Nutrient limitation in the lower Housatonic River estuary

One of the most serious threats to freshwater and marine ecosystems is high rates of anthropogenic nutrient loading, particularly nitrogen (N) and phophorus (P). One of the major freshwater sources of nutrients to Long Island Sound (LIS) is the Housatonic River (HR). Current management plans that call for reducing N inputs without reducing P inputs may change the N: P ratio in the water column and the pattern of algal nutrient limitation and species composition in the tidal portion of the river. To assess the current pattern of algal nutrient limitation in the HR estuary, nutrient bioassays were conducted in spring, summer, and fall at 5 sites throughout the tidal portion and adjacent LIS. Diatoms were a dominant taxon at all sites throughout the sampling period. Other seasonally important taxa include cyanobacteria, cryptophytes, and euglenoids. Phytoplankton in LIS were always strongly N limited and were co-limited by P in spring. During low flow (summer), phytoplankton in the lower HR estuary were N limited. Phytoplankton in the middle reaches showed no evidence of N or P limitation and were likely limited by other factors. In spring, phytoplankton in the upper HR estuary were P limited. Periods of N or P limitation were better correlated with periods of lower concentrations of nitrate or phosphate than with differences in N: P ratio. These results suggest that decreases in N concentration could increase the prevalence of N limitation throughout the estuary that in turn may reduce phytoplankton biomass and alter species composition of the phytoplankton.     

Analysis of recharge-induced geochemical change in a contaminated aquifer

Recharge events that deliver electron acceptors such as O2, NO3, SO4, and Fe3+ to anaerobic, contaminated aquifers are likely important for natural attenuation processes. However, the specific influence of recharge on (bio)geochemical processes in ground water systems is not well understood. The impact of a moderate-sized recharge event on ground water chemistry was evaluated at a shallow, sandy aquifer contaminated with waste fuels and chlorinated solvents. Multivariate statistical analyses coupled with three-dimensional visualization were used to analyze ground water chemistry data (including redox indicators, major ions, and physical parameters) to reveal associations between chemical parameters and to infer processes within the ground water plume. Factor analysis indicated that dominant chemical associations and their interpreted processes (anaerobic and aerobic microbial processes, mineral precipitation/dissolution, and temperature effects) did not change significantly after the spring recharge event of 2000. However, the relative importance of each of these processes within the plume changed. After the recharge event, the overall importance of aerobic processes increased from the fourth to the second most important factor, representing the variability within the data set. The anaerobic signatures became more complex, suggesting that zones with multiple terminal electron-accepting processes (TEAPs) likely occur in the same water mass. Three-dimensional visualization of well clusters showed that water samples with similar chemical associations occurred in distinct water masses within the aquifer. Water mass distinctions were not based on dominant TEAPs, suggesting that the recharge effects on TEAPs occurred primarily at the interface between infiltrating recharge water and the aquifer.     

A paleolimnological record of Holocene climate and environmental change in the Temagami region,northeastern Ontario

The Arcellacean (Thecamoebian) fauna was assessed in five Holocene sediment cores obtained from James and Granite lakes in the Temagami region of northeastern Ontario. In addition, palynological analysis was carried out on two of these cores, one each from James and Granite lakes. The first indication of postglacial colonization by plants was the appearance of rare Cupressaceae pollen, dated to 10,800 yr BP. Plant diversity began to increase by 10,770 yr BP when Pinus spp. and Larix migrated into the area. The first appearance of arcellaceans occurred after 9650 yr BP in assemblages dominated by Centropyxis constricta and opportunistic Centropyxis aculeata. High abundances of charophytes in the cores until 8800 yr BP indicated that macroalgae were proliferating at this time. This deposition is interpreted to have occurred during the draining of an ice-marginal lake following the retreat of the Laurentide Ice Sheet. Based on pollen analysis, warmer conditions associated with the Holocene Hypsithermal prevailed in the area from 6250 to 4115 yr BP. The stable, open Great Lakes – St. Lawrence type forest that developed here at the beginning of the Hypsithermal continues to prevail to the present. The periodic colonization of the lake by beavers (Castor canadensis) acted as a control on water-level and eutrophication through the Holocene. Evidence of eutrophication was indicated in the core samples by the abundance of high levels of the alga Pediastrum and the arcellacean Cucurbitella tricuspis. Eutrophication periodically developed when beavers dammed a site, causing the rate of flow in drainage streams to slow and stagnant conditions occurred. When the site became depleted of the nearby trees, which were preferred by beaver (Betula, Alnus and Populus), the dam would be abandoned, causing the water-level to drop. Stagnant conditions were reduced as flow levels increased, reducing eutrophication and resulting in recovering forest stands. In addition, the lowering water levels would result in encroachment of the forest along the lake shore. This cycle occurred many times in the history of this lake as indicated by fluctuations in the size of arcellacean populations.     

Silicate perovskite analogue ScAlO3: temperature dependence of elastic moduli

The elastic moduli of ScAlO₃ perovskite, a very close structural analogue for MgSiO₃ perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al₂O₃ measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO₃ perovskite, the value of (∂K_S/∂T)_P is −0.033 GPa K⁻¹ and (∂G/∂T)_P is −0.015 GPa K⁻¹. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂K_S/∂T)_P| is typically about twice |(∂G/∂T)_P|. The value of (∂K_S/∂T)_P for ScAlO₃ is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO₃ perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P–V–T equation of state of (Mg,Fe)SiO₃ perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO₃ perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂G/∂T)|_P for ScAlO₃ is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO₃ perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO₃ perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679].

