Theses (Earth and Ocean Sciences)

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    EONS: A new biogeochemical model of Earth's longterm evolution
    (2024-01-31) Horne, Julia; Goldblatt, Colin
    I present Earth’s Oxygenation and Natural Systematics (EONS): a new, fully coupled biogeochemical model of the atmosphere, ocean, and their interactions with the geosphere, which can reproduce major features of Earth’s evolution fol- lowing the origin of life to the present day. The model includes an interactive biosphere, cycles of carbon, nitrogen, phosphorus, and oxygen, and climate. A nominal model run initialized in the Eoarchean resolves emergent surface oxy- genation, nutrient limitations, and climate feedbacks. The modelled atmosphere oxygenates in stepwise fashion over the course of the Proterozoic; a nearly billion year lag after the evolution of photosynthesis at 3.5 Ga is followed by a great oxi- dation event (GOE) at 2.4 Ga, which appears to be caused by the gradual buildup of organic matter on the continents imposing nutrient limitation on the biosphere by removing key nutrients from the ocean system. The simple climate system shows significant temperature shifts punctuate the oxygenation process, implying that major biological transitions possibly destabilized Earth’s climate. I expand upon this finding by adapting the climate system to include non-linearities such as ice-albedo and supergreenhouse feedbacks in order to investigate potential causes of Paleoproterozoic Snowball Earth events. My preliminary findings suggest that Paleoproterozoic glaciations may have preceded the GOE, and are more likely a result of perturbations to atmospheric CO2 than from declining CH4. This work demonstrates that forward modelling the entirety of Earth’s history with relatively few imposed boundary forcings is feasible, that the Earth system is not at steady state, and that our understanding of coupled C-N-P-O cycling as it functions today can explain much of the Earth’s evolution.
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    Quantifying spatiotemporal variability in mesozooplankton distribution and nutritional quality around seamounts within the Canadian Offshore Pacific Bioregion
    (2024-01-04) Labbé, Daniel M.; Dower, John; Sastri, Akash Rene
    Zooplankton are a diverse group of organisms that are the key link between primary producers and higher trophic levels in marine ecosystems. They are an important food source for many fish, marine mammal, and sea bird species, and are a major allochthonous energy source for seamounts. Therefore, zooplankton are an important indicator of ecosystem processes in the open ocean. The southern portion of the Canadian Offshore Pacific Bioregion (OPB), in the Northeast Pacific, is a seamount-rich environment that has recently been a target of conservation efforts by the Canadian Government via the proposal of the Tang.G̲wan - ḥačxwiqak - Tsig̲is (TḥT) Marine Protected Area (MPA). Oceanographic research expeditions (Northeast Pacific Deep-sea Exploration Project; NEPDEP) from 2015 to 2022 surveyed near seamounts to inform the monitoring and management of TḥT and other MPAs in the Canadian OPB. Part of this sampling program included oceanographic and zooplankton sampling. Marine monitoring typically only accounts for variability in zooplankton biomass and/or species abundance, overlooking potential variability in zooplankton nutritional quality. This thesis uses size fractionated zooplankton biomass samples to quantify the spatiotemporal variability in zooplankton biomass and nutritional quality (i.e., energy density, total lipids, and total proteins) in the Canadian OPB. The data presented in this thesis indicate that zooplankton distribution is driven primarily by mesoscale oceanographic processes and not by the presence of seamounts, per se. Interannual variability was the most significant driver of change in the zooplankton community, representing up to a 7-fold difference in biomass and up to a 2-fold difference in zooplankton nutritional quality, where generally warm (cool) water years had less (more) nutritious zooplankton communities. Spatially, there were distinct differences between zooplankton collected within offshore and continental slope regions, which influences the allochthonous energy provided to seamount ecosystems. This data was also used to identify key zooplankton taxa which appear to have a disproportionate effect on total zooplankton nutritional quality. Notably, Neocalanus spp. were associated with lipid-rich zooplankton samples, while gelatinous species (e.g., doliolids and salps) were associated with lower energy density. These data and analyses have important implications for the future monitoring and conservation of the Canadian OPB and provide important baseline characterization of the spatiotemporal variability of the zooplankton community around seamounts in this region.
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    Examining the Feasibility of Sentinel-1 InSAR data for landslide monitoring and failure forecasting in western Canada.
    (2024-01-03) Blenman, Peter; Schaeffer, Andrew; Nissen, Edwin
    Landslides are geological hazards that significantly threaten human life, infrastructure, and biotic habitat in areas with steep slopes. Precursory signs of a landslide can be undetectable or non- existent, making the evacuation of residents unlikely. The ongoing climatic cycles and geological triggers imposed on regions susceptible to landslides exhibit long-term ground movement superimposed with accelerations due to seismicity or precipitation. Landslide monitoring and forecasting aims to understand the structural dynamics of the slide accelerations to estimate when there will be a catastrophic failure. The thesis explores the potential of InSAR technology for monitoring slope movement in the western Canadian Cordillera. The study takes a two- pronged approach: first, investigating the capability of the technique to detect movement on slopes that have already undergone previous landslide activity, focussing on the Garibaldi Volcanic Complex (GVC) as a case study, and second, analyzing five sites that have recently experienced landslides to determine if InSAR technology could have forecasted the failures. The InSAR results presented in the thesis show that ground displacement occurred on the slopes of all the study sites, which corresponded with previous landslide activity. However, InSAR results collected during winter months were less detailed and frequent than those collected using a seasonal approach. The forecasting study discovered that all the sites displayed signs of preceding movement on the slopes, which were successfully detected by InSAR. Furthermore, each site encountered extreme weather conditions, resulting in catastrophic failure. The Elliot Lake and Ecstall River sites experienced seismic activity the same afternoon as the landslide events, potentially connected to glacial loss and retreat. Results obtained during snowfall were less reliable than the summer acquisitions. iv The results from the thesis demonstrate that the Sentinel-1 mission's temporal resolution is inadequate for creating a real-time monitoring system for landslide-prone slopes in western Canada. Factors that trigger landslide acceleration, such as precipitation, seismicity, and geological processes, can occur over decades or hours. Hence, the primary role of Sentinel-1 in landslide monitoring is identifying large-scale moving slopes. Future InSAR platforms could provide a promising solution with high temporal resolution, making landslide forecasting and monitoring a reality.
