Tidally incised valleys on tropical carbonate shelves: An example from the northern Great Barrier Reef, Australia

The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia's Great Barrier Reef, adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than 90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a − 120 m, eustatic sea level low-stand, as what occurred on this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position.

Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley.

Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal-incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the adjacent ocean basins during intermediate (rather than last glacial maximum) low-stand, sea level positions. The model may apply to other rimmed shelves, both modern and ancient.     

Impact of urban stormwater runoff on estuarine environmental quality

Urban stormwater runoff could contribute to the deterioration of water quality of a receiving water body. In this research, field studies and laboratory experiments were conducted to assess the microbial contamination resulting from urban stormwater runoff into the Lake Pontchartrain estuary. Fecal coliform, Escherichia coli and enterococci were used as indicator organisms. The specific objectives of the research were to examine the distribution of the indicator organisms in different environmental elements (water column, suspended particles and sediment) and to further investigate the mechanisms related to their fate. Results of the research indicated satisfactory water quality at the study sites during dry weather periods. However, a significant increase was observed in the concentrations of the indicator organisms in the water columns and sediment at specific study sites following a given stormwater event. Three to seven days were needed for the elevated indicator organisms to return back to their background levels in the water column and sediment, respectively. The mechanism of sedimentation contributed to a reduction in the microbial concentration in the water column, as the indicator organisms were found to attach to the suspended particles in the stormwater. The percentage of fecal coliform, E. coli and enterococci attached to the suspended particles was found to be within the range of 9.8–27.5%, 21.8–30.4%, and 8.4–11.5% of the total indicator organisms in the stormwater loaded into the estuary, respectively. About 75–80% of the total indicator organisms remained free-floating for some distance in the water column before dying off.     

An unsteady wave driver for narrowbanded waves: modeling nearshore circulation driven by wave groups

In this paper, we derive an unsteady refraction–diffraction model for narrowbanded water waves for use in computing coupled wave–current motion in the nearshore. The end result is a variable coefficient, nonlinear Schrödinger-type wave driver (describing the envelope of narrow-banded incident waves) coupled to forced nonlinear shallow water equations (describing steady or unsteady mean flows driven by the short-wave field). Comparisons with experimental data show that good accuracy can be obtained for cases of nonbreaking wave transformation. Numerical simulations show that the interaction of wave groups with longshore topographic nonuniformities generates strong edge wave resonances, providing a generating mechanism for low-order edge waves.     

Animal-sediment relations in shallow water benthic communities

Ninety benthic samples were used to study animal-sediment relations in Tomales Bay, California. While most of the benthic species studied were found more often in a particular type of substrate, individuals of such species were occasionally found in other substrates. There is a striking tendency for species occurring outside of their characteristic environment to be associated with the most diverse assemblages of the foreign substrate. Species low in the order of succession are those species that are found more frequently on other substrates. These phenomena are explained in terms of environmental stability. The concepts involved suggest a means of predicting the sequence of faunal changes following the alteration of the substrate.     

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

Marine geology in the environs of Bouvet Island and the South Atlantic triple juction

It is suggested that Bouvet Island is the surface manifestation of a mantle plume which has resulted in the creation of a chain of seamounts in the South Atlantic and a general shoaling of the region.The strike of two newly defined large fracture zones borth and south of the Bouvet Island pedestal have been utilized to determine a pole of rotation at 12.5°S, 12.5°W for the Africa-Antarctica motion. A pole at 75°S, 13°E has been calculated for the South America-Antarctica motion. At the triple junction the South America-Africa relative motion is 3.3 cm yr^-1 (whole rate) at 075°. The Africa-Antarctica motion is 1.7 cm yr^-1 at 065° and the South America-Antarctica motion is 1.6 cm yr^-1 at 085.     

Some responses of transplanted Mytilus edulis to metal-enriched sediments and sewage sludge

Approximately 25 % of the sewage sludge produced in the United Kingdom (about 10 million wet tonnes anually) is disposed of to British coastal waters at 15 sites. A series of tests is being evaluated in order to determine the effects of this practice in relation to the effects due to other polluting inputs, both natural and man-made, to the marine environment. The results presented here are for one site on the south-west coast of Britain. They indicate that the increased metal content of the tissue and the presence of sulphydryl-rich metal-binding proteins in the lysosomes of digestive cells in transplanted mussels can be correlated with exposure at sites overlying metal-rich sediments and to a lesser extent an area used for the disposal of sewage sludge containing metallic contaminants.     

Book reviews

THE IMPACT OF VOLCANIC HAZARDS AT RABAUL, PAPUA NEW GUINEA (Discussion Paper No. 33) by R. J. Blong and C. Aislabie. 17 × 25 cm, xiv and 207 pages. Institute of National Affairs: Port Morseby 1988 (ISBN 9980 77 086 4).

A PRACTICAL APPROACH TO SEDIMENTOLOGY by R. C. Lindholm. 15 × 23 cm, xviii and 279 pages. Allen & Unwin: London 1987 (ISBN 0 04 551132 2) $A45.00 (soft).

THE ATMOSPHERE AND WEATHER OF SOUTHERN AFRICA by R. A. Preston‐Whyte and P. D. Tyson. 18 × 24 cm, xiii and 375 pages. Oxford University Press: Cape Town 1988 (ISBN 0 19 570 514 9) R56.00 (hard); (ISBN 0 19 570 496 7) R39.14 (soft).

QUATERNARY PALEOCLIMATOLOGY: Methods of Paleoclimatic Reconstruction by R. S. Bradley. 15 × 23 cm, xvii and 472 pages. Allen & Unwin: Boston 1985 (ISBN 0 04 551067 9) $A69.95 (hard); (ISBN 0 04 551068 7) $A39.95 (soft).

