The East Antarctic continental margin, which extends from the Weddell Sea to the Ross Sea, surrou... more The East Antarctic continental margin, which extends from the Weddell Sea to the Ross Sea, surrounds the largest and oldest ice mass on Earth; however, it has only been studied at a few locations because of its remoteness and persistent sea ice. The shelf is 100–150 km wide over most of its length but broadens where major crustal structures intersect it, such as in Prydz Bay where the shelf is 200–300 km wide. This paper reviews what is known presently about the geomorphology of the beststudied sectors of the East Antarctic margin: the deep re-entrant of Prydz Bay and the narrower shelves of George V and Mac.Robertson Land. Only a small proportion of the East Antarctica shelf has been surveyed with multibeam bathymetry, so this review is also dependent on compilations of single-beam bathymetry, seismic-reflection profiles and side-scan sonar data. In particular, we use George V Digital Elevation Model and International Bathymetric Chart of the Southern Ocean. The slope has been more widely studied, with large amounts of seismic-reflection data available.
We report the results of a multivariate analysis of geomorphic features occurring on the global c... more We report the results of a multivariate analysis of geomorphic features occurring on the global continental shelf that were mapped based on the Shuttle Radar Topography Mapping (SRTM30_PLUS) 30-arc sec database. The analysis was based on 11 input variables as follows: (1) the mean continental shelf depth; (2) mean shelf break depth; (3) mean shelf width; (4) percent area of low relief shelf; (5) percent area of medium relief shelf; (6) percent area of high relief shelf; (7) percent area of glacial troughs; (8) percent area of shelf valleys; (9) percent area of basins perched on the shelf; (10) the percent of submarine canyons that are shelf-incising; and (11) the percent area of coral reef. For the analysis the global shelf was divided into 551 reporting blocks, each approximately 500 km in along-shelf length. Eight shelf morphotypes were defi ned by multivariate analysis of the 11 input variables, and they can be grouped into four broad categories: narrow-shallow shelves; wide-fl at shelves; intermediate shelves; and deep-glaciated shelves. There is a negative correlation between shelf width and active plate margins, although there are examples of most shelf morphotypes occurring on both active and passive margins. Glaciation plays a major role in determining shelf geomorphology and characterizes around 21 % of the global shelf. In particular, we fi nd a very strong correlation between mean shelf depth and the percentage area of glacial troughs, indicative of the role played by glaciation and glacial erosion in shaping the global shelf. Coral reef growth is an important factor for one morphotype, which covers 427,000 km 2 or about 1.3 % of all continental shelves. The hypsometric curve for mean shelf depth exhibits a peak at a depth of 40 m that coincides with a persistent position of sea level during the last 500,000 years based on one published sea level curve. The geomorphic characterization and classifi cation of the continental shelf at a global scale could be advanced using predictive modeling tools (for tidal sand banks, for example) but is otherwise dependent upon improved resolution bathymetric data becoming available.
The stability of floating ice shelves is an important indicator of ocean circulation and ice shel... more The stability of floating ice shelves is an important indicator of ocean circulation and ice shelf mass balance. A sub ice-shelf sediment core collected during the Austral summer of 2000-2001 from site AM02 (69°42.8’S, 72°38.4’E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ~80 km south of the floating ice shelf edge and contains a 0.5 m thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub ice-shelf circulation models which predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ~80 km. An increase in sea-ice associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum.
Large-scale bedforms (2–10 m in vertical and 102–103 m in horizontal dimensions) found in wide-mo... more Large-scale bedforms (2–10 m in vertical and 102–103 m in horizontal dimensions) found in wide-mouthed estuaries are described. Different bedform types occur depending upon the local availability of sand. With an increasing sand supply, sand ribbons grade into elongate trains of sand waves and then form sandwave fields. Inshore, headland-associated sand banks are formed which multiply into en-echelon sand banks. Based upon a review of data on directions of sand transport from the Bristol Channel and Thames Estuary, U.K., and from Moreton Bay, Australia, charts of ebb- and flood-dominant transport zones are constructed for lower estuarine environments which have undergone different degrees of infilling. Linear sand banks are seen to delimit partially the boundaries between opposing sand transport zones. Transport paths demonstrate how sediments derived from outside of the estuary are dispersed through ebb and flood transport zones, to supply other areas of net deposition. A comparison between different estuaries reveals that variations in the compexity of ebb- and flood-dominant transport zones and the morphologies of large-scale bedforms are coupled with apparent changes in the relative amounts of sand available to each system. A model for the sequential infilling of estuaries and the evolution of large-scale bedforms is presented and applied to the interpretation of present day examples. Vertical sequences predicted to be generated by such bedform evolution are described and discussed, in terms of their preservation in the geological record.
Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-do... more Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-dominant zones of net bedload transport are present. To test this hypothesis, long-term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind-waves. Extrapolating these values over the ebb- and flood-dominant zones, sand flux to the upper part of the channel is estimated at about 64 × 105 t yr−1 with supply and removal being of nearly equal magnitude. Such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments.
Recently discovered sediment drift deposits on the Antarctic continental shelf provide access to ... more Recently discovered sediment drift deposits on the Antarctic continental shelf provide access to information on the Holocene palaeoceanography of the bottom current regime within deep shelf basins that was previously inaccessible. The George Vth basin on the East Antarctic margin has been identified by oceanographers as an important source of Antarctic Bottom Water, hence the Holocene history of bottom current activity here may be relevant to variations in bottom water export. The analysis of seismic and sediment core data indicates that the Holocene history of sedimentation on the Mertz Drift occurred in response to a progressively changing bottom current regime. The stratigraphic horizons that mark the onset of different phases of deposition are diachronous within the Mertz Drift. Rapidly accumulating, laminated siliceous mud and diatom ooze (SMO) that comprises the bulk was deposited first on the eastern side of the drift (5,000 to 3,500 years BP) and later on the lobate, southwestern side (3,000 and 2,000 years BP). This spatial variation in timing of rapid SMO deposition is attributed to a gradual increase in bottom current speed over the mid- to late Holocene. The deposition of true drift-style sedimentary features is restricted to a small area in the southwestern corner of the Mertz Drift, with the remaining parts characterised by drape and fill deposits. An estimation of the mass flux of sediment reaching the drift suggests that the modern, stronger bottom currents need carry only a very low suspended sediment concentration to explain the measured 10-fold reduction in sediment accumulation rates after 3000 years BP. Cores from the Mertz Drift and from a perched basin located 60 km to the east, contain the same lithologic units and have the same approximate age. Thus the palaeoenvironmental interpretations based on the Mertz Drift can be extrapolated over the entire George Vth Basin region.
