Characteristics of a Shallow River Plume: Observations from the Saco River Coastal Observing System

Interest in the coastal dynamics of river plumes has mainly focused on large rivers, but plumes from the more numerous smaller rivers have important local consequences and may, in aggregate, be significant contributors to coastal circulation. We studied the dynamics of the plume from the Saco River in Saco Bay, Gulf of Maine, over a 3-year period. The transport and salinity in the region are governed by river discharge, tides, winds, and interaction with the Western Maine Coastal Current. The dynamics of the flow field vary with location within the plume and discharge. The far-field dynamics of the Saco River plume are dominated by inertial processes (hence qualifying it as a small-scale river plume), during times of low discharge, with low salinity water present both up and downstream of the river mouth, but are dominated by rotational processes during times of high discharge (thus qualifying it as a large-scale river plume), with buoyant water primarily advected downshelf. Near-field dynamics are governed by weak, subcritical flow during low discharge but strongly inertial, supercritical flow during high discharge. Offshore movement of the plume is not governed by Ekman dynamics but is instead a result of discharge, wind-induced vertical mixing, and the geography of the coastline and adjacent islands.     

Experimental phase relations of a low MgO Aleutian basaltic andesite at XH2O = 0.7–1

We conducted melting experiments on a low MgO (3.29 wt.%) basaltic andesite (54.63 wt.% SiO₂) from Westdahl volcano, Alaska, at XH₂O = 0.7–1 and fO₂ ~ Ni–NiO, at pressures = 0.1–180 MPa and temperatures = 900–1,200 °C. We examine the evolution of the melt along a liquid line of descent during equilibrium crystallization at high H₂O and fO₂ conditions, starting from a high FeOt/MgO, low MgO basaltic andesite. Ti-magnetite formed on the liquidus regardless of XH₂O, followed by clinopyroxene, plagioclase, amphibole, and orthopyroxene. We observe slight but significant differences in the phase stability curves between the XH₂O = 1 and 0.7 experiments. Early crystallization of Ti-magnetite and suppression of plagioclase at higher pressures and temperatures resulted in strongly decreasing melt FeOt/MgO with increasing SiO₂, consistent with a “calc-alkaline” compositional trend, in agreement with prior phase equilibria studies on basalt at similar H₂O and fO₂. Our study helps quantify the impact of small amounts of CO₂ and high fO₂ on the evolution of melts formed during crystallization of a low MgO basaltic andesite magma stored at mid- to shallow crustal conditions. Like the prior studies, we conclude that H₂O strongly influences melt evolution trends, through stabilization of Ti-magnetite on the liquidus and suppression of plagioclase at high P–T conditions.     

Inferring groundwater contributions and pathways to streamflow during snowmelt over multiple years in a discontinuous permafrost subarctic environment (Yukon, Canada)

Research on large northern rivers suggests that as permafrost thaws, deeper groundwater flowpaths become active, resulting in greater baseflow, increased concentrations of weathering ions and reduced concentrations of dissolved organic carbon in the streamflow. In contrast, at the headwater-catchment scale, where understanding of groundwater/surface-water interactions is developed, inter-annual variability in climate and hydrology result in complex hydrological and chemical responses to change. This paper reports on a 4-year runoff investigation in an alpine discontinuous permafrost environment in Yukon, Canada, using stable isotopes, major dissolved ions and hydrometric data, to provide enhanced insight into the inter-annual-variability runoff-generation processes. Stable isotope results suggest that pre-event (old) water stored within the catchment dominates the snowmelt hydrograph, and dissolved ion results reveal that groundwater pathways occur predominantly in the near-surface during freshet. Dissolved organic carbon varies inter-annually, reflecting changing melt patterns, whereas weathering ions generated from deeper flowpaths become diluted. The total snow-water equivalent does not have a major influence on the fraction of snowmelt water reaching the stream or the runoff ratio. Results from multiple years highlight the considerable variability over short time scales, limiting our ability to detect climate-change influences on groundwater at the headwater scale.     

Biofilms and Size Sorting of Fine Sediment During Erosion in Intertidal Sands

The goal of this research was to investigate size-specific retention of clay and silt-sized grains by biofilms in sandy intertidal sediments. Sediment cores were collected from an intertidal flat in Cole Harbour, NS, and eroded at increasing shear stresses (0.08–0.60 Pa) with a Gust microcosm. Half of the cores were eroded without undergoing prior treatment, while sodium hypochlorite was added to the other cores to destroy biofilms. The disaggregated inorganic grain size distribution of sediment resuspended by the Gust microcosm was then obtained with a Multisizer^? 3 Coulter Counter®, and each treated core was compared with its corresponding untreated core. Overall, significantly less total sediment mass was resuspended from untreated cores than from treated cores. At intermediate shear stresses, the sediment resuspended from treated cores contained a greater proportion of fine and medium silts than the sediment resuspended from untreated cores. Very fine silts and clays were not retained preferentially by biofilms. The results show that biofilms stabilize the sediment, but they do not necessarily enhance the proportion of finest sediment sizes, as previously proposed.     