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    Crustal carbonate assimilation limits and CO2 production within arc magmas - The Jurassic Bonanza arc, Vancouver Island, Canada
    (2024-01-02) Morris, Rebecca; Canil, Dante
    The contribution of crustal-derived CO2 into arc magmas is controversial. Traditional views assume most CO2 outgassed at arc volcanoes is slab-derived, but recent studies show that active and ancient systems where arc magmas intrude carbonate may contribute significant crustal CO2. Quantifying CO2 from magma-carbonate reactions within the crust is important as it has implications on how we understand the long-term C cycle. In addition, the mechanism of carbonate assimilation into arc magmas is understudied, especially in the form of well-constrained field exposures that provide in situ reactions. Within this thesis, I document carbonate assimilation limits, CO2 production, and the mechanisms that drive assimilation for different scales of well-exposed magma bodies (m-scale dikes and sills, km-scale pluton) that intrude limestone within the Jurassic Bonanza arc, on Vancouver Island, Canada. Magma-carbonate reactions preserved in m-scale dikes is observed in two forms: 1) as unique orbicular dikes that outcrop above the level of stratigraphy of basalt-limestone reactions, and 2) as reacted margins (or ‘boundary melts’) preserved at basalt-limestone contacts in dikes and sills. Orbicular dikes consist of segregated Ca-rich microcrystalline orbicules, where orbicule compositions are similar to hybrid melts produced from basalt-limestone experiments. Binary mixing models of limestone into basalt confirms the orbicules form from assimilating up to 25 wt.% limestone into basalt, and are capable of producing up to 11 wt.% CO2. I interpret that orbicules formed from basalt-limestone reactions at depth, where produced calcic melts were transported upwards from recharging magma feeding the dikes in the lower section. Rapid cooling within the dikes preserved these segregated calcic melts in situ, where homogenization with the host melt was limited due to their varying viscosities. Reacted margins at basalt-limestone contacts in dikes and sills document boundary melts that are distinctly lighter in colour and texturally unique (glassy ± orbicules). Boundary melt compositions show unique Ca, U, and Sr enrichments, Si depletion, and 87Sr/86Sr that approaches host limestone values. Binary mixing models indicate the boundary melts form from ~20 wt.% limestone assimilation into basalt, suggesting that these reacted margins which make up to 4% of dike and sill volumes are capable of producing ~10 wt.% CO2. Contrasting viscosities between the boundary and interior melts appears to promote uphill diffusion of elements more enriched in the wallrock (U, Sr), and is preserved in these reacted margins that likely cooled in minutes. Extreme Sr enrichment (up to 5500 ppm) far above typical basalt concentrations (~400 ppm) in dike and sill interiors is surmised to occur from meltback of channel walls, where Sr-enriched boundary melts may remelt and concentrate in dike interiors where flow velocities are greatest. Magma-carbonate reactions from a km-scale gabbro pluton that intrudes limestone shows limited carbonate interaction, with the exception of a thin (<2 m) gabbroic chilled margin with elevated U, Sr, REEs, and 87Sr/86Sr ratios. This chilled margin is also in contact with a wide (~50 – 150 m) metamorphic aureole. Modeling of assimilation + concurrent fractional crystallization (AFC) indicates the chilled margin chemistry and mineralogy can be obtained from the uptake of 20 wt.% limestone into gabbro, suggesting this thin reacted veneer is capable of producing ~10 wt.% CO2. A lack of enriched 87Sr/86Sr ratios shows no indication of crustal enrichment from ~10 to >1000 m from the contact, which further constrains that any reaction with carbonate is limited to the outer ~10 m margin of the pluton, accounting for <1% of the total pluton volume. Results indicate that an enhanced extent of magma-carbonate reaction and CO2 production is via a network of shallow m-scale dikes and sills versus deeper km-scale plutons. Additional estimates on CO2 produced within the metamorphic aureole indicate that >89% of crustal-derived CO2 is liberated via wallrock decarbonation and <11% is liberated by magma from carbonate assimilation. Estimates of CO2 were extrapolated to calculate a flux for the Jurassic Bonanza arc that is notably lower than some present-day fluxes from arc volcanoes intersecting carbonate-rich lithologies (i.e., Etna, Popocatépetl). These lower estimates are likely due to thinner (<1 km) carbonate sequences in the Bonanza arc crustal substrate. Nonetheless, this work details the limited extent to which plutons assimilate limestone and caution any historical flux estimate where a consistent mass fraction of carbonate was assimilated into plutons. Experimental data from others further indicates that assimilation into km-scale magma bodies in thick transcrustal sections is limited with depth. The results from this study ultimately provide realistic and quantitative limits on arc-derived CO2 from upper crustal wallrock sources.
