Browsing by Subject "Sulfide"
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Item Biogeochemical Interactions and Cycling of Sulfur, Iron, and Carbon in Sulfate-Impacted Riparian Wetlands and Wild Rice Waters(2018-08) Torgeson, JoshuaSulfide accumulation in the porewater of freshwater aquatic systems has been shown to inhibit the growth of many aquatic macrophytes, including wild rice. While interactions between sulfur (S), carbon (C), and iron (Fe) cycles are recognized, secondary “cryptic” S cycles are much less understood; these cycles favor reduction of sulfate over Fe, contrary to traditional thermodynamic expectations; these “cryptic” cycles have been suggested to occur at Second Creek through models by Ng et al. (2017). Using field observations, hydrologic monitoring, and geochemical analyses, we found that changes in hyporheic flux result in changes in porewater SO42- concentrations. Additionally, we have found that intermediate valence S species may act as primary sinks for excess dissolved sulfide. Our comparison study between a SO42--impacted stream and a less-impacted river demonstrates that the accumulation of porewater sulfide may be suppressed through limited TOC, excess sediment Fe, or through generation of S-intermediates.Item The effect of high sulfate loading on methylmercury production, partitioning, and transport in mining-imapcted freshwater sediments and lakes in northeastern Minnesota(2015-02) Bailey, Logan TimothyMethylmercury (MeHg) is a highly toxic form of mercury with the ability to bioaccumulate in food webs. The bioaccumulation of MeHg leads to elevated MeHg levels in fish tissue and poses a threat to public health. Thus MeHg concentrations in surface waters - which may be a result of water column MeHg production, or sediment MeHg production and subsequent flux from sediment porewater - are of particular concern. The production of MeHg from inorganic mercury (iHg) is primarily a result of sulfate-reducing bacteria (SRB) activity in anoxic aquatic environments.Ongoing and historic mining activity on the Mesabi Iron Range (Minnesota, USA) has led to elevated sulfate levels in the downstream waters of the St. Louis River watershed. In an effort to understand the effect of mining-related sulfur-loading on the production and partitioning of MeHg, sediment samples were collected and analyzed from sulfur impacted and non sulfur-impacted lakes and wetlands within the watershed. Additionally, the water column and inlet and outlet streams of a mesotrophic lake (Lake McQuade) were sampled intensively during summer stratified conditions in order to identify the sources and sinks of MeHg to the lake system and determine the potential for MeHg export downstream.Results suggest that dissolved sulfide plays a large role in governing MeHg dynamics in sulfate-impacted freshwater sediment. Consistent with previous research, net MeHg production appeared to be inhibited in sediments with dissolved sulfide >60 uM. However, these high concentrations of dissolved sulfide were accompanied by increased partitioning of MeHg into the porewater phase, potentially increasing the fraction of MeHg available to be transported into surface waters.Sediment at sulfate-impacted sites was generally characterized by high dissolved sulfide and a low potential for long-term net MeHg production. However, the accumulation of dissolved sulfide in sediment porewaters can be limited by the availability of free labile iron (Fe2+) and consequent iron-sulfide precipitation reactions. In the results presented here, high sulfur-loading at two sites appeared to have consumed the available free labile iron and created conditions which allowed for the accumulation of dissolved sulfide and inhibition of MeHg production in the sediment. However, relatively high sulfur-loading(>100 mg/L) to a third site where iron remains in excess of sulfur in sediment may have led to robust net MeHg production, in absence of inhibitory dissolved sulfide concentrations. Accumulation of MeHg in the hypolimnion of Lake McQuade occurred during summer 2012 during a time when bottom water sulfate was being consumed. Though some uncertainty remains as to the ultimate source of the MeHg, estimates of MeHg inputs and outputs to the hypolimnion suggest that water column production was a primary source of MeHg to the hypolimnion during the stratified summer months. Following the wet spring months when inputs were dominated by upstream flows, the flux of MeHg across the limnetic surface was estimated to be the primary source of MeHg to the epilimnion during the stratified summer months. However, most of MeHg input to the epilimnion was apparently degraded prior to being exported to the outlet stream. Thus, despite mid-summer accumulation of MeHg in the hypolimnion, the combination of stratification and substantial degradation in the epilimnion acted to limit export of MeHg out of Lake McQuade.As a whole, Lake McQuade acted as small net source of MeHg to the surrounding water system during the summer months of 2012. Evidence points to a brief rise in MeHg export immediately following lake turnover in Mid-August due to the release of hypolimnetic MeHg to surface waters during lake mixing.Item Emplacement and Crystallization Histories of Cu-Ni-PGE Sulfide-mineralized Peridotites in the Eagle and