The Neoproterozoic was a time of major change in Earth’s surficial history, including a major rise in atmospheric oxygen, the first appearance of complex metazoan life, and a series of worldwide glaciations. A particular interesting element of these so called “snowball Earth” glacial deposits is the presence of iron formation, a distinctive Precambrian rock type that is largely absent from the post-Paleoproterozoic record. Despite being relatively poorly studied with respect to their geochemistry and sedimentology, Neoproterozoic iron formations are used to support many models for the record of oxygen concentrations of the Earth. The classical example of Neoproterozoic iron formation is the Rapitan iron formation of northwestern Canada. This hematite-jasper iron formation is associated with glaciogenic turbidites and diamictites. Despite being the archetype, the Rapitan iron formation has not been studied in the context of recent ideas about the Neoproterozoic. In this thesis, the stratigraphy, geochemistry, and basin architecture of the Rapitan iron formation are reassessed. Using the REE+Y and the redox-sensitive elements Mo and U, it is shown that the Rapitan iron formation was deposited in a partially restricted basin from biogenically reduced iron under variable redox conditions. Elemental Re and Mo isotopes further show that although oxic and ferruginous conditions predominated during deposition of the iron formation, a transition towards a sulfidic water column locally terminated deposition. Finally, regional stratigraphy and geochemistry show that the iron formation was preferentially deposited in deep, newly formed basins that were protected from significant siliciclastic sedimentation. These basins were delimited by inferred crustal-scale faults trending roughly perpendicular to the axis of the rift basin, and allowed significant changes in thickness and sedimentological character over short distances along strike. These factors help build an overall geotectonic regime under which Neoproterozoic iron formations were deposited: young, deep rift basins that had undergone marine incursion, and were intermittently sealed by an ice shelf, allowing for the generation of an anoxic, iron-rich water column. The absence of the Eu anomaly and the heavy Mo isotopic signature indicate that the open ocean was fully oxygenated at the time of Rapitan iron formation deposition, as opposed to ferruginous as previously suggested.

Multi-proxy approach for the reconstruction of paleo-redox conditions is attempted on a radiocarbon (C-14) dated sediment core near the equatorial Indian Ocean. Based on the behavior and distribution of redox sensitive and productivity proxies, study demonstrates prevalence of anoxic bottom water conditions during LGM due to poorly ventilated bottom waters augmented by high surface productivity resulting in better preservation of organic carbon (OC). During early Holocene, the equatorial Indian Ocean witnessed high sedimentation rates resulting in high organic carbon (OC) with depleted redox sensitive elements thereby causing better preservation of OC. The study underscores poor bottom water ventilation during LGM and preservation of OC as a result of high sedimentation rate in early Holocene.

Elevated levels of geogenic arsenic (As) in groundwater are regarded as the most formidable environmental crisis in the contemporary world and an estimated 30-70 million people in Bangladesh are at risk. Many of the provided options for mitigation have not been well accepted. In recent years, local drillers have been targeting As-low groundwater on the basis of the color of the sediments. A correlation between the color characteristics of the sediments and the groundwater redox conditions and thereby the risk for As mobilization has been established. It is possible to assess the relative possibility of occurrence of high As concentrations in groundwater if the color characteristics of the sediments are known. One main objective of the present study is to validate if targeting As-safe groundwater is a sustainable mitigation strategy. This has been done through monitoring groundwater from tube-wells installed in sediments with different color, depth and redox characteristics. Samples were collected yearly from a set of 17 tube-wells for the period of 2004 to 2008 in Matlab, Bangladesh. The wells were grouped into oxidized and reduced/non-oxidized wells. All of these wells tap water within a depth of 85 m from the surface. Major cations and trace elements including As were analysed by high resolution ICP-OES; and anions were measured by ion chromatography. Groundwater chemistry of the water abstracted from different sediments revealed that no major change had occurred over the period of 5 years. In reduced wells, concentration of major cations and anions varied by <10% over time, and no significant variation was observed in redox sensitive elements, such as Fe, Mn and S. There were some minor variations in the As concentrations in the wells. In the oxidized wells, except well 58, major cations, anions and redox sensitive elements varied by <5%, while the As concentrations were found quite low and stable (<5.2 μg/L). Time series trends thus suggest that the change in groundwater chemistry is insignificant over the period of 5 years from 2004 to 2008. Low As concentrations in the tubewells installed in the oxidized brownish sediments thus validate the mitigation strategy corresponding to the conceptual understanding of the groundwater system in Matlab in southeastern Bangladesh.

Les différentes lithologies sédimentaires du Groupe Belqa présentent un enrichissement en P et en toute une série d'éléments sensibles aux conditions redox. Il est montré que l'enrichissement en U, Cu, Co, Mo, V résulte d'un dépôt syn-sédimentaire sous conditions suboxiques. Par contre, les éléments Cr, Ni, Cd, Zn sont beaucoup plus enrichis et requierent un flux exogénique de métal probablement relié à l'altération de roches ultrabasiques obductées à la même période au nord de cette région, lors de la collision de la plaque arabo-africaine avec la plaque eurasienne. Les phosphates représentent le principal hôte de l'uranium et des terres rares. L'analyse des apatites par ablation laser ICP-MS a permis de montrer leur évolution, depuis le stade sédimentaire-diagénétique, puis pyrométamorphique, jusqu'à l'altération supergène. La libération de l'uranium de la structure de l'apatite lors du pyrométamorphisme et de l'altération supergène permet sa mise à disposition pour la formation de minéralisations. Le pyrométamorphisme, dû à une combustion des niveaux riches en matière organique, est responsable de la déstabilisation des apatites, et de la formation de roches de compositions semblables à des ciments clinker/Portland. L'uranium a également été libéré de la structure de l'apatite par altération supergène. L'altération météorique et les fluides évaporitiques ont permis le lessivage et le transport des éléments (U, V) des roches carbonatées métamorphiques, et des carbonates puis a conduit au dépôt des vanadates d'uranyles dans les carbonates pulvérulents ayant subi une dissolution importante / The different lithologies of the Belqa Group present an enrichment in P and various redox sensitive elements. The U, Cu, Co, Mo, V enrichment results from synsedimentary deposition in suboxic conditions. However, the higher Cr, Ni, Cd, Zn enrichment requires an exogenic metal flux probably related to the terrestrial leaching of ultrabasic rocks obducted during the collision between African-Arabian and Eurasian plates to the north of the study region, at the same time. Phosphates are the main host lithology for uranium and rare earth elements. The analysis of the apatites by laser ablation ICP-MS shows their evolution along the sedimentary-diagenetic and pyrometamorphic stages, up to the supergene weathering. The uranium liberation from the apatite structure during the pyrometamorphism and surficial weathering led to its availability as supergene ore mineralization. Pyrometamorphism resulted from the combustion of organic-rich layers and caused the destabilization of the apatites, and the formation of rocks which have compositions similar to clinker/Portland cements. Uranium has been also released from the structure of the apatite by supergene alteration. Meteoric weathering and the evaporitic groundwater resulted in the leaching and the transport of the elements (U, V) from the metamorphic carbonated hills, and from the surrounding permeable limestone which has undergone dissolution in the near surface zone