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    The Okanagan subterrane: a 'Subduction Initiation Rule' ophiolite in the Canadian Cordillera
    (2023-12-20) McEwen, Gerri L.; Johnston, Stephen T.; Canil, Dante
    The Okanagan subterrane in south-central British Columbia is commonly referred to as the basement to Quesnellia, a composite arc terrane in the Canadian Cordillera. Distinct geochemical, lithologic, and structural characteristics allow for interpretation of the Okanagan subterrane as a forearc ophiolite formed over a long-lived west-dipping subduction zone initiating in western Panthalassa. The forearc ophiolite model has significant impacts on existing Cordilleran models which focus on a North American origin for this terrane. The Okanagan subterrane is comprised of harzburgites, gabbros, basaltic lavas, and ocean basin sediments. The age of the subterrane spans the Middle Devonian through the Early Permian. New stratigraphic, structural, and whole rock geochemical data are presented together with a synthesis of pre-existing data from regional studies to better constrain the origins and evolution of the Okanagan subterrane. The results indicate a transition from quiet shallow ocean basin deposition to the initiation of subduction and construction of a new forearc crustal sequence and mantle wedge. The earliest crustal sequences were formed from the eruption of basalts geochemically similar to those produced at oceanic spreading centres in the Late Devonian to earliest Mississippian. A chemostratigraphic magmatic progression through the Early and Middle Mississippian resulted in lavas becoming increasingly island arc-like in composition. Uplift and erosion dominated through the Pennsylvanian and Permian, punctuated by periods of carbonate deposition and capped by a regional Permo-Triassic unconformity and a Mesozoic volcanic arc. The Okanagan subterrane was affected by two main compressional deformation events, both interpreted as products of ophiolite obduction. The first, in the Permo-Triassic, resulted in tight to isoclinal folding, uplift, and erosion yielding the Permo-Triassic unconformity and the obduction of the Okanagan subterrane onto an extinct island arc within the lower plate. The second, in the Middle Jurassic, resulted in a southwest-verging fold and thrust belt which likely records the subsequent accretion to pericratonic terranes of western Laurentia. Comparing this ophiolite to other supra-subduction zone ophiolites worldwide allows for an interpretation of the Okanagan subterrane as a forearc ophiolite constructed during Late Devonian intra-oceanic subduction initiation.
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    Circulation and deep water renewal in Douglas Channel, British Columbia, Canada
    (2023-09-20) Wan, Di; Foreman, Michael George Garvin; Dosso, Stanley Edward
    Douglas Channel is the main waterway in the fjord system on the west coast of British Columbia (BC), Canada, connecting the town of Kitimat to Queen Charlotte Sound and Hecate Strait. A 200 m depth sill divides Douglas Channel into an outer and an inner basin. This thesis examines the sub-tidal circulation and deep water renewals in Douglas Channel using mooring data collected between 2013 and 2015. It was found that the overall circulation in Douglas Channel has a three-layer structure in winter and a four-layer structure in summer when deep water renewal occurs. The circulation is a mixture of estuarine flow, wind-driven flow, and the barotropic and baroclinic responses to changes to the surface pressure gradients. The surface current velocity fluctuation is dominated by the along-channel wind-driven currents, whereas a counter-wind velocity response is detected at 100--120 m depth. Further investigation of the wind-driven circulation shows that the physically narrow Douglas Channel is dynamically wide and the cross-channel geostrophic balance determines the along-channel velocity layer thicknesses and directions as found in wide channels. This result can be generalized to other flows, such as the surface outflow in the estuarine circulation, and provides a possible explanation to the disagreement between the thicknesses of the surface velocity layer and the stratification layer. At depth, the renewal events are connected to the availability of dense water on the shelf. The examination of the dense water variation leads to evidence that a long-range connection exists between the BC shelf water density and remote influences, expanding the studies of the processes and frequencies that influence upwelling on the BC coast to the quasi-biweekly and intraseasonal bands and the equatorial Pacific and northern California and Oregon regions. These findings will be able to direct future studies in this region, as well as the entire BC shelf upwelling dynamics.
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    Remote sensing investigations of immature faulting and distributed coseismic deformation
    (2023-09-07) Gaudreau, Élyse; Nissen, Edwin
    Anticipating future earthquake behavior requires active faults to be accurately mapped as well as a wealth of knowledge on their physical properties and tectonic histories, but many incipient faults are unknown until after a damaging earthquake strikes. Earthquakes on so-called immature faults are also associated with near-surface slip that is distributed over zones hundreds to thousands of meters wide, rather than focused along a narrow fault with a clear surface expression as are their mature counterparts. This results in an underestimation of seismic hazard, as field surveys cannot capture the full coseismic displacement field, and typical fault slip inversions are unable to model this distributed deformation. In this thesis, a suite of cutting edge geodetic remote sensing techniques --- Interferometric Synthetic Aperture Radar (InSAR), photogrammetry and correlation of historical optical imagery --- are exploited to investigate what controls the localization of surface slip and the consequences of common modeling assumptions such as approximating a fault as a series of infinitely thin planes in an elastic halfspace. In doing so, I also shed light on the active tectonics of three study regions --- northern Alaska, southern California and the Tibetan plateau --- with important implications for seismic hazard assessments. The 1971 Mw 6.6 San Fernando, California, 2018 Mw 6.4 and Mw 6.0 Kaktovik, Alaska, and 2021 Mw 7.4 Maduo, China earthquakes are three examples of major earthquakes to have ruptured immature faults that were previously unknown or considered inactive, demonstrating that incipient faults pose a significant seismic hazard. The coseismic displacement field of the 1971 San Fernando, California earthquake, extracted from historical aerial imagery, reveals in unprecedented detail the complexity of the surface deformation. There is a discrepancy between seismic data and surface (geologic and geodetic) displacement data that may be explained by the strong dynamic stresses inferred from nearby accelerometer recordings. For the 2018 Kaktovik, Alaska earthquakes, a combined analysis of InSAR and seismological data reveals that deformation in this region is accommodated by a complex network of unmapped faults that may be influenced by inherited Cordilleran structures. A survey of well-studied earthquakes from across the Tibetan plateau suggests that inherited structures may have also played a significant role in the 2021 Maduo earthquake, possibly enabling its unusually long rupture and therefore large magnitude. Surface displacements mapped with optical imagery and InSAR show that surface slip during the 1971 San Fernando, California earthquake and multiple recent earthquakes in the Tibetan plateau was distributed over zones hundreds of meters wide. I show that this may partly explain why InSAR-derived coseismic slip models systematically underestimate near-surface slip, with the degree of bias partly dependent on fault complexity and thus likely influenced by fault maturity. I also demonstrate that independently inverted geodetic slip models are unreliable in the shallowest 1 km of the crust, suggesting that the proportion of this shallow slip deficit that can be attributed to distributed, inelastic surface deformation cannot be meaningfully estimated.