Predation Risk Experienced by Mummichog, Fundulus heteroclitus, in Intertidal and Subtidal Salt Marsh Habitats

It is often presumed that salt marshes provide a predation refuge for small fishes, but predation risks have rarely been compared in intertidal and subtidal habitats, making the importance of salt marshes as a predation refuge speculative. We measured relative survival of tethered mummichog (Fundulus heteroclitus) in four habitats in a salt marsh?Ctidal creek system: unvegetated and vegetated intertidal areas and the subtidal creek at high and low tide. At high tide, mummichog in the intertidal zone had significantly higher survival than in the subtidal creek in June through August. Survival rates in unvegetated and vegetated intertidal habitats were not significantly different, suggesting that higher intertidal survival was due to less abundant predators compared with the creek, rather than predators being less effective in vegetation. The lower predation risk experienced by mummichog in the intertidal marsh suggests that access to intertidal habitats will be important for production of small estuarine fishes.     

Seismic performance of highway bridges with fusing bearing components for quasi‐isolation

Modern highway bridges in Illinois are often installed with economical elastomeric bearings that allow for thermal movement of the superstructure, and steel fixed bearings and transverse retainers that prevent excessive movement from service‐level loadings. In the event of an earthquake, the bearing system has the potential to provide a quasi‐isolated response where failure of sacrificial elements and sliding of the bearings can cause a period elongation and reduce or cap the force demands on the substructure. A computational model that has been calibrated for the expected nonlinear behaviors is used to carry out a parametric study to evaluate quasi‐isolated bridge behavior. The study investigates different superstructure types, substructure types, substructure heights, foundation types, and elastomeric bearing types. Overall, only a few bridge variants were noted to unseat for design‐level seismic input in the New Madrid Seismic Zone, indicating that most structures in Illinois would not experience severe damage during their typical design life. However, Type II bearing systems, which consist of an elastomeric bearing and a flat PTFE slider, would in some cases result in critical damage from unseating at moderate and high seismic input. The sequence of damage for many bridge cases indicates yielding of piers at low‐level seismic input. This is caused by the high strength of the fixed bearing element, which justifies further calibration of the quasi‐isolation design approach. Finally, the type of ground motion, pier height, and bearing type were noted to have significant influence on the global bridge response. Copyright © 2013 John Wiley & Sons, Ltd.     

The distribution of water ice in the interior of Comet Tempel 1

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained (∼1±1 μm) water ice particles, which are free from refractory impurities.     

High‐calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites

Abstract— Our analyses of high quality spectra of several S‐type asteroids (17 Thetis, 847 Agnia, 808 Merxia, and members of the Agnia and Merxia families) reveal that they include both low‐ and high‐calcium pyroxene with minor amounts of olivine (<20%). In addition, we find that these asteroids have ratios of high‐calcium pyroxene to total pyroxene of >～0.4. High‐calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies. The silicate mineralogy inferred for Thetis and the Merxia and Agnia family members requires that these asteroids experienced igneous differentiation, producing broadly basaltic surface lithologies. Together with 4 Vesta (and its smaller “Vestoid” family members) and the main‐belt asteroid 1489 Magnya, these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies. Based on this analysis of a small subset of the near‐infrared asteroid spectra taken to date with SpeX at the NASA IRTF, we expect that the number of known differentiated asteroids will increase, consistent with the large number of parent bodies inferred from studies of iron meteorites.     

Olivine‐dominated asteroids and meteorites: Distinguishing nebular and igneous histories

Abstract— Melting models indicate that the composition and abundance of olivine systematically co‐vary and are therefore excellent petrologic indicators. However, heliocentric distance, and thus surface temperature, has a significant effect on the spectra of olivine‐rich asteroids. We show that composition and temperature complexly interact spectrally, and must be simultaneously taken into account in order to infer olivine composition accurately. We find that most (7/9) of the olivine‐dominated asteroids are magnesian and thus likely sampled mantles differentiated from ordinary chondrite sources (e.g., pallasites). However, two other olivine‐rich asteroids (289 Nenetta and 246 Asporina) are found to be more ferroan. Melting models show that partial melting cannot produce olivine‐rich residues that are more ferroan than the chondrite precursor from which they formed. Thus, even moderately ferroan olivine must have non‐ordinary chondrite origins, and therefore likely originate from oxidized R chondrites or melts thereof, which reflect variations in nebular composition within the asteroid belt. This is consistent with the meteoritic record in which R chondrites and brachinites are rare relative to pallasites.