Long length scales of element transport during reaction texture development in orthoamphibole-cordierite gneiss: Thor-Odin dome,British Columbia,Canada

First-order factors controlling the textural and chemical evolution of metamorphic rocks are bulk composition and pressure–temperature–time (P–T–t) path. Although it is common to assume that major element bulk composition does not change during regional metamorphism, rocks with reaction textures such as corona structures record evidence for major changes in effective bulk composition (EBC) and therefore provide significant insight into the scale, pathways, and mechanisms of element transport during metamorphism. Quantifying changes in EBC is essential for petrologic applications such as calculation of phase diagrams (pseudosections). The progressive growth of complex corona structures on garnet and Al₂SiO₅ porphyroblasts in orthoamphibole-cordierite gneiss Thor-Odin dome (British Columbia, Canada) reduced the EBC volume of the rock during metamorphism and therefore had a dramatic effect on the evolution of the stable mineral assemblage. These rocks contain a chemical and textural record of metamorphic reactions and preserve 3D networks (reaction pathways) connecting corona structures. These coronal networks record long (>cm) length scales of localized element transport during metamorphism. P–T, T–X, and P–X pseudosections are used to investigate the control of effective bulk composition on phase assemblage evolution. Despite textural complexity and evidence for disequilibrium, mineral assemblages and compositions were successfully modeled and peak metamorphic conditions estimated at 750°C and 9 kbar. These results illustrate how textural and chemical changes during metamorphism can be evaluated using an integrated petrographic and pseudosection approach, highlight the importance of effective bulk composition choice for application of phase equilibria methods in metamorphic rocks, and show how corona structures can be used to understand the scale of compositional change and element transport during metamorphism.     

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.     

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     

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.     

Maintaining the Healthy Country–Healthy People Nexus through Sociocultural and Environmental Transformations: challenges for the Wik Aboriginal people of Aurukun,Australia

This paper examines the potential for multiple co-benefits to arise through re-establishing the connection between Aboriginal people and their lands. The research project was participatory in its design and implementation, and centred on three short but intensive visits to the Kendall River over a period of 4 years. Interviews with and observations of Kendall River people on country provided qualitative information concerning their wish to reconnect with country, not only to transmit key cultural knowledge through the generations, re-socialise their lands and manage them appropriately but also to help them manage the negative consequences of Wik aggregation in the troubled community of Aurukun. Participants reported that returning to and carrying out activities on country, and the family and country planning resulting from those trips, provided a way to counter feelings of disempowerment and despondency arising from living solely in Aurukun. This paper concludes by arguing that activities that re-engage Aboriginal people with country (if not actually returning to live on country) can serve to build cultural resilience in the face of multiple economic, environmental and social challenges, including those arising from life lived largely in communities such as Aurukun, thereby also likely benefiting their physical and psychosocial health and well-being.     

Effects of oil on mangrove seedlings grown under different environmental conditions

The effects of oil treatments (one-time addition of 120 ml per plant and weekly addition of 15 ml per plant) and environmental conditions at time of oiling (air conditioned laboratory/diffuse light vs hot, direct sunlight) on the survival and growth of Rhizophora mangle and Avicennia germinans seedlings were examined. No oiled A. germinans survived longer than a few weeks. Both one-time and weekly oiling depressed survival, stem growth, leaf production and maximum leaf size in R. mangle. Significant interaction terms between oil treatment and environmental conditions occurred later (50 and 59 weeks) for R. mangle stem growth and earlier (21 and 23 weeks) for leaf production. Relative to other treatment combinations, R. mangle mortality was greatest and growth lowest in the combination of one-time oiling under hot, bright outdoor conditions. Results indicate that synergistic interactions between oil effects and environmental conditions may contribute to the wide range of effects reported in the literature in studies of oil contamination of mangroves.     

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.     

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.