The influence of riparian vegetation on near‐bank turbulence: a flume experiment

Measurements from a fixed‐bed, Froude‐scaled hydraulic model of a stream in northeastern Vermont demonstrate the importance of forested riparian vegetation effects on near‐bank turbulence during overbank flows. Sections of the prototype stream, a tributary to Sleepers River, have increased in channel width within the last 40 years in response to passive reforestation of its riparian zone. Previous research found that reaches of small streams with forested riparian zones are commonly wider than adjacent reaches with non‐forested, or grassy, vegetation; however, driving mechanisms for this morphologic difference are not fully explained. Flume experiments were performed with a 1:5 scale, simplified model of half a channel and its floodplain, mimicking the typical non‐forested channel size. Two types of riparian vegetation were placed on the constructed floodplain: non‐forested, with synthetic grass carpeting; and forested, where rigid, randomly distributed, wooden dowels were added. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 41 locations within the channel and floodplain at near‐bed and 0·6‐depth elevations. Observations of velocity components and calculations of turbulent kinetic energy (TKE), Reynolds shear stress and boundary shear stress showed significant differences between forested and non‐forested runs. Generally, forested runs exhibited a narrow band of high turbulence between the floodplain and main channel, where TKE was roughly two times greater than TKE in non‐forested runs. Compared to non‐forested runs, the hydraulic characteristics of forested runs appear to create an environment with higher erosion potential. Given that sediment entrainment and transport can be amplified in flows with high turbulence intensity and given that mature forested stream reaches are wider than comparable non‐forested reaches, our results demonstrated a possible driving mechanism for channel widening during overbank flow events in stream reaches with recently reforested riparian zones. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of a pumping test of the Snake River Plain aquifer using axial-flow numerical modeling

The Snake River Plain aquifer in southeast Idaho is hosted in a thick sequence of layered basalts and interbedded sediments. The degree to which the layering impedes vertical flow has not been well understood, yet is a feature that may exert a substantial control on the movement of contaminants. An axial-flow numerical model, RADFLOW, was calibrated to pumping test data collected by a straddle-packer system deployed at 23 depth intervals in four observation wells to evaluate conceptual models and estimate properties of the Snake River Plain aquifer at the Idaho National Engineering and Environmental Laboratory. A delayed water-table response observed in intervals beneath a sediment interbed was best reproduced with a three-layer simulation. The results demonstrate the hydraulic significance of this interbed as a semi-confining layer. Vertical hydraulic conductivity of the sediment interbed was estimated to be about three orders of magnitude less than vertical hydraulic conductivity of the lower basalt and upper basalt units. The numerical model was capable of representing aquifer conceptual models that could not be represented with any single analytical technique. The model proved to be a useful tool for evaluating alternative conceptual models and estimating aquifer properties in this application. Electronic Publication     

Temporal and spatial differences in salinity and water chemistry in SW Florida estuaries: Effects of human-impacted watersheds

Three estuaries near Naples, Florida with variably modified watersheds have been investigated to understand the chemical consequences of altering drainage patterns. Blackwater River (near natural drainage, control site), Henderson Creek (moderately modified watershed), and Faka-Union Canal (severe channelization) were sampled for temperature, salinity, δ¹⁸O, δ¹³C of dissolved inorganic carbon (DIC), molality of CO₂ (ΣCO₂), and Mg:Ca and Sr:Ca ratios between freshwater and marine water end members over a 17-mo period. Carbon isotope composition followed similar seasonal patterns as salinity. Freshwater and seawater end members are more negative than the global average, likely reflecting equilibration with local carbon sources derived from mangrove leaf litter and groundwater. δ¹³C responds to differences in primary productivity between estuaries. Henderson Creek has higher primary productivity than Blackwater River (probable due to higher sewage input and agricultural runoff) and has more positive δ¹³C and lower ΣCO₂. δ¹⁸O is affected by seasonal input of freshwater from atmospheric precipitation, evaporation, and groundwater. Late summer and fall rains lower the δ¹⁸O of estuarine water, whereas evaporative conditions in the dry season elevate δ¹⁸O to values that can be more positive upstream than those from the Gulf of Mexico (estuarine inversion). Evaporation produces water in the Gulf of Mexico that is >1‰ more positive than the global sea surface average most of the year. The very negative δ¹⁸O values in Blackwater River and Henderson Creek likely reflect atmospheric and groundwater contribution. Mg:Ca and Sr:Ca ratios of Gulf water from all three estuaries are similar to global averages at low latitudes. Freshwater end members among estuaries are different in that Blackwater River has higher ratios, suggesting a groundwater contribution. Dolomitic rocks in the subsurface likely provide a source of Mg ions.     

