Herbicide contamination and the potential impact to seagrass meadows in Hervey Bay, Queensland, Australia

Low concentrations of herbicides (up to 70 ng l(-1)), chiefly diuron (up to 50 ng l(-1)) were detected in surface waters associated with inter-tidal seagrass meadows of Zostera muelleri in Hervey Bay, south-east Queensland, Australia. Diuron and atrazine (up to 1.1 ng g(-1) dry weight of sediment) were detected in the sediments of these seagrass meadows. Concentration of the herbicides diuron, simazine and atrazine increased in surface waters associated with seagrass meadows during moderate river flow events indicating herbicides were washed from the catchment to the marine environment. Maximum herbicide concentration (sum of eight herbicides) in the Mary River during a moderate river flow event was 4260 ng l(-1). No photosynthetic stress was detected in seagrass in this study during low river flow. However, with moderate river flow events, nearshore seagrasses are at risk of being exposed to concentrations of herbicides that are known to inhibit photosynthesis.     

The Comet Nucleus Tour (Contour); A NASA Discovery Mission

In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished.     

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5 m) and piezometer wells (depth 12.2–79.2 m) demonstrate that the groundwater is mostly the Ca-HCO₃ type and anoxic in nature (mean Eh_SHE = 34 mV). Arsenic concentrations ranged from <10–538 μg/L, with high concentrations only present in the shallow to medium depth (30–50 m) of the aquifer along with high Fe (0.07–9.8 mg/L) and relatively low Mn (0.15–3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO₃, SO₄ and NO₂ along with higher values of DOC and HCO₃, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.     

Carbon isotope fractionation between calcite,graphite and CO2: an experimental study

The partitioning of stable carbon isotopes between calcite, graphite and CO₂ was experimentally determined at temperatures from 500 to 1200 °C and 1 to 15 kbar pressure. Attainment of carbon isotope equilibrium in CO₂-calcite runs was proven by achieving the same fractionation from isotopically opposite directions. The resultant CO₂-calcite fractionation curve for carbon differs from Bottinga's calculation by 1.2 and confirms recent experiments of Chacko et al. and Mattey et al. In CO₂-graphite experiments equilibrium fractions were extrapolated by applying the partial-exchange technique of Northrop and Clayton and by optimizing the contribution of surface reaction in graphite. CO₂-graphite fractionations at temperatures up to 800 °C are in fair agreement with Bottinga's calculation, but yield a surprisingly high fractionation of 5 at upper mantle temperatures. The combination of CO₂-calcite (carbon) and CO₂-graphite fractionation results in a new experimentally determined calcite-grapite fractionation curve, expressed by the equation:

Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: ¹⁸O-depleted rocks (as low as δ¹⁸O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas ¹⁸O-normal rocks (with bulk δ¹⁸O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ¹⁸O eclogites usually have higher hydroxyl-mineral contents than the normal δ¹⁸O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted ¹⁸O compositions have had different fluid histories compared to those with normal δ¹⁸O values. Rocks with depleted ¹⁸O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO₂ inclusions are commonly found in the rocks with normal δ¹⁸O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.     

Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust

Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.     

Metastable staurolite–cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M₁, while coaxial conditions prevailed during later contact metamorphic M₂ growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves conditions during M₁, estimated by AFM diagrams and P–T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite–andalusite assemblages of M₂, suggesting polyphase metamorphism and decompression along a clockwise P–T path for the staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).     

Transport timescales for identifying seasonal variation in Bass Strait, south-eastern Australia

A three-dimensional σ-coordinate ocean model with realistic forcing is used to derive and compare flushing time, residence time and water age distributions in Bass Strait, a broad shallow shelf sea in south-eastern Australia which exhibits seasonal variation in water mass properties and circulation. Results illustrate flushing out of most old water occurs each winter–spring period and the circulation in summer increases age of remnant older water in the Strait. A correlation between water age and salinity is found in winter which is the result of advective “erosion” of the interior water mass. The presence of an area in the south-eastern part of the interior with low flushing is suggested to result from a number of physical factors including the presence of quasi-stationary eddies which entrain age, leading to relatively older water residing there.     

Influence of melt‐freeze‐cycles on the radionuclide transport in homogeneous laboratory snowpack

Radionuclides released to the environment and deposited with or onto snow can be stored over long time periods if ambient temperature stays low, particularly in glaciated areas or high alpine sites. The radionuclides will be accumulated in the snowpack during the winter unless meltwater runoff at the snow base occurs. They will be released to surface waters within short time during snowmelt in spring. In two experiments under controlled melting conditions of snow in the laboratory, radionuclide migration and runoff during melt‐freeze‐cycles were examined. The distribution of Cs‐134 and Sr‐85 tracers in homogeneous snow columns and their fractionation and potential preferential elution in the first meltwater portions were determined. Transport was associated with the percolation of meltwater at ambient temperatures above 0 °C after the snowpack became ripe. Mean migration velocities in the pack were examined for both nuclides to about 0.5 cm hr^?1 after one diurnal melt‐freeze‐cycle at ambient temperatures of ?2 to 4 °C. Meltwater fluxes were calculated with a median of 1.68 cm hr^?1. Highly contaminated portions of meltwater with concentration factors between 5 and 10 against initial bulk concentrations in the snowpack were released as ionic pulse with the first meltwater. Neither for caesium nor strontium preferential elution was observed. After recurrent simulated day‐night‐cycles (?2 to 4 °C), 80% of both radionuclides was released with the first 20% of snowmelt within 4 days. 50% of Cs‐134 and Sr‐85 were already set free after 24 hr. Snowmelt contained highest specific activities when the melt rate was lowest during the freeze‐cycles due to concentration processes in remaining liquids, enhanced by the melt‐freeze‐cycling. This implies for natural snowpack after significant radionuclide releases, that long‐time accumulation of radionuclides in the snow during frost periods, followed by an onset of steady meltwater runoff at low melt rates, will cause the most pronounced removal of the contaminants from the snow cover. This scenario represents the worst case of impact on water quality and radiation exposure in aquatic environments.