A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered... more A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of East Antarctica in an 850 m deep glacial trough off George Vth Land. Radiocarbon dating indicates that a period of rapid deposition on the drift (averaging 290 cm/kyr) occurred in the mid-Holocene, between about 3,000 to 5,000 years before present (yr BP). Slower deposition rates of around 10 cm/kyr, during the past 0-3,000 years and from 5,000 to about 13,000 yr BP, coincides with the deposition of bioturbated, ice-rafted debris (IRD) rich, sandy mud under an energetic bottom current regime. In contrast, the mid-Holocene (3,000-5,000 yr BP) sediments are fine-grained, laminated to cross-laminated with minimal IRD content, and are contemporaneous with a period of warmer marine conditions with less sea ice production. This pattern suggests that bottom currents were weaker than present day in the mid-Holocene, and that the rate of dense bottom water production was reduced at that time. This scenario is consistent with the hypothesis of non-steady state rates of Antarctic bottom water production through the Holocene as recently proposed by Broecker and his colleagues.
This special issue of Continental Shelf Research contains 20 papers giving research results produ... more This special issue of Continental Shelf Research contains 20 papers giving research results produced as part of Australia’s Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and extension research in the Torres Strait region of northeastern Australia was carried out to meet three aims: 1) support the sustainable development of marine resources and minimize impacts of resource use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to effective policy formulation and management decision making. Subjects covered, including commercial and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows and communication of science results to local communities, have broad applications to other similar environments.
Tidal current and elevation data were collected from five oceanographic moorings during October 2... more Tidal current and elevation data were collected from five oceanographic moorings during October 2004 in Torres Strait, northern Australia, to assess the effects of large bedforms (i.e., sand banks) on the drag coefficient (CD) used for estimating bed shear stress in complex shallow shelf environments. Ten minute averages of tidal current speed and elevation data were collected for 18 days
Seagrass communities in the northwest of Torres Strait are known to disappear episodically over b... more Seagrass communities in the northwest of Torres Strait are known to disappear episodically over broad areas. Sediment mobility surveys were undertaken within two study areas during the monsoon and trade wind seasons, in the vicinity of Turnagain Island, to find out if the migration of bedforms could explain this disappearance. The two study areas covered sand bank and sand dune
In order to protect the biological diversity of marine life in Australia's exclusive economic zon... more In order to protect the biological diversity of marine life in Australia's exclusive economic zone (EEZ), the Commonwealth Government has passed the Environment Protection and Biodiversity Conservation (EPBC) Act. The Act is being implemented through the preparation of regional marine plans (that commenced in 2001) and by designing networks of representative marine protected areas (MPAs) in both Commonwealth and State waters. In the absence of direct information about the distribution of seabed biodiversity, appropriate surrogates must be used instead. A major constraint is the short timeframe available to managers to make decisions; only information that is readily accessible and available can be used under these circumstances. Existing seabed bathymetry data were used to produce a geomorphic features map of the Australian EEZ. This map was used in conjunction with existing fish diversity information and other data to derive a benthic bioregionalisation (2005) that subdivides Australia's EEZ into 41 bioregions including 24 biologically unique provinces. Biophysical variables measured at broad spatial scales apart from bathymetry (and derived variables such as seabed slope) include ocean primary production, seabed sediment properties, temperature and sediment mobilisation due to waves and tides. To better characterise habitats on the Australian continental margin, Geoscience Australia has created ''seascape'' maps that integrate multiple layers of spatial data that are useful for the prediction of the distribution of biodiversity. An example is presented from the southwest margin of Australia; here, we used ER-Mapper's unsupervised ISO class algorithm to simultaneously classify six variables, with equal weighting, to yield a range of statistically different classes. The broad spatial coverage provided by the analysis of biophysical data provides information complementary to the knowledge of sites important to threatened or endangered species, to iconic sites of high conservation value and to the available (mostly very limited) direct measurements of biodiversity (hot spots). Seascape and geomorphic habitat maps have assisted managers in Australia with broad-scale marine planning and in the design of representative MPA networks.
Simple, conceptual geomorphic models can assist environmental managers in making informed decisio... more Simple, conceptual geomorphic models can assist environmental managers in making informed decisions regarding management of the coast at continental and regional scales. This basic information, detected from aerial photographs and/or satellite images, can be used to ascertain the relative significance of several common environmental issues, including: sediment trapping efficiency, turbidity, water circulation, and habitat change due to sedimentation for different types of clastic coastal depositional environments. The classification of 780 Australian clastic coastal depositional environments based on their geomorphology is used to derive a coastal regionalisation, comprised of a distinctive suite of environments for each region. Because of the close link between the relative influence of waves and tides and the geomorphology of clastic coastal depositional environments, a basic understanding of the broad geomorphic and sedimentary characteristics by environmental managers will assist them in ascertaining the relative significance of environmental issues in each region. The benefit of this approach is that it provides guidance in tailoring management schemes differently for each region, resulting in more effective and efficient treatment of these issues. Crown
The formation of incised valleys on continental shelves is generally attributed to fluvial erosio... more 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 À 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.
Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene
... more Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia’s coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits.