The Lerokis and Kali Kuning submarine exhalative gold-silver-barite deposits, Wetar Island, Maluku, Indonesia

Wetar, an island in the southern part of the Banda Arc, is made up submarine volcanic rocks, with the oldest rocks exposed being subvolcanic intrusions and flows dated at 12 Ma. Basaltic andesite pillow lavas and intercalated volcaniclastic sedimentary rocks grade upward into more felsic volcanic lavas, tuffs and breccias, and sedimentary rocks and epiclastic mudflows cap the sequence.Gold-silver mineralization occurs at Lerokis and Kali Kuning, 3.5 km apart on the north coast of the island, in stratiform barite sand, clay or silt. The sediments are underlain by Cu-rich massive pyrite in volcanic breccias and overlain by a limestone dated at about 4 Ma. Foot wall volcanic breccias and lavas show intense clay-pyrite alteration indicating temperatures higher than 230°C.In situ geological resources, prior to the inception of mining in 1990, totalled 2.9 Mt at 3.5 g/t Au, 114 g/t Ag and 40% barite at Lerokis and 2.2 Mt at 5.5 g/t Au, 146 g/t Ag and 60% barite at Kali Kuning. Most of the Au occurs as electrum, associated with limonite, jarosite and goethite, and most of the Ag is in tetrahedrite and sulphosalts. High Pb contents, between 0.5% and 1.4% Pb on average, derive from plumbojarosite, sulphosalts and cerrussite, and there is, on average, between 200 ppm to 900 ppm Cu, 0.2% Sb, 0.1% As, 100 ppm to 200 ppm Zn, and 18 ppm Hg. Underlying massive sulphide mineralization is mainly pyrite-marcasite with zones of Cu enrichment. A significant part of the Cu is contained in enargite.Formation likely took place at less than 600 m water depth in a sea floor caldera setting similar to the Kuroko district in Japan. The ferruginous sediments hosting the Au-barite deposits may have originated through erosion like the ochres of Cyprus. These stratiform Au-barite deposits highlight a new style of sea floor mineralization not clearly recognized before. Modern-day analogues have been described from a variety of sea floor settings.     

Long Island Sound: Distributions,trends, and effects of chemical contamination

Trace metals and organic contaminants concentrations are monitored annually in surface sediments, blue mussel tissue, and winter flounder livers at multiple sites in Long Island Sound by National Oceanic and Atmospheric Administration’s National Status and Trends (NS&T) program for Marine Environmental Quality. The NS&T program is also conducting various studies on the bioeffects of contaminants in the sound. Three years of monitoring results indicate organic and elemental contaminants concentrations in sediments and biota at sites in the western portion of the sound are high on a national scale. Possible decreasing trends in cadmium and chlordane in the sound are suggested by the 1986–1988 data for their concentrations in mussels. A comparison between NS&T Mussel Watch results and those of the Environmental Protection Agency’s Mussel Watch, conducted from 1976 through 1978, indicated a decadal increase in copper concentrations and a decrease in lead in the sound. Bioeffects studies in the sound have revealed responses to contamination only in localized zones where contaminant levels are very high.     