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    Investigating Suitable Geochemical Tracers for Monitoring CO2 Sequestration in Offshore Deep-Sea Basalt in the Cascadia Basin
    (2023-08-31) Louis, Emma; Moran, Kate; Coogan, Laurence A.
    Carbon dioxide concentrations in the atmosphere have drastically increased due to human activities, causing accelerating climate change. Carbon dioxide (CO2) removal technologies are now deemed essential to aid in reaching international climate goals to limit further global surface temperature increase. As deep-sea basalt has significant potential to permanently sequester enormous quantities of CO2 but has never been tested, an investigation into monitoring this process was carried out with an emphasis on the use of geochemical tracers to verify the success of CO2 sequestration into deep-sea basalt as part of the Solid Carbon feasibility study. In basalt, CO2 that would otherwise be released into the atmosphere is sequestered through chemically binding the injected CO2 to form carbonate minerals in the pore spaces of the basalt. This research investigates geochemical tracers typically used in hydrogeologic and carbon capture and storage studies to determine their suitability at the temperature and pressure conditions within the basaltic crust in the Cascadia Basin. The most suitable conservative tracer for the planned CO2 injection experiment is SF5CF3, for verifying CO2-rich fluid breakthrough and determining fluid velocities and rates of dilution. The most suitable reactive tracer is concluded to be stable carbon isotopes, for confirming carbon has been removed from formation fluids through precipitation of carbonate minerals. Fluid samples collected before, during, and after injection with long-term osmotically pumped fluid sampling systems and mobile pumping systems could be used to analyze tracer concentrations, in addition to analyzing alkalinity, pH, dissolved inorganic carbon, and major ion and trace element concentrations to inform geochemical changes occurring in-situ in the deep-sea basaltic aquifer. A variety of measurements from sensors connected to the NEPTUNE cabled observatory were also evaluated to collect geochemical data in real time. Finding suitable tracers and geochemical monitoring parameters for the in-situ conditions in deep-sea basaltic crust is necessary for interpreting geochemical changes due to CO2 injection. The monitoring parameters outlined in this research may be useful in future offshore deep-sea basalt CO2 injection scenarios with more understanding of tracer behavior in supercritical CO2/water systems under high pressures and temperatures.
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    Spatial and temporal variability in the geochemistry of the sediments at the Main Endeavour Field, Juan de Fuca Ridge
    (2023-06-01) Mills, Melissa; Coogan, Laurence; Cullen, Jay T.
    Sediments near hydrothermal vents are enriched in metals derived directly from hydrothermal fluids (e.g., Fe, Cu, Zn, Mn) and those associated with scavenging and co-precipitation from seawater with hydrothermally derived Fe-sulfides and Fe-oxyhydroxide minerals (e.g., P, V, Co, Mo, As, REEs). The sediments surrounding active venting have high concentrations of these elements which decrease with distance from the vents due to both hydrothermal plume dilution with seawater and sedimentation of hydrothermal particles. The composition of hydrothermal sediments from the Main Endeavour Field on the Juan de Fuca Ridge, approximately 300 km off the coast of Vancouver Island, was determined using samples collected in sediment traps at three locations along a transect below the hydrothermal plume. These traps were deployed on-axis, 3 km, and 9 km off-axis allowing the spatial variability of the hydrothermal component of the sediments to be assessed. The chemical composition and mass accumulation rates of the hydrothermal component is governed by particle formation in the near vent region and is controlled by particle settling rates with distance from active venting. The concentration and mass accumulation rate of the hydrothermal component of the sediment decreases rapidly with distance, with an order of magnitude decrease between the on-axis and 3 km off axis sediment trap samples, and a further 1-2 orders of magnitude decrease from 3 km off axis to 9 km off axis. Sediment trap samples are also used to create a high-resolution time series of hydrothermal sedimentation over the course of the approximately year sampling period, with each on-axis sample collecting 21 days of sediment and each off-axis sample collecting 12 days of sediment. These samples allowed for an initial assessment of the temporal variation in the chemical composition and mass accumulation rate of the hydrothermal sediment. The variability observed in the hydrothermal component mass accumulation rate suggests that physical oceanographic processes (e.g., flow reversal) impacts the rate of sedimentation. The base of a 50 cm sediment core, collected 2.6 km northwest of the Main Endeavour Field, was dated at ~6,000 years and a high-resolution geochemical reconstruction is used to determine how the hydrothermal component has changed on a 100–1000 year time scale. The comparison of the sediment core to fresh sediment collected by the sediment traps is used to understand how post-depositional changes affect the composition of the hydrothermal component preserved in sediment, as well as the utility of some elements, such as Mn, in reconstructing paleo-hydrothermal sedimentation.