The aim of this study is to assess the global occurrence of large submarine canyons to provide co... more The aim of this study is to assess the global occurrence of large submarine canyons to provide context and guidance for discussions regarding canyon occurrence, distribution, geological and oceanographic significance and conservation. Based on an analysis of the ETOPO1 data set, this study has compiled the first inventory of 5849 separate large submarine canyons in the world ocean. Active continental margins contain 15% more canyons (2586, equal to 44.2% of all canyons) than passive margins (2244, equal to 38.4%) and the canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. This study confirms observations of earlier workers that a relationship exists between canyon slope and canyon spacing (increased canyon slope correlates with closer canyon spacing). The greatest canyon spacing occurs in the Arctic and the Antarctic whereas canyons are more closely spaced in the Mediterranean than in other areas. River-associated, shelf-incising canyons are more numerous on active continental margins (n=119) than on passive margins (n=34). They are most common on the western margins of South and North America where they comprise 11.7% and 8.6% of canyons respectively, but are absent from the margins of Australia and Antarctica. Geographic areas having relatively high rates of sediment export to continental margins, from either glacial or fluvial sources operating over geologic timescales, have greater numbers of shelf-incising canyons than geographic areas having relatively low rates of sediment export to continental margins. This observation is consistent with the origins of some canyons being related to erosive turbidity flows derived from fluvial and shelf sediment sources. Other workers have shown that benthic ecosystems in shelf-incising canyons contain greater diversity and biomass than non-incising canyons, and that ecosystems located above 1500 m water depth are more vulnerable to destructive fishing practices (bottom trawling) and ocean acidification caused by anthropogenic climate change. The present study provides the means to assess the relative significance of canyons located in different geographic regions. On this basis, the importance of conservation for submarine canyon ecosystems is greater for Australia, islands and northeast Asia than for other regions.
Submarine canyons influence oceanographic processes, sediment transport, productivity and benthic... more Submarine canyons influence oceanographic processes, sediment transport, productivity and benthic biodiversity from the continental shelf to the slope and beyond. However, not all canyons perform the same function. The relative influence of an individual canyon on these processes will, in part, be determined by its form, shape and position on the continental margin. Here we present an analysis of canyon geomorphic metrics using an updated national dataset of 713 submarine canyons surrounding mainland Australia. These metrics (attributes) for each canyon are used to classify them into canyon types across a hierarchy of physical characteristics separately for shelf-incising (n= 95) and slope-confined (blind; n = 618) canyons. We find that the canyon metrics describe a wide variety of canyon form and complexity that is consistent with a population of canyons that has evolved at different rates around the Australian margin since the break-up of Gondwana. The large number of slopeconfined canyons is interpreted to reflect dominance of slope mass-wasting processes over erosive turbidity flows from fluvial and shelf sources on an arid continent. The distribution of submarine canyons around the Australian margin is not regular, with clusters occurring in the east, southeast, west and southwest where the margin is steepest. The classification result provides a quantitative framework for describing canyon heterogeneity for application in studies of geological controls on individual canyons, canyon oceanography and canyon biodiversity.
The Queen Charlotte Fault Zone (QCFZ) off western
Canada is the northern equivalent to the San A... more The Queen Charlotte Fault Zone (QCFZ) off western
Canada is the northern equivalent to the San Andreas
Fault Zone, the Pacific–North American plate boundary.
Geomorphologic expression and surface processes associated
with the QCFZ system have been revealed in unprecedented
detail by recent seabed mapping surveys. Convergence of the
Pacific and North American plates along northern British
Columbia is well known, but how the QCFZ accommodates
this convergence is still a subject of controversy. Themultibeam
sonar bathymetry data reveal, for the first time, evidence of a
fault valley with small depressions on the upper slope, offshore
centralHaida Gwaii (Queen Charlotte Islands). The depressions
form where strike-slip right-step offsets have realigned the fault
due to oblique convergence. Core stratigraphy and radiocarbon
dating of sediments within the fault valley and small depressions
suggest that these features are recent in origin. In addition,
the development of the fault valley and dislocation of submarine
canyons control sediment migration from the continental
shelf through to the lower slope. This interpretation of the
geomorphic expression of major plate tectonic processes along
the QCFZ can now be tested with new surveys subsequent to
the October 2012 magnitude 7.7 earthquake.
Faulting commonly influences the geomorphology of submarine canyons that occur on active continen... more Faulting commonly influences the geomorphology of submarine canyons that occur on active continental margins. Here we examine the geomorphology of canyons located on the continental margin off Haida Gwaii, British Columbia, that are truncated on the mid-slope (1,200–1,400 m water depth) by the Queen Charlotte Fault Zone (QCFZ). The QCFZ is an oblique strike-slip fault zone that has rates of lateral motion of around 50–60 mm/yr and a small convergent component equal to about 3 mm/yr. Slow subduction along the Cascadia Subduction Zone has accreted a prism of marine sediment against the lower slope (1,500 to 3,500 m water depth), forming the Queen Charlotte Terrace, which blocks the mouths of submarine canyons formed on the upper slope (200 to 1,400 m water depth). Consequently, canyons along this margin are short (4 to 8 km in length), closely spaced (around 800 m), and terminate uniformly along the 1,400 m isobath, coinciding with the primary fault trend of the QCFZ. Vertical displacement along the fault has resulted in hanging canyons occurring locally. The Haida Gwaii canyons are compared and contrasted with the Sur Canyon system, located to the south of Monterey Bay, California, on a transform margin, which is not blocked by any accretionary prism, and where canyons thus extend to 4,000 m depth, across the full breadth of the slope.
