Roger Francois

Variation in the composition of chromium (Cr) stable isotopes has emerged as a powerful tracer for environmental processes as a result of the Cr isotope fractionation induced during the reduction of Cr(VI) to Cr(III). Comprehensive characterization of the geochemical processes that act on and can change Cr speciation, and of all processes that result in Cr isotope fractionation, is needed in order to make robust inferences based on the variation in Cr isotopic compositions. Our knowledge of Cr geochemistry in the natural environment is limited, with lab-based experiments often not capturing the natural complexity of the environment, in part due to Cr speciation being driven by kinetic factors, resulting in disequilibrium, as well as limited speciation data, partially due to a lack of reproducible methods. This dissertation presents a new speciation method and a wealth of new Cr speciation data resulting from the application of this method, including the first species-specific Cr isotope compositions measured in seawater across an oxic-anoxic boundary. These results provide clear evidence of positive and negative excursions in water column δ53Cr(III) in response to oxygenation and deoxygenation events, respectively, which are likely to be reflected in the authigenic Cr sedimentary record. Modeling of water column data also highlights the role of reservoir effects in muting Cr isotope fractionation relative to the intrinsic fractionation factor associated with a given process, like Cr(VI) reduction. We find non-redox processes such as water mass mixing to be a key process in Saanich Inlet, while in treated stream waters we find evidence of substantial oxidative remobilization of Cr(VI) due to reaction between reactive Cr(III) precipitates and manganese (Mn) oxides. Together, the work in this dissertation identifies the local key processes controlling Cr speciation and isotopic composition in these studied environments, with water mass mixing and in-situ Cr(III) oxidation in particular having a strong effect on the resulting Cr isotope composition, both of which are underrepresented processes in current studies. By contributing this new knowledge on Cr behaviour in the modern environment, we help to facilitate more robust inferences of Cr cycling in past environments.

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To better understand how our climate will change in the future, we look to past analogues of our current climate and rely on indicators of past fluctuations preserved in marine sediment. One such indicator is the isotopic composition of neodymium preserved in authigenic phases of marine sediment, which has been widely applied to reconstruct water mass distribution through time. However, recent studies have suggested that the Nd isotopic composition of authigenic phases associated with foraminifera and bulk marine sediment is influenced by early diagenesis. This dissertation documents through experiments and numerical modelling that the Nd isotopic composition of these authigenic phases is influenced by Nd released to pore water through lithogenic dissolution. To identify the source of Nd to a synthetic authigenic phase, we use an incubation of sediment and seawater from the local Strait of Georgia. By tagging the seawater with a unique isotopic ratio, we show that pore water primarily determines the Nd isotopic composition of the authigenic phase. Then, we examine core top sediment in the Labrador Sea, which acts as a natural analogue of the incubation experiment. The seawater in the region has a more radiogenic Nd isotopic composition than the sediment which fills the basin, providing two natural end members. Measurements of the Nd isotopic composition of foraminifera and the authigenic phase associated with bulk sediment are significantly more unradiogenic than seawater. These measurements indicate that seawater is not the primary source of Nd to the authigenic phases in this region. By constructing a numerical model of Nd concentration and isotopic composition in pore water, we are able to quantify diagenetic Nd added to authigenic phases and apply this numerical model to a location in the Equatorial Atlantic Ocean. The diagenetic biases that shift the ℇNd of foraminiferal records in this region mimic the expected variations in ocean circulation. While the authigenic phase associated with foraminifera and bulk sediment leachates seems to preserve the ℇNd of modern seawater, this proxy is subject to effects of diagenesis that change in time, and therefore we conclude that it is not a reliable recorder of ocean circulation.

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The oxygen concentration of the ocean atmosphere system regulates the nature, activity and diversity of life on Earth. Atmospheric and ocean oxygenation is tightly coupled to the global biogeochemical cycles of C, N, P, S and Fe, as well as climate. Reconstructing the history of oxygen on planet Earth, therefore, is a key component to understanding the evolution of life. Our emergent picture of the evolution of Earth’s surface redox state with its links to the evolution of life and climate relies heavily on interpretations of geochemical information preserved in the rock record. The Cr isotope and Fe-speciation proxies are two widely applied tools used to diagnose redox conditions in both modern and ancient depositional environments. Many aspects of the precise mechanisms that lend the use of these two transition metals as paleoredox proxies, however, remain unclear, confounding accurate reconstructions of paleo-oxygen concentrations that rely on Cr isotope and Fe-speciation data. In this work I studied Cr isotope and Fe speciation proxy systematics to develop more nuanced frameworks for how these two paleoredox proxies may be employed to reconstruct depositional redox states in both modern and past environments. I determined the Cr isotope and Fe mineral composition of modern marine hydrothermal sediments, revealing Cr isotope fractionations that imply deposition from an oxygenated deep ocean. I determined Cr isotope fractionations associated with the reduction of Cr(VI) in modern ferruginous sediments, revealing that the magnitude of Cr isotope fractionation in such environments is linked to the speciation of Fe and the oxygen penetration depth of the sediments. I determined Fe-speciation and trace metal abundances of sediments deposited during oceanic anoxic event 1a (OAE1a), revealing that during this interval the oceans were anoxic and Fe-rich (ferruginous) for more than 1 million years. Lastly, I determined the Fe-speciation of suspended and sedimented material from two modern ferruginous lakes, revealing that the mineral magnetite forms authigenically in the ferruginous water columns. This new knowledge of Cr and Fe proxy systematics will allow for more refined interpretations of paleo oxygen concentrations based on Cr isotope and Fe-speciation signals captured in the rock record through time.

