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Trace Element Variability in Clay Sediments as a Function of Environmental Conditions within the Fluvial to Marine Water Transition Zone
- Author / Creator
- Knudson, Calla J. H.
Estuaries are marginal marine environments of considerable complexity, due to their spatial and temporal variations in hydrodynamic energy, water chemistry, and sediment source and composition. This complexity hinders geologists’ ability to accurately reconstruct paleoenvironments and geological
histories from the rock record. In particular, the geochemical evolution of sediments in the transition from freshwater to marine environments remains poorly understood. Determining the location of the freshwater to brackish-water transition zone in sedimentary rock successions is essential to accurate
paleoenvironmental reconstructions and detailed facies models. The interpretation of estuarine depositional environments is currently predicated on ichnological and micropaleontological analyses. However, the utility of these methods is limited by
the preservation potential and subjectivity of trace fossil and microfossil assemblages. Geochemical techniques have the potential to enhance and refine these interpretations in intervals where ichnological and micropaleontological datasets are inconclusive. Other researchers have made attempts to correlate
specific trace element abundances and ratios in mudstones to paleosalinity conditions. However, these studies do not provide sufficient direct evidence for the relationship between these specific geochemical signatures and their inferred
environmental salinity conditions. In an effort to refine the relationship between environmental salinity and trace element signatures in mudstones, I use quantitative geochemical methods to make observations of trace element enrichment in clay minerals under controlled conditions. Laboratory experiments
were conducted with three common clay minerals that contribute to mudstones: illite, kaolinite, and montmorillonite. The adsorption of caesium cations to each clay at a range of ionic strength conditions was measured to evaluate the efficacy
of caesium as a potential proxy for paleosalinity in mud deposits. Each clay sample was also equilibrated with North Saskatchewan River water and English Bay sea water samples to make empirical observations of element partitioning between aqueous and particulate clay phases under fluvial and marine conditions.
The results of the ionic strength-dependent adsorption experiments suggest that there is an inverse proportional relationship between solution ionic strength and the proportion of caesium that is sorbed to clay. Caesium shows promise as a
salinity indicator and further work on this element in that context is recommended. The partitioning of elements between the aqueous and particulate clay phases differed significantly between the fluvial and marine systems, as well as between discrete suspended sediment load conditions. This discrepancy
suggests that it may be feasible to determine whether fine-grained sediments were deposited in fluvial or marine settings based upon their geochemical signatures, although the effects of climate, suspended sediment load, mineralogy, and organic
matter should be considered. Sediment and surface water samples were collected from the wave-dominated Arcachon Bay and the tidal-fluvial Gironde Estuary, southwestern France. The geochemical signatures of these two modern, marginal
marine environments are defined and compared in an effort to develop an analogue for controls on geochemical signatures in the rock record. I determined the chemical indices of alteration, distribution coefficients of trace elements between the surface waters and sediments, sum totals of rare earth element (REE)
abundances, and various critical element ratios (Eu/Eu*, Th/Co, La/Sc, LaN/YbN, Th/Sc, Zr/Sc). These parameters were compared at various locations to infer relationships between sediment geochemistry and locations within each modern coastal environment. The sediments of Arcachon Bay and the Gironde Estuary are interpreted as the early to intermediate weathering products of a source rock with mixed felsic and mafic composition derived from an active continental margin.
The results of this study further indicate that the composition of the sediment source rock is the primary control on the availability of REEs and trace elements in sediment deposits. However, the spatial variation and preservation of REEs and
trace elements in sediments are dictated by organic matter distributions. These observations suggest that the use of trace elements to infer the syn-depositional conditions of sedimentary rocks is problematic, and that other parameters such as
sediment source compositions, sedimentary processes, and organic matter content need to be incorporated into these models.
- Graduation date
- Fall 2019
- Type of Item
- Master of Science
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