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    Phytoplankton Dynamics in the Subarctic Northeast Pacific Ocean During the 2019 Marine Heatwave
    (2023-05-29) Kafrissen, Sile; Varela, Diana
    Marine phytoplankton are responsible for nearly 50% of all the primary productivity on Earth, and their response to climate change and anomalous climatic events impacts biogeochemical cycling. During the summer of 2019, the NE Pacific Ocean experienced a warming event that caused sea surface temperature anomalies of up to 4 °C. These types of events, called marine heatwaves (MHW), are becoming more common globally but their effects on pelagic primary producers are not yet well understood. In September 2018 and August-September 2019, temperature, salinity, nutrient concentrations, primary producer biomass, and uptake rates of carbon (ρC) and nitrogen (ρNO3) were measured along the Line P transect in the NE Pacific. Additionally, in August-September 2019, nitrate (NO3) and silicon (Si) uptake kinetic experiments were performed at five Line P stations in addition to five stations on the west coast of Vancouver Island to assess potential physiological limitation in phytoplankton from NO3 and Si. In 2019, temperatures in the euphotic zone along Line P were higher and the water column was more stratified at all stations than in 2018. Concentrations of dissolved NO3 and silicic acid (Si(OH)4) were anomalously low in 2019 and nutrient depletion extended approximately 500 km further offshore than in 2018. The Line P station closer to the shelf (P4) had a considerable reduction in chlorophyll-a (chl-a), biogenic silica (bSiO2) and the contribution of diatoms to the entire phytoplankton assemblage. From P4 to P20, the assemblage was dominated by small-celled phytoplankton (<5 um) in both years, but there was a relative increase in 2019. There were particularly unusual observations at the most oceanic station (P26) where the contribution of diatoms, concentrations of chl-a and bSiO2, and ρC, and ρNO3 were anomalously high in 2019 compared to regional averages. In 2019, the uptake of Si(OH)4 appeared to be substrate limited at the majority of stations while only station CS02 on the west coast of Vancouver Island appeared to have physiological limitation by NO3. Based on Michaelis-Menten uptake kinetics, half saturation constants (Km) ranged from 0.01-0.13 for NO3 and 2.33-18.3 for Si(OH)4, suggesting that assemblages are less efficient at Si uptake than NO3 uptake in the NE Pacific. Results from this study are consistent with observations from the other warming anomalies including the 2015 “blob” and the 1997/1998 ENSO event in the NE Pacific Ocean. The similarities observed during these ocean warming events suggest that phytoplankton in the NE subarctic Pacific may become increasingly susceptible to nutrient limitation, particularly from Si(OH)4, with increased stratification. Future investigations should focus on co-limitation studies of Fe and Si(OH)4, uptake rates of regenerated nitrogen sources, timing of bloom onsets, total annual biomass and trophic interactions with zooplankton during MHW events.
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    Modelling the global impact of carbon leaching from marine plastic debris on oceanic and atmospheric tracers
    (2023-05-17) Gurgacz-Safianowicz, Natalia; Weaver, Andrew J.; Eby, Michael
    Since the beginning of its large-scale production in the early twentieth century, plastic remains a critical material used throughout several industries. Despite serving many benefits, plastics are resistant to degradation and instead, accumulate in the ocean and affect marine ecosystems. The leaching of toxic compounds from plastics has been widely researched, but only recently have studies begun to explore dissolved organic carbon as a leachate from marine plastic debris, as well as its effects on microbial communities. Furthermore, research has only recently begun to explore the potential effects of plastic-derived carbon leaching on the global carbon cycle and hence Earth’s climate system. Here we quantify an upper bound estimate for this effect using the UVic Earth System Climate Model with the addition of global and regional carbon fluxes, based on current and projected marine plastic values. The results of these modelling integrations indicate that the addition of the plastic-derived DIC flux results in only very minor changes to the Earth’s climate, emphasizing the need to focus research on plastic production and incineration-associated emissions, which are a greater threat to the climate.
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    The Microbial Ecology of Nitrous Oxide Cycling in Marine Environments: Linking community dynamics to ecosystem processes
    (2023-05-08) Jameson, Brett Douglas; Juniper, S. Kim; Stevens, Catherine J.
    Nitrous oxide (N2O) is an increasingly abundant, atmospheric trace-gas that contributes to climate change and stratospheric ozone depletion. Marine environments act as a net source of N2O to the atmosphere at global scales resulting from the combined microbial processes of nitrification and denitrification. Considerable effort has been directed toward understanding the environmental drivers of N2O production and consumption in the ocean over the past few decades. However, comparatively little is known about the ecological mechanisms that facilitate N2O cycling in marine environments and how this relates to environmental variability. This research attempts to resolve some of these knowledge gaps by leveraging modern molecular tools and biogeochemical rate measurements to identify links between microbial community dynamics and N2O production across a wide range of marine environments. The first data chapter considers Saanich Inlet, a seasonally anoxic fjord located on Vancouver Island, Canada, as a model oxygen deficient zone for investigating patterns of microbial community assembly in relation to variable N2O production rates across spatial and temporal redox gradients. Network analysis of prokaryote 16S rRNA amplicon sequences delineated discrete community subnetworks that were structured around several putative keystone taxa and displayed contrasting water column distributions and roles in N2O cycling. Keystone taxa implicated in coupled carbon, nitrogen, and sulfur cycling were prominent in the low-oxygen subnetwork and correlated well with N2O production from denitrification in waters demonstrating net N2O consumption. Conversely, oxycline subnetworks were characterized by keystone aerobic heterotrophs that correlated with nitrification rates and water column N2O accumulation. This work presents a first assessment of the relationships between microbial community interaction networks and N2O cycling rate processes in the ocean. The remainder of this dissertation focuses on N2O cycling in sediment environments, which can act as net sources or sinks of N2O at local scales. The third chapter resolves an important data gap with respect to N2O fluxes from continental margin sediments underlying the northeast subarctic Pacific (NESAP) oxygen deficient zone, a previously unstudied environment with respect to N2O cycling. This work reports the first sub-millimeter resolution porewater N2O profiles in offshore sediments and employs a profile interpretation model to demonstrate that these environments are a considerable source of N2O to the water column. Finally, experimental manipulations provided evidence that upwelling conditions can stimulate N2O production and efflux from continental shelf sediments. Chapter Four builds on this work by adapting this procedure for work at low N2O concentrations to quantify the N2O sink capacity of minimally impacted mangrove sediments. Molecular data collected from both the mangrove and NESAP continental margin sediments was then used to identify relationships between microbial community dynamics and sediment N2O source/sink status. Mangrove N2O sinks had higher abundances and expression levels of ‘atypical’ N2O reductases (nosZII), suggesting that net N2O consumption in nitrogen-limiting systems may be driven by non-denitrifying N2O scavengers. NosZII was associated with taxonomic groups implicated in dissimilatory nitrate reduction to ammonium (DNRA), a prominent nitrogen conservation pathway in nitrogen-limiting systems. N2O source sediments from the NESAP contained higher abundances of putative ammonia oxidizing Archaea and were associated with elevated expression levels of typical nosZI variants, suggesting likely contributions from both nitrification and denitrification.