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This thesis evaluates the applicability of flow-through time-resolved analysis (FT-TRA) to address problems ranging from determining mineral dissolution kinetics and dissolution regimes, to unraveling the elemental composition of multiple mineral phases in microfossils, to predicting drainage chemistry from mine waste. FT-TRA consists of a gradient pump, which continuously passes eluent of fixed or varying composition through a small flow-through reactor containing a small amount of solid sample. The effluent composition is then analyzed online using an inductively coupled plasma mass spectrometer in time-resolved mode, or is collected in a fraction collector for subsequent offline analysis, depending on the goal of the experiment. It is found that FT-TRA is well suited to study mineral dissolution kinetics. Using forsterite as a case study, it is shown that FT-TRA can be used to rapidly determine mineral dissolution rate parameters. The high temporal resolution data generated by FT-TRA documents in detail, and in real-time the gradual formation of surface leached layers as well as sporadic and abrupt exfoliation events occurring during dissolution. A range in eluent residence times in the reactor can be applied by controlling the eluent flow with the gradient pump, allowing for the empirical determination of the dissolution regime (surface- or transport-controlled), which must be established prior to interpreting mineral dissolution rates measured during the experiment. When combining FT-TRA data with pore scale modeling, dissolution rate constants can still be determined, even when the dissolution experiment is conducted under transport-controlled conditions. The added value of this continuous eluent flow system for assessing the leaching behavior of mine waste is also evaluated. The ability to carry out experiments in a relatively short time period provides a new means to elucidate the mechanism and conditions resulting in the release of toxic metals from mine waste during weathering. Finally, using insight gained from studying mineral dissolution kinetics, the premise on which FT-TRA was used to distinguish the elemental ratios of different biogenic mineral phases in microfossils for paleoceanographic reconstruction is re-evaluated.

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Recent Arctic warming and reduced summer sea ice extent have stimulated increased research into the role of sea ice in the high latitude carbon cycle. Using data collected on a number of field expeditions throughout the Arctic Ocean, I apply a multi-tracer approach to investigate the influence of the sea-ice life cycle on the biological and abiotic export of CO₂ into the sub-surface. The results of this study illuminate the role of sea ice in polar carbon cycling across the perennial sea ice region of the central Canada Basin and in the seasonal ice zone of the Canadian Arctic Archipelago. In the perennial sea ice region, lateral exchanges of shelf derived carbon were found to exert the most important control on carbon distribution in the central Canada Basin, both in the surface mixed layer and in the sub-surface halocline. Stable carbon isotope data suggest that surface water particulate organic carbon is derived, to a large extent, from external inputs from Eurasian rivers. Further, results from a suite of geochemical tracers show that sub-surface accumulation of dissolved inorganic carbon in the halocline reflects an organic matter remineralization signature derived from the shelves and transported into the halocline by dense Pacific winter waters. Within the seasonal ice zone, observations over the winter-spring transition illustrated a highly dynamic carbon cycle, and results from this study provide new insight into the biological, physical and chemical factors which contribute to C cycling in different depth horizons of the ice over this period. Physical constraints on inorganic carbon cycling dominated CO₂ distributions in the majority of the ice column early in the season. As the melt period advanced, sea ice melt dilution led to decreasing CO₂ partial pressures in brine, contributing to pCO₂ under-saturation and CO₂ uptake from the atmosphere as the melt period advanced. In contrast, the carbonate system in bottom ice layers was much more closely tied to the flourishing algal community. Our inorganic carbon system measurements within natural sea ice brine samples further reveal the limitations of current thermodynamic constants used to compute carbonate system equilibrium in sea ice systems.

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Two important questions in the fields of paleoceanography and marine biogeochemistry (the reconstruction of past changes in the strength and geometry of the ocean’s overturning circulation and the quantification of particle flux to the seafloor) are addressed using three isotopes from the U-decay series (²³⁴Th, ²³⁰Th and ²³¹Pa).Two-dimensional scavenging models of the Atlantic and Pacific Ocean were tuned to reproduce the ²³⁰Th and ²³¹Pa seawater activity profiles measured in these oceans and used to establish the distribution of sediment ²³¹Pa/²³⁰Th generated by simple meridional overturning circulation cells. The results indicate that circulation is the main factor controlling the distribution of sediment ²³¹Pa/²³⁰Th in the Atlantic and confirm the use of this proxy as a paleocirculation tracer. In the Pacific, both circulation and boundary scavenging are important in determining the distribution of sediment ²³¹Pa/²³⁰Th. Thorium-234 scavenging and moored sediment traps yield similar particle flux estimates in Saanich Inlet, on the coast of British Columbia. This study highlights the possibility of estimating the flux of organic carbon in coastal waters by simply measuring ²³⁴Th and POC on particles, which would provide a simple and rapid method for large scale monitoring. Measurements of ²³⁴Th and ²³⁰Th dissolved in seawater and adsorbed on three different size classes of particle were used to estimate particle flux in the epipelagic and mesopelagic zone of the ocean at station Papa. The results suggest that a significant fraction of the carbon flux can be associated with very large, rapidly-sinking particles with very low Th activities, and unaccounted for in Th-based flux estimates.

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