Eagle East Intrusions(2018-06) Mulcahy, ConnorThe Eagle and Eagle East intrusions, located about 40 kilometers northwest of Marquette, MI, are two small, partially exposed, sub-vertical, funnel-shaped mafic/ultramafic intrusions emplaced in Paleoproterozoic black slates. Both intrusions host economic Ni-Cu-(PGE) sulfide deposits, the Eagle intrusion in its main body and the Eagle East intrusion in its feeder at depth. The Eagle deposit has been being mined by the Lundin Mining Corporation since 2014, which is now also expanding its operation to mine the Eagle East deposit. Transmitted light petrography, scanning electron microscopy, and electron microprobe analyses were performed on samples from six drill cores in the Eagle system, three from Eagle and three from Eagle East. Lundin additionally provided whole-rock geochemistry for five of these cores at ~1.5m intervals. The concentration of Ni in olivines in the Eagle and Eagle East intrusions were measured by electron microprobe. A bimodal distribution of Ni concentration in olivine - i.e., both enriched and unenriched populations being present - may have been evidence for multiple magma pulses in the Eagle system. However, olivine in both intrusions were determined to be universally Ni-enriched, which means that this line of inquiry was not useful for determining the number of magma pulses. The cumulate nature of samples were determined by whole-rock geochemistry, wherein incompatible trace elements including Zr, and La were used as proxies for the amount of intercumulus material present in a sample, as well as by visual estimation using transmitted-light petrography. Counter to the conclusions of Ding et el. (2010), variations in incompatible trace element ratios in various rock types in the Eagle system were satisfactorily explained by the cumulate nature and high sulfide content of the samples, with no need to invoke multiple parental magmas in the explanation. The intrusive breccia (or “IBRX”) lithology present in both the Eagle and Eagle East intrusions was studied with transmitted light petrography. It was determined to occur in at least two variations. Both variations have a feldspathic lherzolite matrix with subangular clasts, but in one type heavy sulfide mineralization (up to 30% by volume) occurs in the clasts and in the other type the clasts are nearly devoid of sulfides. In both cases the clasts have high pyroxene abundances and are devoid of olivine, but clasts with high sulfide content tend to have more plagioclase and are more heavily altered. In the Eagle East intrusion, sampled clasts were only of the low-sulfide variety. The IBRX clasts may be a slower-cooling version of the PRX lithology also present in the Eagle system. Alternatively, they may be part of an older intrusion that the Eagle system parental magma cannibalized at depth during emplacement. The main body of the Eagle East intrusion was studied by petrographic examination of a core that profiled its depth. While there was no significant change in cumulate rock type, the core did show modest cryptic variation with depth. Notably, a horizon of increased olivine abundance indicated the potential recharge of the intrusion with the same, homogeneous parental magma. The lithological similarity of the Eagle and Eagle East intrusions indicates that they likely formed from the same parental magma. The main petrographic differences between the two intrusions were the poikilitic nature of clinopyroxene and the lower abundance of plagioclase in the Eagle East intrusion. These differences may be explained by the larger size and thus presumed longer cooling time of the Eagle East intrusion.Item Iron-Nickel-Sulfur-Carbon System Under High Pressure, With Implications To Earth’S Mantle(2016-10) Zhang, ZhouFe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications. According to their chemical behavior, Fe-Ni-S-C phases preferentially store siderophile and chalcophile elements (and potentially noble gases). Physically, Fe-Ni-S-C phases have distinctly higher densities, surface tensions, and electrical conductivities, and lower melting points than mantle silicates. Understanding the geochemical and geophysical impacts caused by Fe-Ni-S-C phases requires accurate quantification of the basic properties of Fe-Ni-S-C phases under mantle conditions. This PhD thesis uses both high-pressure experiments and thermodynamic calculations to constrain the melting temperatures and compositions of Fe-Ni-S-C phases in the Earth’s upper mantle mantle, and their potential for deep carbon storage. This study suggests that monosulfides in the upper mantle are mostly molten, even in significant portions of cratonic roots under continental geotherms. Incorporation of carbon depresses the monosulfide solidus by 50-100˚C. Experiments and calculations of reactions between Fe-Ni-S melts and silicates at mantle conditions suggest that Fe-Ni-S melts are Ni-rich (Ni/(Ni+Fe)~0.6) monosulfides ((Fe+Ni)/S~1 or Xs~0.5) under oxidized (FMQ -2 to FMQ) conditions at 2 GPa. With increasing depth in the mantle (thus decreasing fO2), Fe-Ni-S melts become increasingly Ni- and S-poor, characterized by Ni/(Ni+Fe)~0.4, (Fe+Ni)/S~3, and Xs~0。