PROCESSES IN PHYSICAL GEOGRAPHY by R. D. Thompson, A. M. Mannion, C. W. Mitchell, M. Parry and J. R. Townsend. 24 × 19 cm, xii and 380 pages. Longman: London 1986 (ISBN 0 582 30136) $A44.95.

PERSPECTIVES ON A DYNAMIC EARTH by T. R. Paton. 16 × 24 cm, 142 pages. Allen & Unwin: London 1986 (ISBN 0 04 55 0043 6) $A23.95 (soft).

LAND DEGRADATION AND SOCIETY edited by P. Blaikie and H. Brookfield. 15 × 23 cm. xxiv and 296 pages. Methuen: London 1987 (ISBN 0 416 40150 3) $A32.95 (soft).

THE PENGUIN DICTIONARY OF HUMAN GEOGRAPHY by B. Goodall. 13 × 20 cm, 509 pages. Penguin: Harmondsworth 1987 (ISBN 0 14 051095 8) $A19.95 (soft).

MAKING SENSE OF DEVELOPMENT: An Introduction to Classical and Contemporary Theories of Development and Their Application to South‐east Asia by P. W. Preston. 14 × 22 cm, xxi and 318 pages. Routledge & Kegan Paul: London 1986 (ISBN 0 7102 0813 8) $A87.95 (hard).

THE DEMOGRAPHIC DIMENSION IN INDONESIAN DEVELOPMENT by G. J. Hugo, T. H. Hull, V. J. Hull and G. W. Jones. 16 × 25 cm, xxv and 417 pages. Oxford University Press: Singapore 1987 (ISBN 0 19 582699 X) $A49.95 (hard).

GLOBAL ISSUES AND OUTLOOK IN PULP AND PAPER edited by G. F. Schreuder, 18 × 26 cm, ix and 302 pages. University of Washington Press: Seattle 1988 (ISBN 0 295 96642 4) $US40.00 (hard).

AN HISTORICAL GEOGRAPHY OF MODERN AUSTRALIA: The Restive Fringe (Cambridge Studies in Historical Geography No. 11) by J. M. Powell. 16 × 33 cm, xx and 400 pages. Cambridge University Press: Cambridge 1988 (ISBN 0 521 25619 4) $A39.95 (hard).

HISTORICAL GEOGRAPHY: Progress and Prospect edited by M. Pacione. 14 × 22 cm, 306 pages. Croom Helm: London 1987 (ISBN 0 7099 40467) $A27.50 (hard).

THE ROLE OF THEORY IN URBAN PLANNING: A Study of Metropolitan Planning in Perth, Western Australia (Geowest No. 24) by O. Yiftachel. 21 × 29 cm, 102 pages. Department of Geography, University of Western Australia: Nedlands 1987 (ISBN 0 86422 063 4).

SYDNEY INTO ITS THIRD CENTURY: Metropolitan Strategy for the Sydney Region by NSW Department of Environment and Planning. 21 × 29 cm, 72 pages. NSW department of Environment and Planning: Sydney 1988 (ISBN 0 7305 4312 9).

BUS SERVICES IN WESTERN SYDNEY: Private Bus Operators and Government Transport Departments (Monograph Series No. 3) by P. Tranter. 21 × 29 cm, v and 98 pages. Department of Geography and Oceanography, University College, Australian Defence Force Academy: Campbell, ACT 1988 (ISBN 0 7317 0036 8).

AGEING IN VICTORIA: An Electronic Social Atlas by A. L. Howe, P. Newton and P. Sharwood. 30 × 21 cm, 105 pages. National Research Institute for Gerontology and Geriatric Medicine, Mt. Royal Hospital and University of Melbourne: Melbourne 1987 (ISBN 0 958326 6 2) $A10.00 (soft).

READINGS IN AUSTRALIAN GEOGRAPHY: Proceedings of the 21st Institute of Australian Geographers’ Conference, Perth 10–18 May 1986, edited by A. Conacher. 21 × 29 cm, vi and 493 pages. Institute of Australian Geographers (WA Branch) and Department of Geography, University of Western Australia: Perth 1987 (ISBN 0 909678 30 B) $A25.00 (soft).

GEOGRAPHY RESEARCH PROJECTS: A Senior Student's Handbook by H. Binden and H. Williams. 18 × 26 cm, 149 pages. Edward Arnold Australia: Caulfield East 1988 (ISBN 0 7131 8311 X) $A 14.95 (soft).

OUR AMERICAN LAND: 1987 Yearbook of Agriculture, ix and 323 pages. US Government Printing Office, Washington, DC: 1987 $US9.50.

THE ENCYCLOPAEDIC DICTIONARY OF PHYSICAL GEOGRAPHY edited by A. Goudie et al. 17 × 24 cm, xvi and 528 pages. Basil Black‐well: Oxford 1988 (ISBN 0 631 15581 3) $A34.95 (soft).

THE CONSERVATION OF ECOSYSTEMS AND SPECIES by Gareth E. Jones, 14 × 22 cm, 277 pages. Croom Helm: London 1987 (ISBN 0 7099 1463 6) $A87.95 (hard).

SO CAME THEY SOUTH by R. Clancy and A. Richardson. 35 × 35 cm, 176 pages. Shakespeare Head: Sydney 1988 (ISBN 0 7302 0864 8) $A34.95.     

Environmental tracers as indicators of karst conduits in groundwater in South Dakota,USA

Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ¹⁸O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration.