Development and Application of a Method to Identify Salt Marsh Vulnerability to Sea Level Rise

Wetlands are commonly assessed for ecological condition and biological integrity using a three-tiered framework of landscape-scale assessment, rapid assessment protocols, and intensive biological and physiochemical measurements. However, increased inundation resulting from accelerated sea level rise (SLR) is negatively impacting tidal marsh ecosystem functions for US Northeast coastal wetlands, yet relative vulnerability to this stressor is not incorporated in condition assessments. This article assesses tools available to measure coastal wetland vulnerability to SLR, including measurements made as part of traditional rapid condition assessments (e.g., vegetation communities, soil strength), field and remote sensing-based measurements of elevation, VDatum, and Sea Level Affecting Marshes Model (SLAMM) model outputs. A vulnerability metric that incorporates these tools was calibrated and validated using recent rates of marsh vegetation losses (1972–2011) as a surrogate for future vulnerability. The metric includes complementary measures of elevation capital, including the percentage of high vs. low marsh vegetation, Spartina alterniflora height, elevation measurements, and SLAMM outputs that collectively explained 62% of the variability in recent rates of marsh vegetation loss. Stepwise regression revealed that all three elements (elevation, vegetation measures, and SLAMM outputs) explained significant and largely unique components of vulnerability to SLR, with the greatest level of overlap found between SLAMM outputs and elevation metrics. While soil strength varied predictably with habitat zone, it did not contribute significantly to the vulnerability metric. Despite the importance of determining wetland elevation above key tidal datums of mean sea level and mean high water, we caution that VDatum was found to perform poorly in back-barrier estuaries. This factor makes it difficult to compare elevation capital among marshes that differ in tidal range and poses accuracy problems for broad-scale modeling efforts that require accurate tidal datums. Given the pervasive pattern of coastal wetland drowning occurring in the Northeastern USA and elsewhere, we advocate that compilation of regional data on marsh habitats and vulnerability to SLR is crucial as it permits agencies to target adaptation to sites based on their vulnerability or mixture of habitats, it helps match sites to appropriate interventions, and it provides a broader regional context to site-specific management actions. Without such data, adaptation actions may be implemented where action is not necessary and to the disadvantage of vulnerable sites where opportunities for successful adaptation will be missed.     

Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

Sea level rise is a major stressor on many salt marshes, and its impacts include creek widening, ponding, vegetation dieback, and drowning. Marsh vegetation changes have been associated with sea level rise across southern New England, but most of these studies pre-date the current period of rapidly accelerating sea level rise coupled with episodic events of extreme increases in water levels. Here, we combine data from two salt marsh monitoring and assessment programs in Rhode Island that were designed to assess marsh responses to sea level rise and use these data to document temporal and spatial patterns in marsh vegetation during the current period of extreme water level increases. Vegetation monitoring at two Narragansett Bay salt marshes confirms the ongoing decline of the salt meadow species Spartina patens during this period as it becomes replaced by Spartina alterniflora. Bare ground resulting from vegetation dieback was significantly related to mean high water levels and led to the rapid conversion of mixed Spartina assemblages to S. alterniflora monocultures. A broader spatial assessment of RI marshes shows that S. alterniflora dominance increases at lower elevation marshes toward the mouth of Narraganset Bay. Our data provide additional evidence that S. patens continues to decline in southern New England marshes and show that losses can accelerate during periods of extreme high water levels. Unless adaptive management actions are taken, we predict that marshes throughout RI will continue to lose salt meadow habitat and eventually resemble lower elevation marshes that are already dominated by S. alterniflora monocultures.     

Magnesium ion chemistry in the stratosphere

Laboratory measurements of reaction rate constants of magnesium ions and magnesium containing ions with O₃, NO, HNO₃, and H₂O₂ have been carried out in a flowing afterglow experiment. Mg⁺ ions react with O₃ to produce MgO⁺ ions, which in turn react with O₃ to produce Mg⁺ ions. Mg⁺ ions react with HNO₃ and H₂O₂ to produce MgOH⁺ ions. MgOH⁺ ions react rapidly with HNO₃ to produce NO⁺₂ ions and Mg(HO)₂. One can therefore conclude that Mg⁺, MgO⁺, or MgOH⁺ ions could not have significant concentrations in the stratosphere if gas phase magnesium compounds were present. The failure to observe these ions therefore cannot be used as evidence that the stratospheric magnesium, resulting from meteor ablation at higher altitudes, is in condensed phases. This is in contrast to the case for sodium where the ion chemistry is such that the failure to observe hydrated Na⁺ ions proves that gas phase sodium compounds are not present in the stratosphere.     

Estimating deep recharge rates beneath an interlobate moraine using temperature logs

The Sandilands area of southeastern Manitoba contains an interlobate moraine that is a major ground water recharge area. Underlying the highly permeable sediments of the moraine are up to 100 m of till and the subcrop of the Winnipeg Formation, which contains a major sandstone aquifer. Ground water flow within the till is examined using high-resolution temperature profiles and solutions to the differential equation for heat flow in porous media. These analyses indicate that recharge to the sandstone aquifer is occurring at a rate of approximately 2 x 10(-8) m/sec beneath the moraine, which is in agreement with recharge rates determined by conventional ground water hydraulics (10(-7) to 10(-10)(m/sec) and another study using multiple environmental tracers (1 x 10(-9) to 6 X 10(-9) m/sec). The use of temperature to determine ground water flux is not limited by half-lives as many environmental tracers are, and this allows for cost-effective estimation of recharge and discharge rates over longer periods.     