Physical disturbances of the seafloor play a key role in ecosystem function and are postulated to... more Physical disturbances of the seafloor play a key role in ecosystem function and are postulated to exert control over spatial patterns of biodiversity. This review investigates the role of natural physical sedimentological processes that occur in shelf, slope and abyssal environments that also act as disturbances to benthic ecosystems and which, under certain circumstances, give rise to benthic disturbance regimes. Physical sedimentological processes can cause both press (process that causes a disturbance by acting over a timespan that is intolerable to benthos) and pulse (process that causes a disturbance by exceeding a threshold above which benthos are unable to remain attached to the seabed or are buried under rapidly deposited sediment) types of disturbance. On the continental shelf, pulse-type disturbances are due to temperate and tropical storm events, and press-type of disturbances identified here are due to the migration of bedforms and other sand bodies, and sustained periods of elevated turbidity caused by seasonally reversing wind patterns. On the continental slope and at abyssal depths, pulse-type disturbances are due to slumps, turbidity currents; benthic storms may cause either press or pulse type disturbances. A possible press-type of disturbance identified here is inter-annual changes in abyssal bottom current speed and/or direction. It is concluded that: 1) physical sedimentary disturbance regimes may characterize as much as 10% of the global ocean floor; 2) multidisciplinary research programs that integrate oceanography, sedimentology and benthic ecology to collect time series observational data sets are needed to study disturbance regimes; and 3) predictive habitat suitability modeling must include disturbance regime concepts, along with other biophysical variables that define the fundamental niches of marine species, in order to advance.
The East Antarctic continental margin, which extends from the Weddell Sea to the Ross Sea, surrou... more The East Antarctic continental margin, which extends from the Weddell Sea to the Ross Sea, surrounds the largest and oldest ice mass on Earth; however, it has only been studied at a few locations because of its remoteness and persistent sea ice. The shelf is 100–150 km wide over most of its length but broadens where major crustal structures intersect it, such as in Prydz Bay where the shelf is 200–300 km wide. This paper reviews what is known presently about the geomorphology of the beststudied sectors of the East Antarctic margin: the deep re-entrant of Prydz Bay and the narrower shelves of George V and Mac.Robertson Land. Only a small proportion of the East Antarctica shelf has been surveyed with multibeam bathymetry, so this review is also dependent on compilations of single-beam bathymetry, seismic-reflection profiles and side-scan sonar data. In particular, we use George V Digital Elevation Model and International Bathymetric Chart of the Southern Ocean. The slope has been more widely studied, with large amounts of seismic-reflection data available.
We report the results of a multivariate analysis of geomorphic features occurring on the global c... more We report the results of a multivariate analysis of geomorphic features occurring on the global continental shelf that were mapped based on the Shuttle Radar Topography Mapping (SRTM30_PLUS) 30-arc sec database. The analysis was based on 11 input variables as follows: (1) the mean continental shelf depth; (2) mean shelf break depth; (3) mean shelf width; (4) percent area of low relief shelf; (5) percent area of medium relief shelf; (6) percent area of high relief shelf; (7) percent area of glacial troughs; (8) percent area of shelf valleys; (9) percent area of basins perched on the shelf; (10) the percent of submarine canyons that are shelf-incising; and (11) the percent area of coral reef. For the analysis the global shelf was divided into 551 reporting blocks, each approximately 500 km in along-shelf length. Eight shelf morphotypes were defi ned by multivariate analysis of the 11 input variables, and they can be grouped into four broad categories: narrow-shallow shelves; wide-fl at shelves; intermediate shelves; and deep-glaciated shelves. There is a negative correlation between shelf width and active plate margins, although there are examples of most shelf morphotypes occurring on both active and passive margins. Glaciation plays a major role in determining shelf geomorphology and characterizes around 21 % of the global shelf. In particular, we fi nd a very strong correlation between mean shelf depth and the percentage area of glacial troughs, indicative of the role played by glaciation and glacial erosion in shaping the global shelf. Coral reef growth is an important factor for one morphotype, which covers 427,000 km 2 or about 1.3 % of all continental shelves. The hypsometric curve for mean shelf depth exhibits a peak at a depth of 40 m that coincides with a persistent position of sea level during the last 500,000 years based on one published sea level curve. The geomorphic characterization and classifi cation of the continental shelf at a global scale could be advanced using predictive modeling tools (for tidal sand banks, for example) but is otherwise dependent upon improved resolution bathymetric data becoming available.
The stability of floating ice shelves is an important indicator of ocean circulation and ice shel... more The stability of floating ice shelves is an important indicator of ocean circulation and ice shelf mass balance. A sub ice-shelf sediment core collected during the Austral summer of 2000-2001 from site AM02 (69°42.8’S, 72°38.4’E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ~80 km south of the floating ice shelf edge and contains a 0.5 m thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub ice-shelf circulation models which predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ~80 km. An increase in sea-ice associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum.
Large-scale bedforms (2–10 m in vertical and 102–103 m in horizontal dimensions) found in wide-mo... more Large-scale bedforms (2–10 m in vertical and 102–103 m in horizontal dimensions) found in wide-mouthed estuaries are described. Different bedform types occur depending upon the local availability of sand. With an increasing sand supply, sand ribbons grade into elongate trains of sand waves and then form sandwave fields. Inshore, headland-associated sand banks are formed which multiply into en-echelon sand banks. Based upon a review of data on directions of sand transport from the Bristol Channel and Thames Estuary, U.K., and from Moreton Bay, Australia, charts of ebb- and flood-dominant transport zones are constructed for lower estuarine environments which have undergone different degrees of infilling. Linear sand banks are seen to delimit partially the boundaries between opposing sand transport zones. Transport paths demonstrate how sediments derived from outside of the estuary are dispersed through ebb and flood transport zones, to supply other areas of net deposition. A comparison between different estuaries reveals that variations in the compexity of ebb- and flood-dominant transport zones and the morphologies of large-scale bedforms are coupled with apparent changes in the relative amounts of sand available to each system. A model for the sequential infilling of estuaries and the evolution of large-scale bedforms is presented and applied to the interpretation of present day examples. Vertical sequences predicted to be generated by such bedform evolution are described and discussed, in terms of their preservation in the geological record.
Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-do... more Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-dominant zones of net bedload transport are present. To test this hypothesis, long-term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind-waves. Extrapolating these values over the ebb- and flood-dominant zones, sand flux to the upper part of the channel is estimated at about 64 × 105 t yr−1 with supply and removal being of nearly equal magnitude. Such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments.