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    How variability in late Neoproterozoic shallow water carbonate δ13Ccarb values and modern fluvial δ13CDIC values may impact fundemental assumptions in carbon isotope stratigraphy
    (2023-04-28) Wren, Olivia; Husson, Jon
    The carbon isotope composition of ancient shallow water carbonates (δ13Ccarb) serves as the primary archive of ocean chemistry for a majority of Earth history and provides insight into the dynamics of global carbon cycling in deep time. The use of shallow water carbonates in reconstructing ancient ocean chemistry requires that δ13Ccarb values reflect the global mean open-ocean δ13CDIC value (Kump and Arthur, 1999). However, in practice, this assumption is difficult to constrain due to several factors, such as the potential for diagenetic alteration to primary δ13Ccarb values (Ahm et al., 2018; Higgins and Schrag, 2003; Allan and Matthews, 1982), and the possibility that δ13Ccarb values reflect a local, rather than global, marine δ13CDIC signal (Geyman and Maloof, 2019; Patterson and Walter, 1994a; Swart and Eberli, 2005; Swart et al., 2009). It is understood that freshwater input can strongly influence local marine δ13CDIC values and potentially shallow water carbonate δ13Ccarb values in marginal marine environments (Patterson and Walter, 1994a,b), but a global dataset of δ13C values for river DIC has not yet been analyzed. Here, I explore the relationship between fluvial DIC, marginal marine DIC, and shallow water carbonate δ13Ccarb values by analyzing: (1) δ13Ccarb measurements for several stratigraphic sections from a late Neoproterozoic shallow water carbonate platform (Mount Fitzwilliam) and (2) a global compilation of fluvial δ13Ccarb values and associated river chemistry data. Through this analysis, I explore whether ancient shallow water carbonate δ13Ccarb values reflect open-ocean DIC or local processes, what dominant process controls modern fluvial δ13CDIC values in a global synthesis, and whether it is possible to determine the fluvial isotopic influence on ancient shallow water carbonate δ13Ccarb values. The shallow water carbonate δ13Ccarb values measured from Mount Fitzwilliam reveal both facies dependence within single stratigraphic sections and significant lateral variability across stratigraphic sections of putatively similar age. This finding is contrary to the expectation that δ13Ccarb values will be reproducible across laterally displaced sections when values reflect open-ocean DIC. While multiple models are discussed to explain this variability, both field observations and isotope data point to freshwater influence on δ13Ccarb values. The observed variability and facies dependence further complicate the reliability of shallow water carbonate δ13Ccarb values in global carbon cycle reconstructions and stratigraphic correlations. Furthermore, these results emphasize the importance of testing for diagenetic and local signals through the measurement of multiple, parallel sections of ancient shallow water carbonates. The interpretation for a fluvial influence on Mount Fitzwilliam δ13Ccarb values emphasizes the need better understand the relationship between freshwater input and shallow water carbonate δ13Ccarb values. In order to accomplish this, it is first necessary to identify the factors that govern fluvial δ13CDIC values. Analysis of the global fluvial δ13CDIC dataset, which captures 4,730 δ13CDIC measurements from 612 rivers across all seven continents, offers the opportunity to explore the dominant processes that control fluvial DIC and δ13CDIC values. Values of fluvial δ13CDIC in the global synthesis are highly variable, ranging from −27.5‰ to 5.1‰. The mean value from this dataset (−9.3‰) is more negative than the assumed river input value used in classic carbon cycling models (−5‰; Kump and Arthur, 1999). Explanations for the high variability and mean value are explored by comparing fluvial δ13CDIC values with estimated isotopic ranges that reflect the dominant sources of fluvial DIC. Through this analysis, I found that 67% of fluvial δ13CDIC values fit into the biotic carbonate weathering δ13CDIC range (−12.5‰ to −5.5‰). These results show the clear effect of soil respiration in the terrestrial biosphere. The strong role that biotic weathering plays in controlling fluvial δ13CDIC values, and the wide range of δ13CDIC values observed, has serious implications for the interpretation of δ13Ccarb values from shallow water carbonates deposited in near-shore, marginal marine environments, and emphasizes the need to strongly consider the potential for freshwater influence on shallow water δ13Ccarb values in both modern and ancient studies.
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    Glacial Isostatic Adjustment Modelling for Crustal Motion in North America
    (2023-04-27) Brierley-Green, Connor; James, Thomas Sinclair; Leonard, Lucinda
    Due to the expansion and retreat of the large ice sheets that covered most of Canada and parts of the northern United States during the Last Glacial Maximum (LGM), the surface of North America presently exhibits vertical and horizontal crustal motion due to glacial isostatic adjustment (GIA). The purpose of this study was to explore the effects that Earth rheology parameters have on this crustal motion and to find a model that best fits the observations. The Earth is assumed to be spherically symmetric, and this thesis explores the effects of varying the Earth-model parameters and the general limitations of the laterally homogeneous approximation. The GIA models used in this study are randomly generated from a wide range of Earth rheological parameters for a 3-layered mantle viscosity model with the spherically symmetric Preliminary Reference Earth Model (PREM) for continuous density and elastic parameters. The surface loading model is ICE6G_C. A new Earth response calculation method dubbed the hybrid method is presented to calculate as much of the normal mode response as possible while still being accurate and robust. The crustal motion predictions of the randomly generated GIA models were compared to the observed MIDAS velocity fields for selected Global Navigation Satellite System (GNSS) sites across North America. The goodness-of-fit was assessed through a Root-Mean-Square (RMS) calculation of the residual velocities. Three types of best models were produced: one for minimizing the vertical crustal response residuals, one for the horizontal crustal response, and one for the combined vertical and horizontal response. The horizontal and combined response exhibited two optimal viscosity profile ranges that produced small residuals, with the global optimum transitioning between these two optimal ranges between 100 and 120 km thick lithospheres, while the vertical response’s optimal viscosity profile range was relatively consistent across all tested lithosphere thicknesses. The optimal viscosity profile for the vertical response was close to other previously published viscosity profiles like VM5a and VM7, and it was most similar to VM1. The horizontal and combined response viscosity profile before the 100 – 120 km transition was also similar to VM1, but after the transition the viscosity profile shifted substantially, with the parts of the viscosity profile changing by more than an order of magnitude. The best vertical response was for a lithospheric thickness of 100 km. The horizontal and combined responses did not show a well-defined minimum until the viscosity profiles across the 100 – 120 km transition were extrapolated before and after the transition. With this extrapolation, both the horizontal and combined showed a minimum RMS residual at 100 km akin to the vertical response. Using 100 km thickness as the best model for all responses, the RMS of the residuals were 1.012 and 0.684 mm/yr for the vertical and horizontal response respectively and 1.303 (vertical) and 0.791 (horizontal) mm/yr for the combined response. For the null hypothesis (no GIA model), the RMS values of the observations were 3.244 and 1.321 mm/yr for the vertical and horizontal responses, respectively. The vertical and horizontal crustal motions of the best combined response model are more similar to the crustal motions of the best horizontal response model than to the crustal motions of the best vertical response model, suggesting that the combined model favours the horizontal constraints over the vertical constraints. Despite the extensive search through Earth rheology, the residuals of the best models are still relatively large. This indicates the potential limitation of the spherically symmetric approximation and the need to incorporate lateral heterogeneity to produce an improved fit to the observations. It may also indicate that other processes, such as surface hydrological change, contribute significantly to the GNSS-observed crustal motion signal and would need to be considered in a future joint analysis.