Reference Minerals for the Microanalysis of Light Elements

The quantitative determination of light element concentrations in geological specimens represents a major analytical challenge as the electron probe is generally not suited to this task. With the development of new in situ analytical techniques, and in particular the increasing use of secondary ion mass spectrometry, the routine determination of Li, Be and B contents has become a realistic goal. However, a major obstacle to the development of this research field is the critical dependence of SIMS on the availability of well characterized, homogeneous reference materials that are closely matched in matrix (composition and structure) to the sample being studied. Here we report the first results from a suite of large, gem crystals which cover a broad spectrum of minerals in which light elements are major constituents. We have characterized these materials using both in situ and wet chemical techniques. The samples described here are intended for distribution to geochemical laboratories active in the study of light elements. Further work is needed before reference values for these materials can be finalized, but the availability of this suite of materials represents a major step toward the routine analysis of the light element contents of geological specimens.     

Secondary flow in anabranch confluences of a braided,gravel-bed stream

Measurements of the primary and secondary velocity components were máde in two, active, braided river anabranch confluences with a simple Y-shaped plan form, in the gravelly Sunwapta River (D₅₀ of approximately 30 mm). Flow velocity was measured at regularly-spaced intervals using a bidirectional electromagnetic current meter and the measured downstream and cross-stream velocities were converted to primary and secondary velocities to yield the secondary circulation. The primary (downstream) velocity field shows two high velocity streams from the two tributaries which merge (and, in some cases, accelerate) into a single high velocity core over the thalweg. Primary flow velocity declines as the flow expands and diverges at the downstream end of the confluence. The secondary circulation is dominated by two helical cells, back-to-back, plunging over the thalweg and diverging at the bed. This is the first confirmation of this flow structure in confluences, based on field measurements. The strength of the secondary cells declines downstream through each confluence, and laterally away from the thalweg area in cross-section. There is also a tendency for one cell, from the larger of the tributaries, to override the other. The secondary and primary flow structure and strength differs slightly between the two confluences and this is reflected in differences in scour hole form.     

The natural occurrence of aegirine—neptunite solid solution

The occurrence of titanian-aegirines in alkaline rocks has been summarized to illustrate the extent of their stability and the nature of the substitution in these clinopyroxenes. The chemistry and substitution of Ti in aegirines is discussed with special reference to the newly observed aegirine — neptunite (Na₂Fe₂ ²⁺TiSi₄O₁₂) solid solution. The available data indicate that neptunite should be included in the pyroxene mineral group as an alkali clinopyroxene.     

A stress-transfer model for the development of extension fracture boudinage

The term boudinage is used to describe a wide variety of extensional structures in deformed rocks. This paper is mainly concerned with boudinage resulting from through-layer extension fractures followed by separation of the layer segments, thus forming boudins with more or less rectangular cross-sections. In principle, this process is similar to the break up of fibres in fibre-reinforced composite materials extended parallel to the fibre direction. Both processes are controlled by the transfer of stress from the matrix to the fibre (or layer) and a mathematical model for fibre-matrix stress transfer (the ‘fibre-loading’ model) is well established. We have used this as a basis for developing a stress transfer model for boudinage. The only difference in the basic mathematical formulation results from geometric differences between the two systems; the geometric expressions in the fibre-loading model have, therefore, been rederived for the layer-matrix case.Stress-transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre. This behaviour, which is clearly shown by finite-element models of boudinage structure, suggests that extension fracture boudinage develops by successive ‘mid-point’ fracturing. According to stress-transfer theory, the process will continue until a layer is reduced to segments (boudins) all of which are shorter than some critical length (for which the tensile fracture strength of the layer is equal to the tensile stress at the mid-point). In practice, successive fracturing will be influenced by two other factors: (1) in nature the controlling material properties (tensile fracture strength, elastic moduli) will not be single-valued but will have a distribution reflecting local variations in lithology and microstructure and (2) major pre-deformation flaws may be present in a layer which will control the ‘starting length’ of layer segments. These factors are incorporated with the stress-transfer theory into a statistical (Monte Carlo) model for extension fracture boudinage which results in a prediction of boudin aspect ratios. The predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within Lower Carboniferous slates at Tintagel, Cornwall.The stress-transfer model implies that boudin-defining fractures occur sequentially so that inter-boudin gap lengths will be unequal. Strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled. A much improved strain estimate is possible based on sequentially closing the inter-boudin gaps. The stress-transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture. Other than our own, we know of no published data on boudin aspect ratio distributions. Hence, further elaboration of the model is not possible until more field data is available. We hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models.     

Feldspar crystallization trends in leucite-bearing and related assemblages

Feldspar chemical variations in representative leucite-bearing and related rocks from well-known localities in Italy, Germany, Uganda and Australia demonstrate that phenocrystal core to rim variations may not represent the feldspar crystallization trend in the host lava and only the groundmass feldspar zoning trend is a reliable indicator of crystal-liquid relationships. Textural relationships indicate that coexisting plagioclase and alkali feldspar crystallized sequentially, the latter after the former, rather than cotectically.Groundmass alkali feldspar show Ca-, Na-depletion and K-enrichment zoning trends. Plagioclase crystallization follows Ca-depletion, Na and K-enrichment trends. Typically, Sr and Ba solid solubility is significant, particularly in groundmass feldspar.The alkali feldspar variation trend from groundmass assemblages is not consistent with the theoretical phase relationships in the system NaAlSiO₄-KAlSiO₄ CaAl₂Si₂O₈-SiO₂ (The phonolite pentahedron) proposed by Carmichael et al. (1974).Factors believed to be important in controlling feldspar crystallization trends are the Sr-Ba feldspar components, the role of the coexisting pyroxene and the presence of F, Cl and/or their alkali compounds.