Combining the results from the previous studies and current measurements on ScAlO₃ perovskite, we extracted the parameters (q and γ₀) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K_S(T), unlike V(P,T) data, tightly constrain the value of q. It is concluded that ScAlO₃ has ‘normal’ γ₀ (1.3) and high q (3.6). The high value of q indicates that ScAlO₃ has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).     

Microstructurally controlled trace element (Zr,U–Pb) concentrations in metamorphic rutile: An example from the amphibolites of the Bergen Arcs

As a common constituent of metamorphic assemblages, rutile provides constraints on the timing and conditions of rock transformation at high resolution. However, very little is known about the links between trace element mobility and rutile microstructures that result from synmetamorphic deformation. To address this issue, here we combine in situ LA-ICP-MS and sensitive high-resolution ion microprobe trace element data with electron back-scatter diffraction microstructural analyses to investigate the links between rutile lattice distortions and Zr and U–Pb systematics. Furthermore, we apply this integrated approach to constrain further the temperature and timing of amphibolite facies metamorphism and deformation in the Bergen Arcs of southwestern Norway. In outcrop, the formation of porphyroblastic rutile in dynamically hydrated leucocratic domains of otherwise rutile-poor statically hydrated amphibolite provides key contextual information on both the ambient conditions of hydration and deformation and the composition of the reactive fluid. Rutile in amphibolite recorded ambient metamorphic temperatures of ~590–730°C during static hydration of the granulitic precursor. By contrast, rutile from leucocratic domains in the directly adjacent shear zone indicates that deformation was accompanied by a localized increase in temperature. These higher temperatures are recorded in strain-free rutile (~600–860°C) and by Zr concentration measurements on low-angle boundaries and shear bands (620–820°C). In addition, we also observe slight depletions of Zr and U along rutile low-angle boundaries relative to strain-free areas in deformed grains from the shear zone. This indicates that crystal–plastic deformation facilitated the compositional re-equilibration of rutile upon cooling to slightly below the peak temperature of deformation. Cessation of deformation at mid-crustal conditions near ~600°C is recorded by late stage growth of small (<150 µm) rutile in the high-strain zones. U–Pb age data obtained from the strain-free and distorted rutile grains cluster in distinct populations of 437.4 ± 2.7 Ma and c. 405–410 Ma, respectively. These different ages are interpreted to reflect the difference in closure for thermally induced Pb diffusion between undeformed and deformed rutile during post-deformation exhumation and cooling. Thus, our results provide a reconstruction of the thermochronological history of the amphibolite facies rocks of the Lindås Nappe and highlight the importance of integration of microstructural data during application of thermometers and geochronometers.     

A modelling framework to simulate field‐scale nitrate response and transport during snowmelt: The WINTRA model

Modelling nutrient transport during snowmelt in cold regions remains a major scientific challenge. A key limitation of existing nutrient models for application in cold regions is the inadequate representation of snowmelt, including hydrological and biogeochemical processes. This brief period can account for more than 80% of the total annual surface runoff in the Canadian Prairies and Northern Canada and processes such as atmospheric deposition, overwinter redistribution of snow, ion exclusion from snow crystals, frozen soils, and snow‐covered area depletion during melt influence the distribution and release of snow and soil nutrients, thus affecting the timing and magnitude of snowmelt runoff nutrient concentrations. Research in cold regions suggests that nitrate (NO₃) runoff at the field‐scale can be divided into 5 phases during snowmelt. In the first phase, water and ions originating from ion‐rich snow layers travel and diffuse through the snowpack. This process causes ion concentrations in runoff to gradually increase. The second phase occurs when this snow ion meltwater front has reached the bottom of the snowpack and forms runoff to the edge‐of‐the‐field. During the third and fourth phases, the main source of NO₃ transitions from the snowpack to the soil. Finally, the fifth and last phase occurs when the snow has completely melted, and the thawing soil becomes the main source of NO₃ to the stream. In this research, a process‐based model was developed to simulate hourly export based on this 5‐phase approach. Results from an application in the Red River Basin of southern Manitoba, Canada, shows that the model can adequately capture the dynamics and rapid changes of NO₃ concentrations during this period at relevant temporal resolutions. This is a significant achievement to advance the current nutrient modelling paradigm in cold climates, which is generally limited to satisfactory results at monthly or annual resolutions. The approach can inform catchment‐scale nutrient models to improve simulation of this critical snowmelt period.     

State-of-the-Practice Review of In Situ Thermal Technologies

In situ thermal-based soil and aquifer remediation technologies (e.g., electrical resistance heating [ERH], conductive heating, and steam-based heating) have undergone rapid development and application in recent years. These thermal technologies offer the promise of more rapid and thorough treatment of nonaqueous phase liquid (NAPL) source zones; however, their field-scale application has not been well documented in the technical literature. A state-of-the-practice review of the application of these technologies was conducted in this study. Available documents from 182 applications were reviewed, which included 87 ERH, 46 steam-based heating, 26 conductive heating, and 23 other heating technology applications conducted between 1988 and 2007. Approximately 90% of the 182 applications were implemented after 1995 and about half since 2000. More specifically, this review identified the geologic settings in which these technologies were applied, chemicals treated, design parameters, operating conditions, and performance metrics. The results of this study are summarized in a table linking this information to five generalized geologic scenarios. Practitioners considering thermal technologies for their site can identify the geologic scenario that most closely resembles their site and then can quickly see which technologies have been applied in that setting, the designs employed, operating conditions, and the performance achieved.