Flow models for back-arc spreading

A secondary flow model for back-arc spreading is developed in this paper that shows some of the characteristics of observed back-arc spreading. Back-arc spreading has formed marginal seas around the west and southwest rim of the Pacific. The episodic spreading and different directions of opening are not completely understood; however, there does appear to be a limited lifetime (< 17 m.y.) and when one episode of spreading ends, there is a lag time (6–10 m.y.) before another adjacent one begins. This suggests that back-arc flow is caused by secondary flow induced by subduction. Simple scaling arguments with physically reasonable values suggest that forced and free convection effects will be nearly equal. A two-dimensional, finite difference model is developed and several numerical experiments lasting 160 m.y. with a varying subduction rate are discussed. These experiments show stress surges lasting 10–20 m.y. and a series of eddies and counter-eddies behind the trench with a spatial scale of 300–400 km. This supports the idea that back-arc spreading is the result of transient eddies induced by the subducting slab.     

Enhanced boulder weathering under late-lying snowpatches

A Schmidt test hammer was employed to assess the aggregate surface hardness of samples of boulders dug out from under late-lying snowpatches at sites in Switzerland, Scotland and Norway. The results were compared with an equivalent set of readings made on boulders at nearby snow-free control sites. The results in every case reveal that the aggregate surface hardness of boulders buried by late-lying snow is significantly less (at the 0·001 level) than that of boulders at the corresponding control sites. This result indicates that late-lying snow significantly enhances rock weathering, probably because boulder and rock surfaces are exposed to prolonged wetting by percolating meltwater under late-lying snowbeds.     

Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey)

The Kiziltepe ophiolitic thrust sheet in the Bolkar Mountains of Turkey occurs between two subparallel ophiolite belts bounding the Tauride carbonate platform and represents a remnant of the Cretaceous Neo-Tethyan oceanic lithosphere. It is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra-oceanic subduction zone in the Neo-Tethys 92-90 Ma. Blueschist-facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing P/T with cooling. Regional tectonic constraints suggest a Maastrichtian age for the timing of this blueschist-facies metamorphism. Sodic amphibole-rich veins and crossite/Mg-riebeckite rims on hornblende suggest that growth of blueschist-facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries. We infer a counterclockwise P-T-t trajectory during which metamorphism was accompanied/succeeded by rapid uplift along the northern edge of the Tauride belt in Late Cretaceous-early Tertiary time.     

Nitrogen losses through sediment denitrification in Boston Harbor and Massachusetts Bay

Sediment denitrification is a microbial process that converts dissolved inorganic nitrogen in sediment porewaters to N₂ gas, which is subsequently lost to the atmosphere. In coastal waters, it represents a potentially important loss pathway for fixed nitrogen which might otherwise be available to primary producers. Currently, data are lacking to adequately assess the role of denitrification in reducing or remediating the effects of large anthropogenic nitrogen loads to the coastal zone. This study describes the results of 88 individual measurements of denitrification (as a direct flux of N₂ gas) in sediment cores taken over a 3-yr period (1991–1994) from six stations in Boston Harbor, nine stations in Massachusetts Bay, and two stations in Cape Cod Bay. The dataset is unique in its extensive spatial and temporal coverage and includes the first direct measurements of denitrification for North Atlantic shelf sediments. Results showed that rates of denitrification were significantly higher in Boston Harbor (mean=54, range<5–206 μmol N₂ m^?2 h^?1) than in Massachusetts Bay (mean=23, range<5–64 μmol N₂ m^?2 h^?1). Highest rates occurred in areas with organic-rich sediments in the harbor, with slower rates observed for low-organic sandy sediments in the harbor and at shallow shelf stations in the bay. Lowest rates were found at the deepest shelf stations, located in Stellwagen Basin in Massachusetts Bay. Observed rates were correlated with temperature, sediment carbon content, and benthic macrofaunal activity. Seasonally, highest denitrification rates occurred in the summer in Boston Harbor and in the spring and fall in Massachusetts Bay, coincident with peak phytoplankton blooms in the overlying water column. Despite the fact that sediment denitrification rates were high relative to rates reported for other East Coast estuaries, denitrification losses accounted for only 8% of the annual total nitrogen load to Boston Harbor, a consequence perhaps, of the short water-residence times (2–10 d) of the harbor.