Recently discovered sediment drift deposits on the Antarctic continental shelf provide access to ... more Recently discovered sediment drift deposits on the Antarctic continental shelf provide access to information on the Holocene palaeoceanography of the bottom current regime within deep shelf basins that was previously inaccessible. The George Vth basin on the East Antarctic margin has been identified by oceanographers as an important source of Antarctic Bottom Water, hence the Holocene history of bottom current activity here may be relevant to variations in bottom water export. The analysis of seismic and sediment core data indicates that the Holocene history of sedimentation on the Mertz Drift occurred in response to a progressively changing bottom current regime. The stratigraphic horizons that mark the onset of different phases of deposition are diachronous within the Mertz Drift. Rapidly accumulating, laminated siliceous mud and diatom ooze (SMO) that comprises the bulk was deposited first on the eastern side of the drift (5,000 to 3,500 years BP) and later on the lobate, southwestern side (3,000 and 2,000 years BP). This spatial variation in timing of rapid SMO deposition is attributed to a gradual increase in bottom current speed over the mid- to late Holocene. The deposition of true drift-style sedimentary features is restricted to a small area in the southwestern corner of the Mertz Drift, with the remaining parts characterised by drape and fill deposits. An estimation of the mass flux of sediment reaching the drift suggests that the modern, stronger bottom currents need carry only a very low suspended sediment concentration to explain the measured 10-fold reduction in sediment accumulation rates after 3000 years BP. Cores from the Mertz Drift and from a perched basin located 60 km to the east, contain the same lithologic units and have the same approximate age. Thus the palaeoenvironmental interpretations based on the Mertz Drift can be extrapolated over the entire George Vth Basin region.
A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered... more A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of East Antarctica in an 850 m deep glacial trough off George Vth Land. Radiocarbon dating indicates that a period of rapid deposition on the drift (averaging 290 cm/kyr) occurred in the mid-Holocene, between about 3,000 to 5,000 years before present (yr BP). Slower deposition rates of around 10 cm/kyr, during the past 0-3,000 years and from 5,000 to about 13,000 yr BP, coincides with the deposition of bioturbated, ice-rafted debris (IRD) rich, sandy mud under an energetic bottom current regime. In contrast, the mid-Holocene (3,000-5,000 yr BP) sediments are fine-grained, laminated to cross-laminated with minimal IRD content, and are contemporaneous with a period of warmer marine conditions with less sea ice production. This pattern suggests that bottom currents were weaker than present day in the mid-Holocene, and that the rate of dense bottom water production was reduced at that time. This scenario is consistent with the hypothesis of non-steady state rates of Antarctic bottom water production through the Holocene as recently proposed by Broecker and his colleagues.
This special issue of Continental Shelf Research contains 20 papers giving research results produ... more This special issue of Continental Shelf Research contains 20 papers giving research results produced as part of Australia’s Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and extension research in the Torres Strait region of northeastern Australia was carried out to meet three aims: 1) support the sustainable development of marine resources and minimize impacts of resource use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to effective policy formulation and management decision making. Subjects covered, including commercial and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows and communication of science results to local communities, have broad applications to other similar environments.
Tidal current and elevation data were collected from five oceanographic moorings during October 2... more Tidal current and elevation data were collected from five oceanographic moorings during October 2004 in Torres Strait, northern Australia, to assess the effects of large bedforms (i.e., sand banks) on the drag coefficient (CD) used for estimating bed shear stress in complex shallow shelf environments. Ten minute averages of tidal current speed and elevation data were collected for 18 days
Seagrass communities in the northwest of Torres Strait are known to disappear episodically over b... more Seagrass communities in the northwest of Torres Strait are known to disappear episodically over broad areas. Sediment mobility surveys were undertaken within two study areas during the monsoon and trade wind seasons, in the vicinity of Turnagain Island, to find out if the migration of bedforms could explain this disappearance. The two study areas covered sand bank and sand dune
In order to protect the biological diversity of marine life in Australia's exclusive economic zon... more In order to protect the biological diversity of marine life in Australia's exclusive economic zone (EEZ), the Commonwealth Government has passed the Environment Protection and Biodiversity Conservation (EPBC) Act. The Act is being implemented through the preparation of regional marine plans (that commenced in 2001) and by designing networks of representative marine protected areas (MPAs) in both Commonwealth and State waters. In the absence of direct information about the distribution of seabed biodiversity, appropriate surrogates must be used instead. A major constraint is the short timeframe available to managers to make decisions; only information that is readily accessible and available can be used under these circumstances. Existing seabed bathymetry data were used to produce a geomorphic features map of the Australian EEZ. This map was used in conjunction with existing fish diversity information and other data to derive a benthic bioregionalisation (2005) that subdivides Australia's EEZ into 41 bioregions including 24 biologically unique provinces. Biophysical variables measured at broad spatial scales apart from bathymetry (and derived variables such as seabed slope) include ocean primary production, seabed sediment properties, temperature and sediment mobilisation due to waves and tides. To better characterise habitats on the Australian continental margin, Geoscience Australia has created ''seascape'' maps that integrate multiple layers of spatial data that are useful for the prediction of the distribution of biodiversity. An example is presented from the southwest margin of Australia; here, we used ER-Mapper's unsupervised ISO class algorithm to simultaneously classify six variables, with equal weighting, to yield a range of statistically different classes. The broad spatial coverage provided by the analysis of biophysical data provides information complementary to the knowledge of sites important to threatened or endangered species, to iconic sites of high conservation value and to the available (mostly very limited) direct measurements of biodiversity (hot spots). Seascape and geomorphic habitat maps have assisted managers in Australia with broad-scale marine planning and in the design of representative MPA networks.
