Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

The vulnerability of saltmarshes to lateral erosion at their margin depends on the local biogeomorphological properties of the substrate. In particular, the 3D architecture of pore and root systems is expected to influence shear strength, with repercussions for the wider-scale stability of saltmarshes. We apply X-ray computed microtomography (μCT) to visualize and quantify subsurface structures in two UK saltmarshes at Tillingham Farm, Essex (silt/clay rich substrate) and Warton Sands (sand-rich substrate), with four types of ground cover: bare ground, Spartina spp, Salicornia spp and Puccinellia spp. We extracted μCT structural parameters that characterize pore and root morphologies at each station, and compared them with field measurements of shear strength using a principal component analysis and correlation tests. The 3D volumes show that species-dependent variations in root structures, plant colonization events and bioturbation activity control the morphology of macropores, while sediment cohesivity determines the structural stability and persistence of these pore structures over time, even after the vegetation has died. Areas of high porosity and high mean pore thickness were correlated to lower values of shear strength, especially at Tillingham Farm, where well-connected vertical systems of macropores were associated with current or previous colonization by Spartina spp. However, while well-connected systems of macropores may lower the local deformation threshold of the sediment, they also encourage drainage, promote vegetation growth and reduce the marsh vulnerability to hydrodynamic forces. The highest values of shear strength at both sites were found under Puccinellia spp, and were associated with a high density of mesh-like root structures that bind the sediment and resist deformation. Future studies of marsh stability should ideally consider time series of vegetation cover, especially in silt/clay-dominated saltmarshes, in order to consider the potential effect of preserved buried networks of macropores on water circulation, marsh functioning and cliff-face erosion.     

Sediment production and yield from an alluvial gully in northern Queensland,Australia

Sediment production, transport and yield were quantified over various timescales in response to rainfall and runoff within an alluvial gully (7 · 8 ha), which erodes into dispersible sodic soils of a small floodplain catchment (33 ha) along the Mitchell River, northern Australia. Historical air photographs and recent global positioning system (GPS) surveys and LiDAR data documented linear increases in gully area and volume, indicating that sediment supply has been relatively consistent over the historic period. Daily time lapse photography of scarp retreat rates and internal erosion processes also demonstrated that erosion from rainfall and runoff consistently supplied fine washload (< 63 µm) sediment in addition to coarse lags of sand bed material. Empirical measurements of suspended sediment concentrations (10 000 to >100 000 mg/L) and sediment yields (89 to 363 t/ha/yr) were high for both Australian and world data. Total sediment yield estimated from empirical washload and theoretical bed material load was dominated by fine washload (< 63 µm). A lack of hysteresis in suspended sediment rating curves, scarp retreat and sediment yield correlated to rainfall input, and an equilibrium channel outlet slope supported the hypothesis that partially or fully transport‐limited conditions predominated along the alluvial gully outlet channel. This is in contrast to sediment supply‐limited conditions on uneroded floodplains above gully head scarps. While empirical data presented here can support future modelling efforts to predict suspended sediment concentration and yield under the transport limiting situations, additional field data will also be needed to better quantify sediment erosion and transport rates and processes in alluvial gullies at a variety of spatial and temporal scales. Copyright © 2013 John Wiley & Sons, Ltd.     

Environmental scanning electron microscopy (ESEM) and nanoindentation investigation of the crack tip process zone in marble

This study explores the interaction between crack initiation and nanomechanical properties in the crack-tip fracture process zone of Carrara marble. Specimens with preexisting cracks were loaded in a uniaxial testing machine until the process zone appeared at the tips of the preexisting cracks. ESEM analysis reveals an increase in microcrack density in the process zone with increased loading of the specimen. Nanoindentation testing comprised of lines and grids of single nanoindentations located both near and far from the process zone shows a decrease in both indentation modulus and indentation hardness near grain boundaries in intact material, and with closeness to the process zone. Ultimately, the study confirms that the crack-tip process zone manifests itself as an area of reduced indentation hardness and indentation modulus in marble.     

Refractory Metal Nuggets – Formation of the first condensates in the Solar Nebula

As gas flowed from the solar accretion disk or Solar Nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner Solar Nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, “infall” atmosphere around the inner Solar Nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased.In the first hundred thousand years after the formation of the Solar Nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) – the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 and 100,000 years after the formation of the Solar Nebula. It is possible that conditions were conducive to RMN/CAI formation over an 80,000 year timescale. The “infall” atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems.The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner Solar Nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event.Observational evidence suggests that RMNs were the nucleation particles for CAIs. As a consequence, the observed bimodal distribution of ²⁶Al in CAIs, where some CAIs have ²⁶Al while others do not, is probably due to the injection ²⁶Al during the short CAI formation period, where ²⁶Al was not present when the first CAIs were formed.     

