Unraveling the Diagenetic Cycling and Sequestration of Rare Earth Elements (REEs) in Marine Sediments to Understand REE Accumulation in Ancient Shales Public Deposited



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  • The rare earth elements (REEs) have been established as powerful tracers for a range of physiochemical processes occurring in the natural environment. They also hold significant economic importance as many technological advancements are reliant upon the REEs for their unique magnetic, luminescent, and electrochemical characteristics. In sedimentary settings, understanding the cycling of the REEs is necessary to fully evaluate the application of the REEs as geochemical tracers and to understand the individual factors that may influence the magnitude and signature of REE accumulations. The association of the REEs with the major element cycles of iron (Fe) and phosphorus (P) have been extensively studied in modern sedimentary environments, but there remain uncertainties on the long-term accumulation and signature of the REEs. In particular, our knowledge of how the preserved record of the REEs in ancient marine black shales is derived from the cycling of REEs observed within modern sediments remains unclear. For these marine black shales, this gap in our knowledge has important ramifications for the potential use of the REEs as geochemical tracers of processes such as groundwater flow or hydraulic fracturing activity as well as the potential sourcing of REEs for economic extraction. In this dissertation, I attempt to refine our understanding of REE accumulation in ancient black shales, and how the original signals of the diagenetic cycling of REEs may be preserved, by using sequential extraction techniques to isolate individual reactive components in the sediments. Based on documented associations of the REEs and authigenic phosphate minerals in sedimentary systems, I first investigated the preserved record of P in two ancient black shales to unravel the environmental parameters that lead to the sequestration of P as carbonate fluorapatite (CFA) (Chapter 2). This authigenic mineral form is considered the main sink term of reactive P from the marine system, and the preserved record of CFA and other components of the P system may help understand the cycling of P during the deposition of these ancient black shales. Using the preserved signals of P cycling in the individual components of the P system of the ancient black shales, I estimated paleo-benthic P fluxes to evaluate how the sedimentary cycling of P may have impacted the nutrient delivery to the depositional basin. During the deposition of the Marcellus black shale formation in the Devonian Appalachian Basin, I estimated paleo-benthic inputs of P from the depositional area of the formation that were within the ranges of P delivered from large river systems such as the pre-anthropogenic Mississippi and Mackenzie rivers. These estimates suggest that a return flux of P from the sediments could have been a significant source of nutrients to the basin and provide support for a positive feedback mechanism suggested for epeiric, anoxic basins where a benthic flux of P could help stimulate primary productivity in the surface, leading to a greater drawdown of oxygen in deeper waters, and helping to maintain anoxic conditions in the basin that may enhance organic matter preservation. In the next chapter, I add in the signals of the sedimentary REE cycle preserved in ancient black shales in association with the preserved components of the P cycle (Chapter 3). In addition to the two blacks shales analyzed in Chapter 2 for the individual components of the P system, I analyzed the whole-rock REE content of black shales ranging in age from Jurassic to Devonian and representing a range of depositional environments. In these black shales, authigenic carbonates and fluorapatites hosted the majority of the REEs in sediment intervals with active authigenic mineral formation. Based on published relationships between REEs in diagenetic coatings and pore waters in modern sediments, I inferred from the preserved REE patterns of authigenic phases in black shales that the recorded signature reflects diagenetic zonation, where middle REE-enrichments might record the signal of Fe-(oxyhydr)oxide dissolution and heavy REE- enrichments might record the later diagenetic signal of methanogenesis. In sedimentary intervals where authigenic phases were not prevalent, which represents the majority of the black shale samples investigated, the REE signal was determined by remnant organic matter hosting the light REEs and siliciclastic material hosting the heavy REEs. Although these samples resembled typical globally average shale patterns of the REEs, a closer inspection of the components of the REE system suggest that average values of shale may be an amalgamation of different REEs phases. While the connections between the cycling of P and the REEs have been investigated in Chapters 2 and 3, the connections with the cycling of Fe, which have also been demonstrated to strongly influence the REEs, are absent due to the labile nature of the reactive Fe components that are not preserved in the black shales. Thus, in the final chapter of the dissertation, I describe the REE content measured in reactive Fe and P phases from modern sediments near the island of South Georgia (sub-Antarctic) that receive a large input of reactive Fe through glacial output (Chapter 4). Despite this large input of reactive amorphous Fe-(oxyhydr)oxide phases, the majority of the REEs measured in the sediments were hosted in refractory siliciclastics, with less than 15 % of the REEs in labile (extractable) phases. The low sedimentary REE pool associated with the labile amorphous Fe phase likely reflects deposition under a very shallow (< 300 m) water column that precludes scavenging of REEs from seawater. This inference is consistent with measurements I performed on the dissolved REEs in the water column at South Georgia, that show relatively lower concentrations compared to deeper waters in the Southern Ocean that experience greater degrees of particulate scavenging. For the REEs that were measured within the labile amorphous Fe and P phases, however, I found that the fidelity of the preserved REE record is also impacted by the ability of the REEs to transfer from labile phases to more permanently stable mineral phases. With increasing sediment depth, the REEs demonstrated a transfer from relatively amorphous phases of Fe-(oxyhydr)oxides to more crystalline carbonate components of the sediments, preserving a characteristic middle-REE-enrichment of Fe-(oxyhydr)oxides. Additionally, the REEs also showed a transfer from more labile, amorphous forms of Fe- (oxyhydr)oxides such as ferrihydrite/lepidocrocite into more crystalline forms such as goethite/hematite phases with depth. Collectively, the chapters of this dissertation describe the formation of REE signatures from the initial signals of diagenesis in marine sediments to the preserved record observable today in ancient marine black shales. The major element cycles of Fe and P strongly influence the cycling of REEs both in the origination of REE signatures (e.g., middle-REE-enrichments from Fe-(oxyhydr)oxides) and preservation of diagenetic signals (e.g., middle- to heavy-REE-enrichments in authigenic carbonates and fluorapatites). The insights learned about REE accumulation in marine black shales, which I present in this dissertation might potentially assist in evaluating which phases of the black shales are accessed during groundwater or hydraulic fracturing studies as well as guiding targets for potential REE extraction.
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