Dissertation Abstract
Stable strontium isotopes in the marine and terrestrial environment
Stevenson, Emily I 2012
Department of Earth Sciences, University of Oxford (United Kingdom), 210 pp.
The work reported in this thesis develops and applies a new isotope tracer, stable strontium isotopes, to address questions concerning changes in global climate that occur in response to continental weathering processes, such as glaciations, and to constrain the modern marine geochemical strontium (Sr) cycle. Stable Sr isotopes are a relatively new geochemical proxy, and as such their geochemical behavior needs quantified and evaluated in the archives from which they are analyzed before inferences can be made regarding their interpretation. It is only then that we can truly interpret stable Sr variability (both in the modern day and geological past) in both marine and terrestrial environments.
Here we find that foraminifera, coccoliths and corals (common calcium carbonate archives for Sr analysis) acquire stable strontium values lighter than that of modern day seawater, (approximately 0.11, 0.05, and 0.2 per mil. lighter than seawater at ~25°C respectively) a measureable offset which can be used to constrain the modern Sr outputs from the ocean and provide a better understanding of the modern Sr geochemical cycle.
Using foraminifera as a sedimentary archive the first marine stable strontium isotope record of seawater over the last two glacial cycles has been constructed. This was used to investigate changing carbonate inputs and outputs in the ocean over the last two glacial periods in Earths history (Modern to 145 k yr ago). Modeling of a large excursion of the stable strontium isotopes to heavier values during Marine Isotope Stage Three reveals that this is more likely to be due to local changes in seawater or post-depositional alteration, rather then whole ocean changes.
In the terrestrial environment, stable strontium isotopes have been measured in the dissolved load of rivers draining the Himalayas to further understand and deconvolve carbonate versus silicate weathering processes. It is found that, in general, rivers draining carbonate catchments possess lighter stable strontium isotopic values than those from rivers draining silicates.
The results from this thesis have greatly improved our knowledge of stable Sr isotope behavior in a variety of marine carbonates, both in the modern ocean and in the past. This information has contributed detail both on biomineralisation of Sr and calcium as well as helping to constrain the geochemical cycle of Sr by allowing the Sr carbonate output of the oceans to be constrained. This is important so that correct interpretations can be made on the relative contributions of silicate and carbonate weathering from the continents to the oceans both today and in the past
Here we find that foraminifera, coccoliths and corals (common calcium carbonate archives for Sr analysis) acquire stable strontium values lighter than that of modern day seawater, (approximately 0.11, 0.05, and 0.2 per mil. lighter than seawater at ~25°C respectively) a measureable offset which can be used to constrain the modern Sr outputs from the ocean and provide a better understanding of the modern Sr geochemical cycle.
Using foraminifera as a sedimentary archive the first marine stable strontium isotope record of seawater over the last two glacial cycles has been constructed. This was used to investigate changing carbonate inputs and outputs in the ocean over the last two glacial periods in Earths history (Modern to 145 k yr ago). Modeling of a large excursion of the stable strontium isotopes to heavier values during Marine Isotope Stage Three reveals that this is more likely to be due to local changes in seawater or post-depositional alteration, rather then whole ocean changes.
In the terrestrial environment, stable strontium isotopes have been measured in the dissolved load of rivers draining the Himalayas to further understand and deconvolve carbonate versus silicate weathering processes. It is found that, in general, rivers draining carbonate catchments possess lighter stable strontium isotopic values than those from rivers draining silicates.
The results from this thesis have greatly improved our knowledge of stable Sr isotope behavior in a variety of marine carbonates, both in the modern ocean and in the past. This information has contributed detail both on biomineralisation of Sr and calcium as well as helping to constrain the geochemical cycle of Sr by allowing the Sr carbonate output of the oceans to be constrained. This is important so that correct interpretations can be made on the relative contributions of silicate and carbonate weathering from the continents to the oceans both today and in the past