Sources and transformations of organic matter in surface soils and sediments from a tidal estuary (North Inlet, South Carolina, USA)

Surface soil and sediment samples collected along a forest-brackish marsh-salt marsh transect in a southeastern U.S. estuary were separated into three different fractions (sand, macro-organic matter, and humus) based on size and density. Elemental, stable carbon isotope, and lignin analyses of these samples reveal important contrasts in the quantity, composition, and sources of organic matter, between forest and marsh sites. Elevated nitrogen contents in humus samples suggest nitrogen incorporation during humification is most extensive in forest soils relative to the marsh sites. The lignin compositions of the macro-organic and humus samples reflect the predominant type of vegetation at each site. Lignin phenol ratios indicate that woody and nonwoody litter from, gymnosperm and angiosperms trees (pines and oaks) is the major source of vascular plant-derived organic matter in the forest site and that angiosperm, grasses (Juncus andSpartina) are the major sources of lignin at the marsh sites. The phenol distributions also reveal that oxidative degradation of lignin is most extensive in the forest and brackish marsh zones whereas little lignin decay occurs in the salt marsh samples. In forest soils, most organic matter originates from highly altered forest vegetation while at the brackish marsh site organic matter is a mixture of degradedJuncus materials and microbial/algal remains. Organic matter in the salt marsh appears to be composed of a more complex mixture of sources, including degradedSpartina detritus as well as algal and microbial inputs. Microbial methane oxidation appears to be an important process and a source of¹³C depleted organic carbon in subsurface sediments at this site.     

Disc–halo interaction – I. Three-dimensional evolution of the Galactic disc

The results of a three-dimensional model for disc–halo interaction are presented here. The model considers explicitly the input of energy and mass by isolated and correlated supernovae in the disc. Once disrupted by the explosions, the disc never returns to its initial state. Instead it approaches a state where a thin H  i disc is formed in the Galactic plane, overlaid by thick H  i and H  ii gas discs with scaleheights of 500 pc and 1–1.5 kpc, respectively. The upper parts of the thick H  ii disc (the diffuse ionized medium) act as a disc–halo interface, and its formation and stability are directly correlated to the supernova rate per unit area in the simulated disc.     

Detecting and quantifying sources of non-stationarity via experimental semivariogram modeling

Conventional geostatistics often relies on the assumption of second order stationarity of the random function (RF). Generally, local means and local variances of the random variables (RVs) are assumed to be constant throughout the domain. Large scale differences in the local means and local variances of the RVs are referred to as trends. Two problems of building geostatistical models in presence of mean trends are: (1) inflation of the conditional variances and (2) the spatial continuity is exaggerated. Variance trends on the other hand cause conditional variances to be over-estimated in certain regions of the domain and under-estimated in other areas. In both cases the uncertainty characterized by the geostatistical model is improperly assessed. This paper proposes a new approach to identify the presence and contribution of mean and variance trends in the domain via calculation of the experimental semivariogram. The traditional experimental semivariogram expression is decomposed into three components: (1) the mean trend, (2) the variance trend and (3) the stationary component. Under stationary conditions, both the mean and the variance trend components should be close to zero. This proposed approach is intended to be used in the early stages of data analysis when domains are being defined or to verify the impact of detrending techniques in the conditioning dataset for validating domains. This approach determines the source of a trend, thereby facilitating the choice of a suitable detrending method for effective resource modeling.     

Complex geophysical wake flows

Idealized studies of island wakes often use a cylinder-like island to generate the wake, whereas most realistic studies use a close representation of the oceanic bathymetry immersed in a complex representation of the “ambient” geophysical flows. Here, a system of multiple islands was placed into numerical and experimental channels, in order to focus on the complexity of the archipelago wake, including (a) the influence of small neighboring islands and (b) the role of the island-shelf. The numerical geostrophic and stratified channel was built using a three-dimensional primitive equation model, considering a realistic representation of the Madeira archipelago bathymetry, with prescribed initial and boundary conditions. Results from the simulations show that the neighboring islands alter the near-field wake. Small eddies generated by the neighboring islands lead to destabilization of the shear layers of the larger island. Laboratory experiments carried out in the Coriolis rotating tank corroborated this near-field disruptive mechanism. The neighboring island perturbation effect was present whatever the direction of the incoming flow, but under different regimes. North–south wakes produced geostrophic eddies (≥ R _d), whereas west–east wakes produced (exclusively) ageostrophic submesoscale eddies (< < R _d) which traveled offshore with wave-like motion. The archipelago shelf contributed to the asymmetric vertical migration of oceanic vorticity. Cyclonic vorticity dominated the surface dynamics, whereas anticyclonic circulation prevailed at the bottom part of the linearly stratified upper layer. This study identifies several likely wake scenarios induced by the Madeira archipelago, and may serve as guide for future multiscale numerical studies and in situ campaigns.     