Simple, conceptual geomorphic models can assist environmental managers in making informed decisio... more Simple, conceptual geomorphic models can assist environmental managers in making informed decisions regarding management of the coast at continental and regional scales. This basic information, detected from aerial photographs and/or satellite images, can be used to ascertain the relative significance of several common environmental issues, including: sediment trapping efficiency, turbidity, water circulation, and habitat change due to sedimentation for different types of clastic coastal depositional environments. The classification of 780 Australian clastic coastal depositional environments based on their geomorphology is used to derive a coastal regionalisation, comprised of a distinctive suite of environments for each region. Because of the close link between the relative influence of waves and tides and the geomorphology of clastic coastal depositional environments, a basic understanding of the broad geomorphic and sedimentary characteristics by environmental managers will assist them in ascertaining the relative significance of environmental issues in each region. The benefit of this approach is that it provides guidance in tailoring management schemes differently for each region, resulting in more effective and efficient treatment of these issues. Crown
The formation of incised valleys on continental shelves is generally attributed to fluvial erosio... more 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 À 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.
Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene
... more Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia’s coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits.
The aim of this study is to assess the global occurrence of large submarine canyons to provide co... more The aim of this study is to assess the global occurrence of large submarine canyons to provide context and guidance for discussions regarding canyon occurrence, distribution, geological and oceanographic significance and conservation. Based on an analysis of the ETOPO1 data set, this study has compiled the first inventory of 5849 separate large submarine canyons in the world ocean. Active continental margins contain 15% more canyons (2586, equal to 44.2% of all canyons) than passive margins (2244, equal to 38.4%) and the canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. This study confirms observations of earlier workers that a relationship exists between canyon slope and canyon spacing (increased canyon slope correlates with closer canyon spacing). The greatest canyon spacing occurs in the Arctic and the Antarctic whereas canyons are more closely spaced in the Mediterranean than in other areas. River-associated, shelf-incising canyons are more numerous on active continental margins (n=119) than on passive margins (n=34). They are most common on the western margins of South and North America where they comprise 11.7% and 8.6% of canyons respectively, but are absent from the margins of Australia and Antarctica. Geographic areas having relatively high rates of sediment export to continental margins, from either glacial or fluvial sources operating over geologic timescales, have greater numbers of shelf-incising canyons than geographic areas having relatively low rates of sediment export to continental margins. This observation is consistent with the origins of some canyons being related to erosive turbidity flows derived from fluvial and shelf sediment sources. Other workers have shown that benthic ecosystems in shelf-incising canyons contain greater diversity and biomass than non-incising canyons, and that ecosystems located above 1500 m water depth are more vulnerable to destructive fishing practices (bottom trawling) and ocean acidification caused by anthropogenic climate change. The present study provides the means to assess the relative significance of canyons located in different geographic regions. On this basis, the importance of conservation for submarine canyon ecosystems is greater for Australia, islands and northeast Asia than for other regions.
Submarine canyons influence oceanographic processes, sediment transport, productivity and benthic... more Submarine canyons influence oceanographic processes, sediment transport, productivity and benthic biodiversity from the continental shelf to the slope and beyond. However, not all canyons perform the same function. The relative influence of an individual canyon on these processes will, in part, be determined by its form, shape and position on the continental margin. Here we present an analysis of canyon geomorphic metrics using an updated national dataset of 713 submarine canyons surrounding mainland Australia. These metrics (attributes) for each canyon are used to classify them into canyon types across a hierarchy of physical characteristics separately for shelf-incising (n= 95) and slope-confined (blind; n = 618) canyons. We find that the canyon metrics describe a wide variety of canyon form and complexity that is consistent with a population of canyons that has evolved at different rates around the Australian margin since the break-up of Gondwana. The large number of slopeconfined canyons is interpreted to reflect dominance of slope mass-wasting processes over erosive turbidity flows from fluvial and shelf sources on an arid continent. The distribution of submarine canyons around the Australian margin is not regular, with clusters occurring in the east, southeast, west and southwest where the margin is steepest. The classification result provides a quantitative framework for describing canyon heterogeneity for application in studies of geological controls on individual canyons, canyon oceanography and canyon biodiversity.
The Queen Charlotte Fault Zone (QCFZ) off western
Canada is the northern equivalent to the San A... more The Queen Charlotte Fault Zone (QCFZ) off western
Canada is the northern equivalent to the San Andreas
Fault Zone, the Pacific–North American plate boundary.
Geomorphologic expression and surface processes associated
with the QCFZ system have been revealed in unprecedented
detail by recent seabed mapping surveys. Convergence of the
Pacific and North American plates along northern British
Columbia is well known, but how the QCFZ accommodates
this convergence is still a subject of controversy. Themultibeam
sonar bathymetry data reveal, for the first time, evidence of a
fault valley with small depressions on the upper slope, offshore
centralHaida Gwaii (Queen Charlotte Islands). The depressions
form where strike-slip right-step offsets have realigned the fault
due to oblique convergence. Core stratigraphy and radiocarbon
dating of sediments within the fault valley and small depressions
suggest that these features are recent in origin. In addition,
the development of the fault valley and dislocation of submarine
canyons control sediment migration from the continental
shelf through to the lower slope. This interpretation of the
geomorphic expression of major plate tectonic processes along
the QCFZ can now be tested with new surveys subsequent to
the October 2012 magnitude 7.7 earthquake.
Faulting commonly influences the geomorphology of submarine canyons that occur on active continen... more Faulting commonly influences the geomorphology of submarine canyons that occur on active continental margins. Here we examine the geomorphology of canyons located on the continental margin off Haida Gwaii, British Columbia, that are truncated on the mid-slope (1,200–1,400 m water depth) by the Queen Charlotte Fault Zone (QCFZ). The QCFZ is an oblique strike-slip fault zone that has rates of lateral motion of around 50–60 mm/yr and a small convergent component equal to about 3 mm/yr. Slow subduction along the Cascadia Subduction Zone has accreted a prism of marine sediment against the lower slope (1,500 to 3,500 m water depth), forming the Queen Charlotte Terrace, which blocks the mouths of submarine canyons formed on the upper slope (200 to 1,400 m water depth). Consequently, canyons along this margin are short (4 to 8 km in length), closely spaced (around 800 m), and terminate uniformly along the 1,400 m isobath, coinciding with the primary fault trend of the QCFZ. Vertical displacement along the fault has resulted in hanging canyons occurring locally. The Haida Gwaii canyons are compared and contrasted with the Sur Canyon system, located to the south of Monterey Bay, California, on a transform margin, which is not blocked by any accretionary prism, and where canyons thus extend to 4,000 m depth, across the full breadth of the slope.