4 at 8 GPa, and Ni/(Ni+Fe)~0.2, Xs~0.05 and (Fe+Ni)/S~10 at 12 GPa. Carbon solubility in Fe-Ni-S melts determined by high-pressure experiments suggests that carbon solubility decreases exponentially with increasing Xs. Based on mantle Fe-Ni-S melt compositions, C-S relations in carbon-saturated melts, and the typical mantle P-T-fO2 profile and sulfur abundance (200 ppm), it is suggested that significant amounts (40-100%) of deep carbon could potentially be stored in Fe-Ni-S melts in the Earth’s reduced deep upper mantle.Item Supporting data for "Sulfur geochemistry impacts population oscillations of wild rice (Zizania palustris)"(2020-07-10) LaFond-Hudson, Sophia; Johnson, Nathan W; Pastor, John; Dewey, Brad; lafo0062@d.umn.edu; LaFond-Hudson, SophiaWild rice populations decline with exposure to elevated sulfate due to production of sulfide in anoxic sediment. Using self-sustaining wild rice mesocosms, we collected data on the population response to sulfate, as well as iron and litter, which both may modify the production and availability of sulfide to plants. Wild rice also experiences natural population oscillations due to delays in release of nitrogen from decomposing litter. We use this data to investigate how sulfate-induced population declines interact with stable litter-driven population cycles. Population data was collected 2014-2019, and geochemical data (iron, sulfide, pH) was collected in 2019, after 5 years of of a factorial design treatment (sulfate, iron, litter).Item Temperature dependent sulfate transport in aquatic sediments(2014-11) DeRocher, Will D.Sulfate, released to overlying waters from natural sources and human activity, has the potential to be reduced to sulfide within the anoxic environments of aquatic sediments and negatively impact the growth of aquatic vegetation. Wild Rice is of particular concern within Minnesota as it is both an economic and cultural resource within the state. This study was conducted to characterize the temperature dependence of sulfate transport, via diffusion, between overlying waters and sediment porewaters through the use of laboratory experimentation and mathematical analysis to study the transient response to changes in the overlying water concentration. Two riverine sediments with contrasting organic carbon content from the St. Louis River watershed in northern Minnesota were characterized for their bulk geochemistry and incubated under laboratory conditions to observe the temperature dependence of ion transport between overlying water and sediment porewaters. Two identical sets of laboratory microcosms, incubated under warm and cold conditions, were subjected to a sulfate loading phase in which the overlying water was spiked with sodium sulfate to induce a concentration gradient between the sediment porewaters and overlying water. At the end of the sulfate loading phase, the sulfate gradient was reversed by replacing the overlying water with fresh water, causing sulfate to diffuse out of the sediment, back into the overlying water. During the sulfate recovery phase, sodium bromide was spiked into the overlying water. Bromide, acting as an inert chemical tracer, provided a diffusion-only baseline with which to compare to reactive sulfate. The anion concentrations in the overlying waters were closely monitored to quantify changes in the concentration through the sulfate loading and recovery phases. Non-destructive porewater samples were collected using Rhizon® soil moisture samplers to measure concentrations of sulfate, bromide, ferrous iron, pH, and sulfide at discrete depths in the sediment during key times after changes in surface boundary conditions.Averaged results from both the high and low organic sediments showed sulfate transport occurred 49% faster out of the overlying waters into the sediments at 23°C when compared to 4.5°C. Estimated rates of sulfate reduction at 4.5°C were on average, 40% of those estimated at 23° C. After seven weeks of recovery from the sulfate loading, porewater sulfate concentrations in the warm microcosms had dropped back to ambient levels while slightly elevated sulfate levels were still noticed within the cold microcosm porewater. Even though more sulfate diffused into the warm sediments, the cold sediments retained the sulfate for a significantly longer period of time after the change in boundary layer conditions due to the retarded rates of diffusion and reaction. The longer the sediment is exposed to elevated sulfate levels a greater potential exists for the wild rice seed within the sediment to be exposed to sulfide.Item Wild rice geochemistry and reproductive life stage data from experimental pots amended with sulfate, 2016(2019-10-24) LaFond-Hudson, Sophia L; Johnson, Nathan W; Pastor, John; Dewey, Brad; lafo0062@d.umn.edu; LaFond-Hudson, Sophia LWild rice, an annual aquatic plant produces fewer, smaller seeds with less nitrogen when exposed to sulfide, but does not produce decreased vegetative biomass. We compared the timing and duration of reproductive life stages in sulfate-amended plants to unamended plants to see how sulfide affects reproductive phenology. We recorded the life stage of plants starting with initiation of reproduction until senescence and measured seed count, mass and nitrogen content. Additionally, we sampled the geochemistry of porewater, sediment, and root surfaces to understand how plant life stage may control sediment redox conditions relevant to sulfide.