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    Constraining Climate Model Projections of 21st-Century Global and Regional Warming
    (2023-04-11) Liang, Yongxiao; Gillett, Nathan Peter; Monahan, Adam Hugh
    Different climate models predict different amounts of future warming over the 21st century. Such uncertainty of future warming projections can be narrowed down by emergent constraints identified based on the relationships between projected warming across climate models and observable features of simulated past climate or climate change. For global means of projected 21st-century warming, using the observed historical global mean near-surface air temperature (GSAT) trend as a constraint results in a relatively low warming relative to unconstrained projections. Using climatological cloud metrics, robust historical predictors with reduced influence of internal variability, to constrain future warming produces a relatively high warming. Such different ranges of constrained projections can be likely explained by the influence of internal variability in the constraint. By removing the unforced internal variability in historical GSAT trends, this study identifies a relatively higher 21st-century warming range than a constrained projection based on the raw GSAT trend, and brings GSAT trend constrained projections into much closer agreement with projections constrained using climatological cloud metrics. Regarding regional constraint of projected 21st-century warming, this study demonstrates the skill of global metrics relative to regional ones, and justifies the climatology cloud metrics alone can robustly constrain regional warming over extratropical Northern Hemisphere.
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    N2O Production in the Global Ocean:Controls by Nitrification and Denitrification Pathways
    (2023-01-31) Pierce, Rebecca; Hamme, Roberta; Christian, James Robert
    Nitrous oxide (N2O) is a potent greenhouse gas with a high global warming potential, but the underlying processes behind N2O production and consumption are not well understood. The ocean is known to be a strong source of N2O to the atmosphere, yet ocean N2O production is poorly constrained at anywhere from 10 to 53% of the total N2O atmospheric source. In order to narrow down estimates of net N2O production in the ocean, I have created a steady state model to explore the uncertainties involved in the biological processes that contribute to N2O production and consumption (nitrification and denitrification). Model results show that oxygen and organic matter supply strongly control N2O production and consumption rates, and also demonstrate strong dependence on parameters such as the oxygen threshold for oxic versus suboxic remineralization, the oxygen concentration dependence of N2O reduction to N2, the proportion of suboxic and oxic remineralization in relation to oxygen, and the attenuation length scale of sinking organic matter. Additionally, I find that global net N2O production rates are heavily influenced by the oxygen data input into the model: interpolation of oxygen data or averaging of monthly oxygen data to an annual mean can lead to underestimation of denitrification. As suboxic conditions are required for denitrification to produce (and consume) N2O, precise and accurate oxygen data is integral to constraining estimates of global ocean N2O production. The results of the base case scenario of the model, as well as from a number of sensitivity experiments, indicate that the source and sink terms for N2O production are larger than suggested by many past models. N2O production and consumption by denitrification are high in oxygen deficient zones (19.2 Tg-N y-1 and 8.37 Tg-N y-1, respectively) while global total nitrification production is low (0.740 Tg-N y-1). N2O production by denitrification is primarily responsible for generating relatively high net N2O production rates in the global ocean (11.5 Tg-N y-1 for the base case scenario), and likely provide a greater proportion of global N2O emissions to the atmosphere than many previous estimates. Model representation of denitrification production and consumption rates is highly sensitive to changes in oxygen concentration and parameters related to oxygen, suggesting that altered ocean environments due to future climate change may have a large effect on N2O production in the ocean. This research isolates the uncertainties that most impact N2O production and consumption processes and suggests pathways of future research to narrow down these uncertainties. In particular, I recommend more in-situ oxygen sampling with high precision and accuracy (especially in oxygen deficient zones), experimental research to constrain important parameters, and separation of N2O production by nitrification, N2O production by denitrification, and N2O consumption by denitrification in future model development (as in this model).
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    The State Dependency of Climate Sensitivity and Cloud Responses to CO2 Doubling
    (2023-01-31) Smith, Brandon; Goldblatt, Colin
    Cloud feedbacks are a large source of uncertainty in paleoclimate studies and in the constraint of climate sensitivity, and it is thought that climate sensitivity depends on climate state because of these feedbacks. Here we evaluate the state dependence of both climate sensitivity and cloud responses to CO2 doubling for a palate of climates with varying solar luminosity and CO2 concentrations, yet near-equivalent global mean surface temperatures. We find that cloud responses to CO2 doubling and climate sensitivity are largely dependent on the magnitude of radiative forcing from doubling CO2 alone, and are therefore independent of climate state. We also determine that the most important responses in clouds occur in the mid-latitude storm tracks, owing to inhibited baroclinic activity as a consequence of a more equable climate under higher CO2 concentrations.
