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Doctor of Philosophy (PHD)
Marine Geology and Geophysics (Marine)
Date of Defense
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This dissertation aims to study a variety of processes by which minerals and rocks chemically alter, either via heating, cementation or recrystallization. The focus of this work is to investigate the behavior of isotopic compositions during these processes, and to apply this proxy to study natural systems. The primary focus is studying the behavior of the clumped isotope temperature proxy (∆47) during these processes. The clumped isotope proxy, which measures the excess of 13C–18O bonds in a mineral compared to a random distribution of bonds, is sensitive to temperature in such a way that allows it to independently calculate crystallization temperatures without a priori knowledge of fluid composition from which the mineral formed. This proxy has many potential benefits in the study of cementation and recrystallization, as it allows for independent determination of mineral formation temperatures; allowing temperature-sensitive factors, such as variations in water oxygen isotopic composition, to be constrained. Calcium carbonate was precipitated in the lab at known temperatures and the resulting material’s ∆47 values were measured using mass spectrometry. This experiment produced an in-house calibration, which can be applied to natural materials. In order to ensure the precipitation experiments were conducted as close to equilibrium as possible, a study was undertaken to investigate the rate of isotopic equilibration for oxygen isotopes and clumped isotopologues. From this kinetics study, a kinetic difference between 13C and 12C-bound oxygen exchange rates was determined, which results in non-linear behavior between the δ18O and ∆47 values during equilibration. This effect is likely present during biomineralization and other natural processes, possibly playing a role in so-called “vital effects” that have described in the literature. A series of experiments were conducted investigating isotopic behavior over the aragonite-calcite transition at elevated temperatures. Results show that at the elevated temperatures relevant to the aragonite-calcite transition, the mineral is able to exchange carbon and oxygen with available CO2, which provides a vector for altering the δ13C and δ18O, as well as introducing isotopically ‘young’ atmospheric radiocarbon. When heated in a vacuum, a measureable change in δ18O is still present even at significantly lower temperatures than is necessary to facilitate the mineral change. This indicates exchange with some other source of oxygen, such as skeletal water. Analyses of skeletal water before and after heating demonstrated a significant shift in water values, confirming that there is isotopic exchange between the two media. The clumped isotopic composition of the biogenic aragonite used in these experiments altered at a lower temperature and at a faster rate than the mineral transition, suggesting little relationship between the two processes other than the experimental heating. The solid-state exchange which affects the ∆47 values occurs at a lower temperature than published values for calcite as well, it is possible that the presence of skeletal water in the aragonite plays a role in lowering the energetic barrier associated with this kind of exchange. The heat associated with certain Neolithic cooking techniques is sufficient to facilitate all the above-mentioned processes; isotopic measurements of shell midden constituents, which may have been cooked, could therefore be affected by these reactions and thus unsuitable for some forms of geochemical analysis. The ∆47 values of biogenic aragonite are more sensitive to heating than the mineral transition, this observation complicates a common assumption used by geochemists that any process which would alter the geochemical composition of an aragonite mineral would also invariably affect its mineralogy. Because this is not true in cases of heating, special thought must be given to ensure that samples whose ∆47 values are being measured were not heated sufficiently to facilitate these solid-state exchange reactions. Measurements of authigenic minerals in the ANDRILL-2A core indicated that the majority of cements and recrystallized biogenic minerals formed in the presence of a fluid with negative δ18O values compared to seawater. Measurements of the corresponding formation waters described a hypersaline brine with similar δ18O values, which is understood to have formed via the freezing of seawater. These minerals, which uniformly record in their ∆47 values cooler crystallization temperatures than present burial temperatures, were likely formed at a shallower, and thus cooler, burial depth. From these lines of evidence, we determine that a fluid with the isotopic composition of the brine was present for a significant fraction of the site’s depositional history. Time-integrated models of the depositional history simulated different rates of recrystallization and different times of brine infiltration, these models suggest that the brine may have been present for over 10 million years. Two cores recovered by the Ocean Drilling Program (ODP) on Expedition 166: Bahamas Margin Transect were analyzed using clumped isotopes. Time-integrated models describing early diagenesis suggest that at low latitudes, where the difference between surface and benthic temperatures is most extreme, clumped isotopes are very sensitive to early recrystallization due to the substantial change in equilibrium composition between initial formation and recrystallization temperatures. The uppermost sediments at both sites show a trend towards cooler values downcore, the more platform-proximal core records warmer temperatures, deeper in the core, which we interpret as reflecting recrystallization of sediments in the geothermal gradient. This deeper recrystallization is associated with more positive reconstructed fluid δ18O values, indicative of co-evolving pore fluid and carbonate oxygen isotope values in the shifting equilibrium state during burial. In this manuscript, we demonstrate that the clumped isotope proxy is sensitive to many forms of alteration, and is in fact more sensitive than many mineralogical indicators often used to indicate pristine compositions. Because clumped isotopes are a non-conservative parameter, they cannot saturate formational fluids, the result of this being that clumped isotopes are equally sensitive to initial recrystallization at shallow depths as they are to recrystallization during deeper burial. The clumped isotope proxy is ideal for studying systems wherein oxygen isotopes and crystallization temperatures cannot be assumed, and we have applied it to study several sediment cores with different diagenetic conditions. These measurements, when coupled with numerical simulations of these sites’ depositional histories and potential recrystallization histories, help to constrain the rate at which authigenic minerals form during burial, as well as the fluid compositions present during mineral formation.
Clumped isotope; diagenesis; aragonite-calcite; geochemistry
Staudigel, Philip Tauxe, "The Application of Clumped Isotopes in the Study of Marine Diagenesis" (2018). Open Access Dissertations. 2188.
Available for download on Thursday, September 10, 2020