of which is attributed to vapour-phase transport. Here we examine the vapour-phase in four geothermal systems in Iceland; the vapour-phase is always lighter than the brine with enrichment factors of some εV-L -2.9‰. This is an important first step towards understanding the mechanisms behind vapour transport and isotopic effects.

About the Ph.D. student

Rebecca started her Earth Science studies at the University of Oxford, UK which culminated in a Masters of Earth Science in 2011. She then went on to work as a laboratory- and field- assistant in geochemistry and volcanology before moving and starting a PhD at the University of Iceland in August 2012. Much of the research was carried out in collaboration with Durham University, including more than two years spent there as a visiting student.

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