Geology of the Continental Margin of Enderby and Mac. Robertson Lands, East Antarctica: Insights from a Regional Data Set

In 2001 and 2002, Australia acquired an integrated geophysical data set over the deep-water continental margin of East Antarctica from west of Enderby Land to offshore from Prydz Bay. The data include approximately 7700 km of high-quality, deep-seismic data with coincident gravity, magnetic and bathymetry data, and 37 non-reversed refraction stations using expendable sonobuoys. Integration of these data with similar quality data recorded by Japan in 1999 allows a new regional interpretation of this sector of the Antarctic margin. This part of the Antarctic continental margin formed during the breakup of the eastern margin of India and East Antarctica, which culminated with the onset of seafloor spreading in the Valanginian. The geology of the Antarctic margin and the adjacent oceanic crust can be divided into distinct east and west sectors by an interpreted crustal boundary at approximately 58° E. Across this boundary, the continent–ocean boundary (COB), defined as the inboard edge of unequivocal oceanic crust, steps outboard from west to east by about 100 km. Structure in the sector west of 58° E is largely controlled by the mixed rift-transform setting. The edge of the onshore Archaean–Proterozoic Napier Complex is downfaulted oceanwards near the shelf edge by at least 6 km and these rocks are interpreted to underlie a rift basin beneath the continental slope. The thickness of rift and pre-rift rocks cannot be accurately determined with the available data, but they appear to be relatively thin. The margin is overlain by a blanket of post-rift sedimentary rocks that are up to 6 km thick beneath the lower continental slope. The COB in this sector is interpreted from the seismic reflection data and potential field modelling to coincide with the base of a basement depression at 8.0–8.5 s two-way time, approximately 170 km oceanwards of the shelf-edge bounding fault system. Oceanic crust in this sector is highly variable in character, from rugged with a relief of more than 1 km over distances of 10–20 km, to rugose with low-amplitude relief set on a long-wavelength undulating basement. The crustal velocity profile appears unusual, with velocities of 7.6–7.95 km s⁻¹ being recorded at several stations at a depth that gives a thickness of crust of only 4 km. If these velocities are from mantle, then the thin crust may be due to the presence of fracture zones. Alternatively, the velocities may be coming from a lower crust that has been heavily altered by the intrusion of mantle rocks. The sector east of 58° E has formed in a normal rifted margin setting, with complexities in the east from the underlying structure of the N–S trending Palaeozoic Lambert Graben. The Napier Complex is downfaulted to depths of 8–10 km beneath the upper continental slope, and the margin rift basin is more than 300 km wide. As in the western sector, the rift-stage rocks are probably relatively thin. This part of the margin is blanketed by post-rift sediments that are up to about 8 km thick. The interpreted COB in the eastern sector is the most prominent boundary in deep water, and typically coincides with a prominent oceanwards step-up in the basement level of up to 1 km. As in the west, the interpretation of this boundary is supported by potential field modelling. The oceanic crust adjacent to the COB in this sector has a highly distinctive character, commonly with (1) a smooth upper surface underlain by short, seaward-dipping flows; (2) a transparent upper crustal layer; (3) a lower crust dominated by dipping high-amplitude reflections that probably reflect intruded or altered shears; (4) a strong reflection Moho, confirmed by seismic refraction modelling; and (5) prominent landward-dipping upper mantle reflections on several adjacent lines. A similar style of oceanic crust is also found in contemporaneous ocean basins that developed between Greater India and Australia–Antarctica west of Bruce Rise on the Antarctic margin, and along the Cuvier margin of northwest Australia.     

A literature review of the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on OTEC heat exchangers

An investigation has been made of available data on the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on Ocean Thermal Energy Conversion (OTEC) heat exchangers. Pertinent oceanographic data is lacking at or near potential OTEC sites for the calculation of the degree of saturation of seawater with respect to calcium carbonate. Consequently, only “extrapolated” saturation values can be used. These indicate that near surface seawater is probably supersaturated, with respect to the calcium carbonate phases calcite and aragonite, at all potential OTEC sites. The deep seawater that would be brought to the surface at the potential Atlantic Ocean sites is also likely to be supersaturated with respect to calcium carbonate. The deep seawater at the potential Pacific Ocean sites may be slightly undersaturated.The fact that OTEC heat exchangers will be operating in seawater, which is supersaturated with respect to calcium carbonate, means that if nucleation of calcite or aragonite occurs on the heat exchanger surfaces, significant growth rates of calcium carbonate scale may be expected. The potential for calcium carbonate nucleation is highest at cathodic metal surface locations, which are produced as the result of aluminum corrosion in seawater. Consequently, corrosion and scale formation may be closely related. What the possible effects of biofouling may be on this process are not known.     