Physical disturbances of the seafloor play a key role in ecosystem function and are postulated to... more Physical disturbances of the seafloor play a key role in ecosystem function and are postulated to exert control over spatial patterns of biodiversity. This review investigates the role of natural physical sedimentological processes that occur in shelf, slope and abyssal environments that also act as disturbances to benthic ecosystems and which, under certain circumstances, give rise to benthic disturbance regimes. Physical sedimentological processes can cause both press (process that causes a disturbance by acting over a timespan that is intolerable to benthos) and pulse (process that causes a disturbance by exceeding a threshold above which benthos are unable to remain attached to the seabed or are buried under rapidly deposited sediment) types of disturbance. On the continental shelf, pulse-type disturbances are due to temperate and tropical storm events, and press-type of disturbances identified here are due to the migration of bedforms and other sand bodies, and sustained periods of elevated turbidity caused by seasonally reversing wind patterns. On the continental slope and at abyssal depths, pulse-type disturbances are due to slumps, turbidity currents; benthic storms may cause either press or pulse type disturbances. A possible press-type of disturbance identified here is inter-annual changes in abyssal bottom current speed and/or direction. It is concluded that: 1) physical sedimentary disturbance regimes may characterize as much as 10% of the global ocean floor; 2) multidisciplinary research programs that integrate oceanography, sedimentology and benthic ecology to collect time series observational data sets are needed to study disturbance regimes; and 3) predictive habitat suitability modeling must include disturbance regime concepts, along with other biophysical variables that define the fundamental niches of marine species, in order to advance.
ABS'IRAcr Submarine canyons in the Mediterranean and Black Seas stand out as globally different b... more ABS'IRAcr Submarine canyons in the Mediterranean and Black Seas stand out as globally different based on studies of global canyon geomorphology; they are more closely spaced, more dendritic (more limbs per unit area), shorter, have the smallest mean area, are among the most steep and have a smaller depth range than canyons that occur in other regions of the world. Here we present the results of a multivariate analysis of submarine canyon geomorphology to explore in more detail the apparently unique attributes of Mediterranean canyons. We fmd that Mediterranean canyons can be divided into six Classes, dominated by "Class 4" that is characterized by small area, close canyon spacing and a relatively high percentage of shelf incising canyons. On a global basis, Class 4 canyons are found to occur mainly (68%) on active continental margins. Examples of other regions in the world containing large numbers of Class 4 canyons are described.
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Papers by Peter T Harris
part of Australia’s Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a
three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and
extension research in the Torres Strait region of northeastern Australia was carried out to meet three
aims: 1) support the sustainable development of marine resources and minimize impacts of resource
use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and
economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to
effective policy formulation and management decision making. Subjects covered, including commercial
and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows
and communication of science results to local communities, have broad applications to other similar
environments.
shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great
Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the
development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp.
107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other
obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of
Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these
deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on
the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked
carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An
explanation for the consistent along-coast cyclone transport pathway is presented based on previously
published hydrodynamic modelling results. These models illustrate how currents generated by the
passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the
eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a
net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over
the entire length of northern Australia’s coastline. This process provides an explanation for the observed
sediment transport patterns on modern tropical continental shelves, as well as a basis for the
interpretation of ancient tropical shelf deposits.
guidance for discussions regarding canyon occurrence, distribution, geological and oceanographic significance
and conservation. Based on an analysis of the ETOPO1 data set, this study has compiled the first inventory of
5849 separate large submarine canyons in the world ocean. Active continental margins contain 15% more
canyons (2586, equal to 44.2% of all canyons) than passive margins (2244, equal to 38.4%) and the canyons are
steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. This
study confirms observations of earlier workers that a relationship exists between canyon slope and canyon
spacing (increased canyon slope correlates with closer canyon spacing). The greatest canyon spacing occurs in
the Arctic and the Antarctic whereas canyons are more closely spaced in the Mediterranean than in other
areas.
River-associated, shelf-incising canyons are more numerous on active continental margins (n=119) than on
passive margins (n=34). They are most common on the western margins of South and North America where
they comprise 11.7% and 8.6% of canyons respectively, but are absent from the margins of Australia and
Antarctica. Geographic areas having relatively high rates of sediment export to continental margins, from
either glacial or fluvial sources operating over geologic timescales, have greater numbers of shelf-incising
canyons than geographic areas having relatively low rates of sediment export to continental margins. This
observation is consistent with the origins of some canyons being related to erosive turbidity flows derived
from fluvial and shelf sediment sources.
Other workers have shown that benthic ecosystems in shelf-incising canyons contain greater diversity and
biomass than non-incising canyons, and that ecosystems located above 1500 m water depth are more
vulnerable to destructive fishing practices (bottom trawling) and ocean acidification caused by anthropogenic
climate change. The present study provides the means to assess the relative significance of canyons located in
different geographic regions. On this basis, the importance of conservation for submarine canyon ecosystems
is greater for Australia, islands and northeast Asia than for other regions.
from the continental shelf to the slope and beyond. However, not all canyons perform the same function. The
relative influence of an individual canyon on these processes will, in part, be determined by its form, shape and
position on the continental margin. Here we present an analysis of canyon geomorphic metrics using an updated
national dataset of 713 submarine canyons surrounding mainland Australia. These metrics (attributes) for each
canyon are used to classify them into canyon types across a hierarchy of physical characteristics separately for
shelf-incising (n= 95) and slope-confined (blind; n = 618) canyons. We find that the canyon metrics describe
a wide variety of canyon form and complexity that is consistent with a population of canyons that has evolved
at different rates around the Australian margin since the break-up of Gondwana. The large number of slopeconfined
canyons is interpreted to reflect dominance of slope mass-wasting processes over erosive turbidity
flows from fluvial and shelf sources on an arid continent. The distribution of submarine canyons around the
Australian margin is not regular, with clusters occurring in the east, southeast, west and southwest where the
margin is steepest. The classification result provides a quantitative framework for describing canyon heterogeneity
for application in studies of geological controls on individual canyons, canyon oceanography and canyon
biodiversity.
