A Study of Novel Magnesium Isotope Tracers in Geological, Paleoceanographic and Biogeochemical Systems
Date
2017-10-31
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Type
Thesis
Degree Level
Doctoral
Abstract
This thesis explores Mg isotopes as tracers in geological and biological systems. Chapters 2 and 3 investigate Mg isotopes as tracers of fluid flow during dolomitization. The studied carbonate is the variably dolomitized Late Ordovician Red River Formation in the Williston Basin. Three types of dolomite are described (burrow, matrix, saddle) with all three yielding identical d26Mg values at the hand-sample scale, indicating that the dolomite formed from the same fluid, or was overprinted by successive dolomitizing events. A broader study was undertaken to determine whether large dolomite bodies might preserve gradients in d26Mg values at the basin-scale. Because dolomite preferentially sequesters light isotopes of Mg during its formation, the dolomitizing fluid (and product dolomite) should become progressively enriched in heavy isotopes in the direction of fluid flow. The studied dolomite body is the interconnected network of Thalassinoides burrows in the ‘C’ member carbonate of the Red River, where it is found that d26Mg values increase radially away from the center of the Williston Basin. As such, dolomitizing fluids must have originated in the deep center of the basin, using the burrows to flow up toward the basin margins. This interpretation is strengthened by decreasingly radiogenic 87Sr/86Sr ratios in the direction of fluid flow. While the timing of dolomitization is unknown, the formation of overpressurized fluids in the deep center of the basin could have been triggered by a late Paleozoic thermal event, and/or the far-field tectonic effects of the Antler Orogeny, which caused crustal fluids to ascend upwards into the bottom of the Williston Basin through a system of down-to-the-basement vertical faults, which still exist in the basin center today.
Chapter 4 presents a technique for determining original limestone d26Mg values in marine carbonates with trace dolomite. The study section is a carbonate platform in Nevada recording the Late Ordovician (Hirnantian) glaciation event. The d26Mg value for limestone deposited during the sea-level lowstand, and bracketing highstands, is calculated from a graphical mixing technique using chemostratigraphic data. Carbonate deposited during the lowstand has the highest d26Mg value, consistent with precipitation of primary aragonite during the Hirnantian glaciation in a tropical-shelf setting, while limestone deposited before and after the glaciation yields lower d26Mg values, consistent with precipitation of calcite. While diagenetic effects on d26Mg values in carbonate sediment are difficult to predict, a negative shift in d44/40Ca, a positive shift in d13C, and the highest Sr/Ca ratios in the study section, strengthen the case for aragonite deposition during the sea-level lowstand. This result is significant in that the studied carbonates formed in the midst of a ‘calcite sea’, which identifies a period of seawater composition favoring abiotic precipitation of calcite, rather than aragonite, in shallow marine settings. This study emphasizes the problem of misinterpreting stratigraphic trends in Mg, Ca, and C isotope profiles as genuine reflections of global-scale changes in the geochemical cycling of these elements.
Chapter 5 examines Mg-cycling in the ‘critical zone’ with the aim of deciphering whether or not first-order streams exhibit plant-fractionated Mg isotope signatures. This has implications for the d26Mg value of the weathering flux to the oceans, which is a component of the ocean Mg-cycle. There is no evidence in a studied sugar maple stand for Mg isotope fractionation associated with uptake from soil solutions into mature trees and seedlings. There is, however, large within-tree fractionation between tissues. While sugar maple in the field exhibit no uptake related fractionation effects, seedlings grown in the laboratory yield contrasting results, exhibiting higher d26Mg values than the nutrient source. The difference is attributed to the absence of arbuscular mycorrhizal fungi in the artificial soils in which the seedlings were grown in the laboratory. It is speculated that these fungi are important vectors in the uptake of Mg into tree fine roots in natural settings, and that the fungi do not fractionate Mg isotopes. It is also found that d26Mg values and Mg/Ca ratios of soil solutions and stream water, and acid leaches of the C horizon mineral soil are identical to each other, and to the aboveground vegetation as well, indicating that plant-recycled fluxes of Mg and Ca, filtering down through the soil profile from the surface, appear to completely overwhelm the much smaller annualized input fluxes of these elements from atmospheric deposition and soil mineral weathering.
Description
Keywords
Magnesium, Mg isotopes, dolomite, dolomitization, diagenesis, Williston Basin, Red River Formation, Late Ordovician, Hirnantian, calcite, aragonite, sugar maple, arbuscular mycorrhizal fungi, critical zone, Mg-cycling, biogeochemistry
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Geological Sciences
Program
Geology