Water column characterisation on the Azores platform and at the sea mounts south of the archipelago

This study provides data concerning the hydrography and water chemistry of the Atlantic region between 29-38° N and 27-31° W, and establishes background values for dissolved Cu, Cd, Pb and As. Three water masses were identified: the Eastern North Atlantic Central Water (ENACW), the Mediterranean Water (MW) and the North Atlantic Deep Water (NADW). The ENACW exhibits a clear meridional gradient of temperature and salinity, with comparatively high values at the southern sites and lower values on the Azores platform. The ENACW, which includes the euphotic zone, also had comparatively high concentrations of oxygen and lower concentrations of nutrients and metals. The Cu, Cd and Pb results suggest that new background concentrations for OSPAR Region V (the Wider Atlantic) should be established as follows: 0.15-13nM for Cu, 0.05-1.4nM for Cd and 0.03-5nM for Pb. The background concentrations of As for OSPAR Region V should be 7-28nM.     

Enrichment of rare earth elements as environmental tracers of contamination by acid mine drainage in salt marshes: A new perspective

Rare earth elements (REE) were analyzed in surface sediments from the Guadiana Estuary (SW Iberian Pyrite Belt). NASC (North American Shale Composite) normalized REE patterns show clearly convex curvatures in middle-REE (MREE) with respect to light- and heavy-REE, indicating acid-mixing processes between fluvial waters affected by acid mine drainage (AMD) and seawater. However, REE distributions in the mouth (closer to the coastal area) show slightly LREE-enriched and flat patterns, indicating saline-mixing processes typical of the coastal zone. NASC-normalized ratios (La/Gd and La/Yb) do not discriminate between both mixing processes in the estuary. Instead, a new parameter (E(MREE)) has been applied to measure the curvature in the MREE segment. The values of E(MREE)>0 are indicative of acid signatures and their spatial distribution reveal the existence of two decantation zones from flocculation processes related to drought periods and flood events. Studying REE fractionation through the E(MREE) may serve as a good proxy for AMD-pollution in estuarine environments in relation to the traditional methods.     

Building sandstone surface modification by biofilm and iron precipitation: emerging block‐scale heterogeneity and system response

Stone surfaces are sensitive to their environment. This means that they will often respond to exposure conditions by manifesting a change in surface characteristics. Such changes can be more than simply aesthetic, creating surface/subsurface heterogeneity in stone at the block scale, promoting stress gradients to be set up as surface response to, for example, temperature fluctuations, can diverge from subsurface response. This paper reports preliminary experiments investigating the potential of biofilms and iron precipitation as surface‐modifiers on stone, exploring the idea of block‐scale surface‐to‐depth heterogeneity, and investigating how physical alteration in the surface and near‐surface zone can have implications for subsurface response and potentially for long‐term decay patterns. Salt weathering simulations on fresh and surface‐modified stone suggest that even subtle surface modification can have significant implications for moisture uptake and retention, salt concentration and distribution from surface to depth, over the period of the experimental run. The accumulation of salt may increase the retention of moisture, by modifying vapour pressure differentials and the rate of evaporation. Temperature fluctuation experiments suggest that the presence of a biofilm can have an impact on energy transfer processes that occur at the stone surface (for example, buffering against temperature fluctuation), affecting surface‐to‐depth stress gradients. Ultimately, fresh and surface‐modified blocks mask different kinds of system, which respond to inputs differently because of different storage mechanisms, encouraging divergent behaviour between fresh and surface‐modified stone over time. Copyright © 2014 John Wiley & Sons, Ltd.