Canada is the northern equivalent to the San Andreas
Fault Zone, the Pacific–North American plate boundary.
Geomorphologic expression and surface processes associated
with the QCFZ system have been revealed in unprecedented
detail by recent seabed mapping surveys. Convergence of the
Pacific and North American plates along northern British
Columbia is well known, but how the QCFZ accommodates
this convergence is still a subject of controversy. Themultibeam
sonar bathymetry data reveal, for the first time, evidence of a
fault valley with small depressions on the upper slope, offshore
centralHaida Gwaii (Queen Charlotte Islands). The depressions
form where strike-slip right-step offsets have realigned the fault
due to oblique convergence. Core stratigraphy and radiocarbon
dating of sediments within the fault valley and small depressions
suggest that these features are recent in origin. In addition,
the development of the fault valley and dislocation of submarine
canyons control sediment migration from the continental
shelf through to the lower slope. This interpretation of the
geomorphic expression of major plate tectonic processes along
the QCFZ can now be tested with new surveys subsequent to
the October 2012 magnitude 7.7 earthquake.
part of Australia’s Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a
three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and
extension research in the Torres Strait region of northeastern Australia was carried out to meet three
aims: 1) support the sustainable development of marine resources and minimize impacts of resource
use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and
economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to
effective policy formulation and management decision making. Subjects covered, including commercial
and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows
and communication of science results to local communities, have broad applications to other similar
environments.
shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great
Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the
development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp.
107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other
obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of
Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these
deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on
the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked
carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An
explanation for the consistent along-coast cyclone transport pathway is presented based on previously
published hydrodynamic modelling results. These models illustrate how currents generated by the
passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the
eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a
net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over
the entire length of northern Australia’s coastline. This process provides an explanation for the observed
sediment transport patterns on modern tropical continental shelves, as well as a basis for the
interpretation of ancient tropical shelf deposits.
guidance for discussions regarding canyon occurrence, distribution, geological and oceanographic significance
and conservation. Based on an analysis of the ETOPO1 data set, this study has compiled the first inventory of
5849 separate large submarine canyons in the world ocean. Active continental margins contain 15% more
canyons (2586, equal to 44.2% of all canyons) than passive margins (2244, equal to 38.4%) and the canyons are
steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. This
study confirms observations of earlier workers that a relationship exists between canyon slope and canyon
spacing (increased canyon slope correlates with closer canyon spacing). The greatest canyon spacing occurs in
the Arctic and the Antarctic whereas canyons are more closely spaced in the Mediterranean than in other
areas.
River-associated, shelf-incising canyons are more numerous on active continental margins (n=119) than on
passive margins (n=34). They are most common on the western margins of South and North America where
they comprise 11.7% and 8.6% of canyons respectively, but are absent from the margins of Australia and
Antarctica. Geographic areas having relatively high rates of sediment export to continental margins, from
either glacial or fluvial sources operating over geologic timescales, have greater numbers of shelf-incising
canyons than geographic areas having relatively low rates of sediment export to continental margins. This
observation is consistent with the origins of some canyons being related to erosive turbidity flows derived
from fluvial and shelf sediment sources.
Other workers have shown that benthic ecosystems in shelf-incising canyons contain greater diversity and
biomass than non-incising canyons, and that ecosystems located above 1500 m water depth are more
vulnerable to destructive fishing practices (bottom trawling) and ocean acidification caused by anthropogenic
climate change. The present study provides the means to assess the relative significance of canyons located in
different geographic regions. On this basis, the importance of conservation for submarine canyon ecosystems
is greater for Australia, islands and northeast Asia than for other regions.
from the continental shelf to the slope and beyond. However, not all canyons perform the same function. The
relative influence of an individual canyon on these processes will, in part, be determined by its form, shape and
position on the continental margin. Here we present an analysis of canyon geomorphic metrics using an updated
national dataset of 713 submarine canyons surrounding mainland Australia. These metrics (attributes) for each
canyon are used to classify them into canyon types across a hierarchy of physical characteristics separately for
shelf-incising (n= 95) and slope-confined (blind; n = 618) canyons. We find that the canyon metrics describe
a wide variety of canyon form and complexity that is consistent with a population of canyons that has evolved
at different rates around the Australian margin since the break-up of Gondwana. The large number of slopeconfined
canyons is interpreted to reflect dominance of slope mass-wasting processes over erosive turbidity
flows from fluvial and shelf sources on an arid continent. The distribution of submarine canyons around the
Australian margin is not regular, with clusters occurring in the east, southeast, west and southwest where the
margin is steepest. The classification result provides a quantitative framework for describing canyon heterogeneity
for application in studies of geological controls on individual canyons, canyon oceanography and canyon
biodiversity.
Canada is the northern equivalent to the San Andreas
Fault Zone, the Pacific–North American plate boundary.
Geomorphologic expression and surface processes associated
with the QCFZ system have been revealed in unprecedented
detail by recent seabed mapping surveys. Convergence of the
Pacific and North American plates along northern British
Columbia is well known, but how the QCFZ accommodates
this convergence is still a subject of controversy. Themultibeam
sonar bathymetry data reveal, for the first time, evidence of a
fault valley with small depressions on the upper slope, offshore
centralHaida Gwaii (Queen Charlotte Islands). The depressions
form where strike-slip right-step offsets have realigned the fault
due to oblique convergence. Core stratigraphy and radiocarbon
dating of sediments within the fault valley and small depressions
suggest that these features are recent in origin. In addition,
the development of the fault valley and dislocation of submarine
canyons control sediment migration from the continental
shelf through to the lower slope. This interpretation of the
geomorphic expression of major plate tectonic processes along
the QCFZ can now be tested with new surveys subsequent to
the October 2012 magnitude 7.7 earthquake.