Ghost fishing in European waters: Impacts and management responses

In this paper we review levels of net loss, what happens to the gear once it has been lost, and the resulting levels of ‘ghost catches’ made in passive net fisheries in the EU. We also consider ghost catches resulting from lost gear in other types of fisheries, and the extent to which the value of ghost catches has been quantified. We consider why fishing gear is lost, and profile common management responses. We present a cost benefit model to assess the relative cost effectiveness of different management measures, and suggest that gear retrieval programmes may provide less value for money than other management responses.     

Microcoleus autumnalis and filamentous algae-dominated mats and chlorophyll-a increase with agricultural land use but respond differently to associated nutrient and sediment enrichment

ABSTRACT

This study investigated the impact of pastoral land use and nutrient and fine sediment inputs on Microcoleus autumnalis and filamentous algae-dominated mats, and benthic chlorophyll-a in streams (lower North Island, New Zealand). Surveying and sampling was undertaken monthly at 61 sites spanning a wide gradient in catchment cover and environmental conditions. Two boosted regression tree models were built. The first models included pastoral land cover and five environmental variables as predictors. In the second model pastoral land cover was replaced by nutrient/sediment data. The abundance of the two mat types and chlorophyll-a increased when pastoral land cover was between 20% and 70% (model 1). Replacement of pastoral land cover by nutrient/sediment data (model 2) slightly improved the model fit for all three periphyton variables. Microcoleus autumnalis-dominant mats increased with dissolved inorganic nitrogen concentrations up to ca. 0.6?mg?L^?1, and in streams with more frequent flushes. In contrast, filamentous algal-dominated mats increased with turbidity, and in streams with less frequent flushes. Chlorophyll-a generally followed the response of the dominant periphyton type. Increased knowledge on responses of specific periphyton types, rather than total biomass, to environmental variables is essential to guide effective management strategies.     

Imposex still evident in eastern Australia 10 years after tributyltin restrictions

A survey of imposex in Thais orbita (Neogastropoda) along the New South Wales (NSW) coast, Australia, found imposex was still widespread 10 years after a partial ban on tributyltin (TBT) based antifouling paints. Transplant experiments also revealed that imposex was induced in T. orbita over a period of 9 weeks in Sydney Harbour, NSW. A comparison of the imposex data with that collected shortly after the introduction of restrictions on TBT revealed a general decline in imposex frequency occurring at open coast sites, an exception being the site at South West Rocks. Sites located within harbour/bay areas did not display a decline in imposex frequency but a decline in the severity of imposex was apparent. The observed decrease in imposex values in T. orbita may reflect a general decrease of butyltin contamination within the coastal environment. However, the persistence of contamination "hotspots" within harbours and bays remains a concern.     

Glacio-lacustrine stratigraphy,aquifer characterization and contaminant transport: a case study in South Lake Tahoe,California, USA

A hydrogeologic model that has been used by many researchers and consultants to describe an area of South Lake Tahoe, California, USA impacted by MTBE contamination describes a relatively homogeneous unconfined aquifer comprised of poorly sorted glacial outwash deposits, within which water-supply wells are able to exert significant alteration in natural groundwater flow. A re-examination of the area’s hydrogeology is presented, which supports a layered heterogeneous aquifer system constructed of alternating fine and coarser-grained glacio-lacustrine depositional units. This re-evaluation was accomplished through a review of lithologic logs across an area of approximately 1 km², combined with observations of significant hydraulic head differences and knowledge of the depositional environments controlled by Pleistocene Lake Tahoe high stands. Many of the fine-grained units observed at depths from 6 to 15 m, although relatively thin, are generally continuous and serve as significant barriers to groundwater flow. The vertical migration of contamination across these fine-grained units to deeper groundwaters was facilitated by cross-screened monitoring wells installed as part of site investigation activities. This conclusion highlights the importance of geologic characterization and proper monitoring well construction at contaminated site investigations.