Life Histories, Salinity Zones, and Sublethal Contributions of Contaminants to Pelagic Fish Declines Illustrated with a Case Study of San Francisco Estuary, California, USA

Human effects on estuaries are often associated with major decreases in abundance of aquatic species. However, remediation priorities are difficult to identify when declines result from multiple stressors with interacting sublethal effects. The San Francisco Estuary offers a useful case study of the potential role of contaminants in declines of organisms because the waters of its delta chronically violate legal water quality standards; however, direct effects of contaminants on fish species are rarely observed. Lack of direct lethality in the field has prevented consensus that contaminants may be one of the major drivers of coincident but unexplained declines of fishes with differing life histories and habitats (anadromous, brackish, and freshwater). Our review of available evidence indicates that examining the effects of contaminants and other stressors on specific life stages in different seasons and salinity zones of the estuary is critical to identifying how several interacting stressors could contribute to a general syndrome of declines. Moreover, warming water temperatures of the magnitude projected by climate models increase metabolic rates of ectotherms, and can hasten elimination of some contaminants. However, for other pollutants, concurrent increases in respiratory rate or food intake result in higher doses per unit time without changes in the contaminant concentrations in the water. Food limitation and energetic costs of osmoregulating under altered salinities further limit the amount of energy available to fish; this energy must be redirected from growth and reproduction toward pollutant avoidance, enzymatic detoxification, or elimination. Because all of these processes require energy, bioenergetics methods are promising for evaluating effects of sublethal contaminants in the presence of other stressors, and for informing remediation. Predictive models that evaluate the direct and indirect effects of contaminants will be possible when data become available on energetic costs of exposure to contaminants given simultaneous exposure to non-contaminant stressors.     

Chironomid-based palaeotemperature estimates for northeast Finland during Oxygen Isotope Stage 3

Quantitative palaeotemperature estimates for the earlier part of Oxygen Isotope Stage (OIS-) 3 are inferred from subfossil chironomid remains. The high-latitudinal study site of Sokli, northeast Finland, provides for a unique lacustrine deposit covering the earlier part of OIS-3, and the chironomid remains found in the sediments show that a shallow lake with a diverse fauna was present at the study site throughout the record. Using a Norwegian calibration data set as a modern analogue, mean July air temperatures are reconstructed. The chironomid-inferred July air temperatures are surprisingly high, reaching values similar to the current temperature at the study site. Other proxies that were applied to the sediments included the analysis of botanical and zoological macro-remains, and our results concur with temperature estimates derived from climate indicator taxa. Summer temperatures for interstadial conditions, reconstructed with climate models, are as high as our proxy-based palaeotemperatures.     

Development and testing of a physically based,three-dimensional model of surface and subsurface hydrology

We present a numerical, catchment-scale model that solves flow equations of surface and subsurface flow in a three-dimensional domain. Surface flow is described by the two-dimensional parabolic approximation of the St. Venant equation, using Manning’s equation of motion; subsurface flow is described by the three-dimensional Richards’ equation for the unsaturated zone and by three-dimensional Darcy’s law for the saturated zone, using an integrated finite difference formulation. The hydrological component is a dynamic link library implemented within a comprehensive model which simulates surface energy, radiation budget, snow melt, potential evapotranspiration, plant development and plant water uptake. We tested the model by comparing distributed and integrated three-dimensional simulated and observed perched water depth (PWD), stream flow data, and soil water contents for a small catchment. Additional tests were performed for the snow melting algorithm as well as the different hydrological processes involved. The model successfully described the water balance and its components as evidenced by good agreement between measured and modelled data.     

Influence of climate change on interannual variation in contaminant body burden in Gulf of Mexico oysters

As part of NOAA's National Status and Trends Mussel Watch Program, oysters were sampled along the Gulf of Mexico coast each winter from 1986 to 1992 and analyzed for trace metal, polynuclear aromatic hydrocarbon (PAH), and pesticide body burden. We identified contaminant variables for which large-scale climate processes played an important role in establishing the interannual variation in body burden by examining cases where body burdens rose or fell more or less in unison over broad geographic regions and distinguished these concordant changes from cases where bays varied independently. Of the 11 metals analyzed, nine had scales of concordancy 100 km. Ni and Se, however, had among the largest scales of concordancy in the study, 1200 km. That is, oyster body burdens in bays as far apart as 1200 km tended to rise and fall in unison from one year to the next. Interannual variations in body burden of organic contaminants had a much stronger regional component. All but two of the 11 organic contaminants had scales of concordancy of 200 km or greater and six exceeded 400 km. Concordancy was strongest either in the southern, northwestern, or north-central Gulf, depending upon the contaminant. For all contaminants, bays tended to vary independently in the northeastern Gulf. For three contaminants, total chlordanes, dieldrin and Cd, regional concordancy may originate from a widespread decrease in use and, therefore, input. These contaminants declined nearly monotonically over the 7 years. For others, including Zn and many of the PAHs and pesticides, the El Niño/Southern Oscillation (ENSO) cycle may be important in establishing the interannual variability in contaminant body burden: (1) ENSO has the geographic scale required; (2) a subtropical influence is required to explain the similarity in interannual variation between south Texas and southern Florida; (3) ENSO-related climate responses in the Gulf follow the southwestern/northeastern trend that would establish the northwestern Gulf focus in concordancy so prominent in many of the contaminants; and (4) deviations in yearly mean body burden from the Gulf-wide mean track the Southern Oscillation Index in some cases.     

Response of chironomids to late quaternary environmental change in the Taitao Peninsula,southern Chile Julieta Massaferro and Stephen J. Brooks. Journal of Quaternary Science Vol 17 No 2 2002, pages 101–111.

The original article to which this Erratum refers was published in Journal of Quaternary Science Vol 17 No 2 2002, pages 101–111 Copyright © 2002 John Wiley & Sons, Ltd.