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    Time Varying Changes and Uncertainties in the CMIP6 Ocean Carbon Sink from Global to Local Scale
    (2023-01-06) Gooya, Parsa; Swart, Neil C.; Hamme, Roberta Claire
    As a major sink for anthropogenic carbon, the oceans slow the increase of carbon dioxide in the atmosphere and regulate climate change. Future changes in the ocean carbon sink, and its uncertainty at a global and regional scale, are key to understanding the future evolution of the climate. Here we report on the changes and uncertainties in the historical and future ocean carbon sink using output from the Coupled Model Intercomparison Project Phase 6 (CMIP6) multimodel ensemble and compare to an observation based product. We show that the ocean carbon sink is concentrated in highly active regions - 70 percent of the total sink occurs in less than 40 percent of the global ocean. High pattern correlations between the historical and projected future carbon sink indicate that future uptake will largely continue to occur in historically important regions. We conduct a detailed breakdown of the sources of uncertainty in the future carbon sink by region. Consistent with CMIP5 models, scenario uncertainty dominates at the global scale, followed by model uncertainty, and then internal variability. We demonstrate how the importance of internal variability increases moving to smaller spatial scales and go on to show how the breakdown between scenario, model, and internal variability changes between different ocean regions, governed by different processes. Using the CanESM5 large ensemble we show that internal variability changes with time based on the scenario, breaking the widely employed assumption of stationarity. As with the mean sink, we show that uncertainty in the future ocean carbon sink is also concentrated in the known regions of historical uptake. Patterns in the signal-to-noise ratio have implications for observational detectability and time of emergence, which we show to vary both in space and with scenario. We show that the largest variations in emergence time across scenarios occurs in regions where ocean sink is less sensitive to forcing - outside of the highly active regions. In agreement with CMIP5 studies, our results suggest that to detect changes in the ocean carbon sink as early as possible, and to efficiently reduce uncertainty in future carbon uptake, modelling and observational efforts should be focused in the known regions of high historical uptake, including the Northwest Atlantic and the Southern Ocean.
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    Algorithmic Hallucinations of Near-Surface Winds: Statistical Downscaling with Generative Adversarial Networks to Convection-Permitting Scales
    (2022-12-21) Annau, Nicolaas J.; Monahan, Adam H.; Cannon, Alex J.
    Providing timely and accurate small-scale information about weather and climate is challenging – especially for variables strongly controlled by processes that are unresolved by low-resolution (LR) models. Motivated by this challenge, this thesis employed emerging artificial intelligence methods from the fields of computer vision and deep learning for the statistical downscaling of surface wind variables to convection-permitting scales. Specifically, Generative Adversarial Networks (GANs) were conditioned on LR inputs to deterministically generate possible arrangements of high-resolution (HR) surface wind patterns using super-resolution (SR). SR models were trained over several subregions in the contiguous United States and southern Canada, allowing for a systematic analysis of the methods. In addition to matching the statistical properties of the target dataset, GANs generate fields with impressive realism and computational efficiency, making them attractive for operational statistical downscaling. However, objectively assessing the realism of the SR models required a careful selection of evaluation metrics. Using power spectra successfully revealed how altered GAN configurations changed spatial structures in the generated fields, where biases in variability originate, and the role of including additional LR covariates in the SR methods. Inspired by recent work in the computer vision field, a novel methodology that separates spatial frequencies in HR fields was used in an attempt to optimize the SR GANs further. This method, called frequency separation, ultimately deteriorated the realism of the generated HR fields. However, frequency separation revealed how spatial structures are influenced by the metrics used to optimize the SR methods. A covariate-sensitivity analysis was also performed, illustrating how physical relationships between the large and small scales are mirrored in the GANs. Furthermore, a methodology was used to evaluate serial dependence in the SR methods. Even without explicitly including temporal information while training, serial dependence in the fine-scale structures of the generated fields was present. Although this thesis did not use fully-stochastic SR models (as has been done in existing work), the results provide valuable insights into variability, biases, covariates, and the optimization problem of SR.
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    Changes in Cross-Equatorial Ocean Heat Transport Impact Regional Climate and Precipitation Sensitivity
    (2022-12-01) Oghenechovwen, Oghenekevwe C.; Singh, Hansi
    Do changes in how cross-equatorial energy transport is partitioned between the ocean and atmosphere impact the hemispheric climate response to forcing? To find out, we alter the cross-equatorial ocean heat transport in a state-of-the-art GCM and ascertain how changes in energy transport and its partitioning impact hemispheric climate and precipitation sensitivity following abrupt CO2-doubling. We further evaluate the applicability our results in CMIP6-class ESMs, where AMOC facilitates the northward cross-equatorial ocean heat transport. In our experiments, changes in ocean cross-equatorial energy transport trigger compensating changes in atmospheric energy transport through changes in the Hadley cells and a shift in the Intertropical Convergence Zone. However, the climate sensitivity in each hemisphere is linearly related to the ocean heat transport convergence, not atmospheric energy transport convergence, due to the impact of ocean heating on evaporation and atmospheric specific humidity. Similarly, we also find that ocean heat transport convergence controls the hemispheric precipitation sensitivity through the impact of ocean heating on surface evaporation. This relationship is also evident in CMIP6 models, where we find differences in hemispheric precipitation sensitivity to be related to the Atlantic Meridional Overturning Circulation (AMOC). Changes in the AMOC control hemispheric differences in upper ocean heat content, which then affect how the hydrologic cycle responds to CO2 forcing in each hemisphere. These results suggest that ocean dynamics impact the hemispheric climate response to CO2 forcing, particularly how much regional precipitation changes with warming.