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Low temperature,low pressure deformation and metamorphism of the Vangorda massive sulphide orebody,Yukon, Canada

The Vangorda orebody is a small stratiform massive sulphide orebody located in Anvil District, Yukon, Canada. The orebody consists of fineto medium-grained semi-massive and massive sulphides with a common sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena, and minor chalcopyrite. The host rocks and the sulphide lithofacies have been complexly deformed during two phases of deformation (D₁ and D₂) and associated metamorphism (M₁ and M₂). The effects of d₁ and M₁ are penetratively overprinted by D₂ and M₂. D₂ and M₂ resulted in tight to isoclinal F₂ folding of the orebody, remobilisation of the sulphides, recrystallisation and development of shear zones along the limbs of the F₂ folds. Chlorite thermometry and sulphide thermobarometry have been carried out on the host phyllites and on the sulphides. Chlorite was analysed from the S₁ and S₂ foliations in the phyllites to determine M₁ and M₂ temperatures, respectively. However, no difference was found between chlorite compositions in these foliations and a mean temperature of 363 °C was calculated from the tetrahedral A1_IV occupancy. Arsenopyrite thermometry yielded a comparable mean temperature of 336 °C. Sphalerite inclusions in M₂ pyrite porphyroblasts from D₂ shear zones were analysed for pressure using the sphalerite + hexagonal pyrrhotite + pyrite barometer. Inclusions were analysed in an attempt to determine if relic m₁ sphalerite, and hence pressure signature, was preserved. Inclusion compositions appear to reflect only M₂ conditions and yielded a mean pressure of 4.0 kb. Sphalerite + hexagonal pyrrhotite assemblages were analysed from D₂ shear zones to determine the M₂ pressure using the sphalerite + hexagonal pyrrhotite barometer. These calculations yielded a mean pressure of 6.1 kb. The M₂ temperatures and pressures calculated using these calibrations are in good agreement with those estimated from petrogenetic relationships.     

Fractional crystallization and zoning in igneous feldspars: ideal water-buffered liquid fractionation lines and feldspar zoning paths

Diagrams giving plagioclase and sanidine fractionation paths and liquid fractionation lines under conditions of ideal water-buffered fractional crystallization in the ternary feldspar system were constructed graphically using topological reasoning, and experimental data and calculated phase relationships from the literature. The liquidus lines and solidus or solvus paths are unique at constant P and a _{H 2}O. The composition of a liquid evolves with time and moves along a fractionation line by removal of successive crystal fractions, whereas the compositions of each of the crystal fractions lie on and define a solidus or solvus path. Most but not all such water-buffered lines and paths differ only slightly from those in which water is free to build up during crystallization and a _{H 2}O to increase, as in many rocks. Liquid compositions lying along liquidus fractionation lines are not normally preserved, unless erupted as aphyric lavas. The solidus or solvus paths may be preserved either as overgrowth zones in crystals (zoning paths) or as a series of crystal fractions in layered intrusions. The topologies of the lines and paths depend mainly on the nature of the two-feldspar boundary line separating the plagioclase and sanidine fields which is a function of P _{H 2}O or a _{H 2}O at constant P; increases in either progressively lower the liquidi and solidi and cause larger intersections of the solidi with the solvus. One-feldspar solidus paths at high P and a _{H 2}O are simple, whereas they are complex and may bend back on themselves at low P _{H 2}O or low a _{H 2}O at high P. Two-feldspar paths may be simultaneous (cotectic) or sequential (peritectic). The former are simple and do not meet at high P and a _{H 2}O, the critical solution line lying in the gap; they are complex and may bend back or overlap at low P _{H 2}O or low a _{H 2}O at high P, the position of the critical solution line being hard to determine. Liquids which have simultaneously fractionated two feldspars may fractionate only one towards the end, crystallization changing from subsolvus to hypersolvus. Sequential paths may involve overgrowth of an early feldspar by a later one, usually sanidine overgrowths on plagioclase, but plagioclase overgrowths on sanidine occur. These complexities explain in part the difficulties of unravelling the textural and compositional relationships of ternary feldspars in water-poor felsic igneous rocks (even in the absence of alteration or complex magma dynamics) and of trying to deduce phase relationships from natural occurrences of feldspars.C.R.P.G. contribution number 948