Global climate is strongly influenced by fluctuations in atmospheric CO₂ concentrations. Weathering of silicate rocks consumes
CO₂, transports cations to the oceans, and thus plays a critical role in both seawater chemistry and climate. A major product of silicate
weathering is magnesium, which in the oceans is homogeneous in both concentration and isotopic composition (δ26Mg[subscript SW] = -0.82‰).
This homogeneity reflects a balance between continental weathering input by rivers (δ26MgRiver ~ -1.09‰) and groundwaters, and removal by
high-temperature hydrothermal oceanic crust alteration (δ²⁶Mg[subscript SW] - δ²⁶Mg[subscript HT] ~ 0.0‰), dolomite formation, and
authigenic alumino-silicate clay formation during low-temperature alteration of the oceanic crust. Since the oceanic residence time of Mg
is significantly longer than the ocean mixing time, temporal variations in Mg isotopic composition of seawater (δ²⁶Mg[subscript SW])
recorded by marine calcites, such as foraminifera, can reflect a global picture of Cenozoic climate that is driven by an imbalance between
the source and sink. A review of previous geochemical proxies and the terrestrial processes of magnesium are discussed in Chapter 1. In
Chapter 2, we present an improved method for trace level Mg measurements in natural carbonates. The method was developed for analysis of
planktonic foraminifera samples in order to generate a Mg isotopic record for Cenozoic seawater, however it can also be applied to
seawater and other natural samples with a high ionic strength sample matrix. There were three major analytical challenges that needed to
be overcome: 1. Maintain a precision of ±0.1‰ with a mass consumption as low as ~10 ng of Mg; 2. Maintain minimal blanks (≤0.1 ng) on Mg
and other matrix elements (Na, K, Ca, etc.); 3. Purify samples to have complete separation of Mg from matrix elements with 100% recovery.
We achieved this through cation-exchange column chromatography and analysis on a Multi-Collector Inductively-Coupled Plasma Mass
Spectrometer (Thermo-Scientific "Neptune" MC-ICP-MS). We quantified our precision and accuracy through measurements of CAM1, natural
seawater, and the JCP-1 coral reference standard and compared with other published studies. In addition, elemental ratios were determined
by Quadrupole-ICP-MS (Q-ICP-MS; Florida State University) or by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES;
University of Cambridge) to verify that the sample had been properly chemically cleaned and that it had not undergone diagenesis that
would alter the Mg isotope ratios. In chapter 3, we present data from 66 core-top planktonic foraminifera samples from the Holocene epoch
to investigate the use of the Mg isotopic composition of foraminifera as a potential proxy for the Mg isotope record of Cenozoic seawater
and to quantify the fractionation between seawater and foraminiferal calcite (Δ²⁶Mg[subscript Foram-Seawater]). The average δ²⁶Mg of the
planktonic foraminifera in our samples is -4.79 ± 0.83‰ (2σ), which includes 10 species with varying size-fractions from 4 sampling sites.
Our study demonstrates that there is limited inter-species variability in δ²⁶Mg. We also observed no differences between the δ²⁶Mg of
foraminifera cleaned with and without the reductive cleaning step (hydrazine + citric acid). This suggests that the Mg isotopic
composition of the high-Mg calcite bands, which are preferentially dissolved during reductive cleaning, is similar to that of the low-Mg
bands. In addition, the discrimination against Mg is higher for foraminiferal calcite formation compared to precipitation of inorganic
calcite, suggesting that biomineralization should induce additional fractionation of Mg isotopes. In chapter 4, we present a new record of
planktonic foraminifera (n = 104) spanning the Cenozoic Era (0 to 70 Ma), which shows a ~1‰ decrease in δ²⁶Mg towards present day values.
The significance of this decrease is challenged by the high variability in our core-top foraminifera calibration (δ²⁶Mg = -4.63±0.57‰
(n=48, 2σ)). However, box model scenarios demonstrate that such a decrease in δ²⁶Mg could be driven by a decrease in the dolomite
formation flux from ~4.0 Tmol/yr to present-day values of 0.84 Tmol/yr. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial
fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2016. / October 3, 2016. / cenozoic, foraminifera, isotopes, magnesium / Includes bibliographical references. / William M. Landing, Professor Co-Directing Dissertation; Vincent J. M. Salters, Professor
Co-Directing Dissertation; John G. Dorsey, University Representative; Munir Humayun, Committee Member; Angela N. Knapp, Committee
Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_405651 |
| Contributors | Dial, Angela R. (Angela Renee) (authoraut), Landing, William M. (professor co-directing dissertation), Salters, Vincent J. M. (professor co-directing dissertation), Dorsey, John G. (university representative), Humayun, Munir (committee member), Knapp, Angela N. (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Earth, Ocean, and Atmospheric Science (degree granting departmentdgg) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (134 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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