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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995
ISBN 0 642 17399 0
How much sediment is supplied to the Australian shelf each year by rivers and wind? The answer to this question is unknown: indeed, estimates of the supply of detrital sediments to the Australian shelf are rare in the scientific literature. The central portion of Australia, covering an area of 3.62 million km2 (or 47% of the continent's land surface) is either desert or contains drainage which flows into inland basins and does not reach the sea. About 40% of the Australian continent is mantled by wind-blown sand and dust storms most probably supply sediments to the shelf. The amount of wind-blown sediment actually reaching the shelf is unknown.
The best known studies of river sediment supplies are those of Belperio and Searle (1988) for the rivers of north-east Queensland draining into the Great Barrier Reef province, and by BMR (1989) and Jansen et al. (1979) for the Murray River. For the rest of Australia, only isolated studies have been carried out in small catchment areas often in relation to dam construction (eg the Ord and Snowy rivers), or in assessments of the effect of forestry and agriculture practices on river sediment loads (summarised in Figure 5). In Australia, river sediment loads are particularly affected by extreme rainfall episodes. The sediment discharged into the sea during one extreme flood event may exceed greatly the amount supplied over several years of 'normal' river conditions. This situation implies that, in order for accurate assessments to be made, river sediment loads must be monitored over long time intervals spanning perhaps tens of years before accurate sediment loads may be determined (Rieger & Olive 1988).
Figure 5: Location of shelf areas where surficial sediments contain greater than or less than 50% calcium carbonate. River catchment areas and estimated total sediment loads supplied to the coast.
The sediment loads supplied to the coastal zone by many rivers are trapped in estuaries; hence, little or no sediment may reach the deeper waters of the continental shelf. In general, coarse sand-sized grains do not escape from any of the estuaries along the southern coast of Australia, with a few minor exceptions (eg the Shoalhaven River in New South Wales: Roy & Thom 1981). For instance, all of the sand supplied by the Murray River is trapped within an estuarine lake (Lake Alexandrina) and only a portion of the fine-grained silts and clays are exported to the continental shelf. River catchments in the seasonally wet northern parts of Australia generally have a higher sediment yield (to the order of 100-300 t/km2/yr) than those in the south (to the order of 10-30 t/km2/yr). Nevertheless, data are unavailable for about 50% of catchments (Figure 5).
The catchments with data on sediment yields (Figure 5) indicate that the total river discharge of sediment to the coastal zone is 106 million t/yr. The amount of sediment discharged by the Ord River (Western Australia) (34.9 million t/yr) is considered to have been affected by overgrazing (Wasson 1987): a pre-European settlement discharge being probably about 10% of this (ie 3.5 million t/yr). Extrapolating the available sediment yield data to the surrounding catchments, an estimate of the pre-European settlement discharge from Australia is in the order of 150 million t/yr (average sediment yield of 38 t/km2/yr). This sediment load is a very small amount by world standards. The sediment load of the continent of Africa is 530 million t/yr in comparison, and that of South America is 1800 million t/yr (Milliman & Meade 1983). On the Australian continental shelf, therefore, fluvial (river-based) supply has in the past provided only relatively small point sources of sediment.
The distribution of sediment types has been mapped for about 70% of Australia's continental shelf and no investigations have been made of the remaining 30% (Figure 5). Details of different sediment distribution patterns are not provided here, but in general, carbonate content in sediments exceeds 50% on the outer shelf, whereas inner shelf sediments commonly contain more than 50% non-calcareous sediments (both detrital and palimpsest) (Figure 5). In southern Australia this non-calcareous inner shelf sediment is generally coarse-grained, relict or palimpsest quartz sand. For example, the large patches of sediment having more than 50% carbonate on the inner parts of the New South Wales, Lacepede and Rottnest shelves are composed mostly of relict or palimpsest quartzose sands. In northern Australia, the inner shelf, non-calcareous sediments are commonly terrigenous silts and clays supplied by rivers. An example of this is in the Great Barrier Reef province (Belperio & Searle 1988).
Outer shelf carbonate-rich sediments are also commonly relict in origin. On the New South Wales Shelf and on the North West Shelf, for example, the carbonate-rich sediment is a mixture of relict and recent biological grains. In contrast, Holocene reefal carbonate buildups in the Great Barrier Reef commonly exceed 10 m and locally are as much as 30 m in thickness. However, the spatial significance of reefs is not great since only about 5% of the Great Barrier Reef shelf is actually covered with reefs (95% of the shelf is characterised by inter-reefal carbonate and terrigenous sediments). The biological organisms producing carbonate-rich sediments vary with latitude, reflecting the ocean's temperature. Hence, whereas hermatypic corals and the calcareous green alga Halimeda characterise tropical shelf sediments north of 24°S, bryozoa dominate the cooler water sediments along the southern margins of Australia (particularly south of latitude 38°S: Marshall & Davies 1978).
Because of the diverse origins of shelf sediment grains it is difficult to quantify with any certainty the rate at which biological sediment is accumulating on the continental shelf. Sediment cores and seismic profiling studies have shown that sediments deposited during the last 6500 years or so (related to the present Holocene sea level; see above) commonly have a thickness of about 1 m. Given the combination of detrital, relict and palimpsest content averaging 50% with the remaining 50% having been biologically produced over the past 6500 years, and a shelf area of 2 million km2, a crude estimate of biogenic supply is approximately 150 million t/yr. In other words, detrital river input and biogenic carbonate input of sediment are roughly equal in terms of gross supply to the continental shelf.
Once they have been introduced into the continental shelf environment, sediments respond to and attain equilibrium with different energy regimes related to the nature of currents characterising a given area. Shelf currents may occur at a number of different spatial and temporal scales: the scale of turbulence (0.2-5 sec); the scale of wave orbital currents (5-20 sec); the scale of tidal currents (6 h); and the scale of storm events (6-10 days). Superimposed upon these 'events' will be other currents, such as wind-driven ocean currents, which may flow at a steady rate for months without changing significantly in speed or direction. Accordingly, shelves may be classed relatively as 'high' energy or 'low' energy based on the degree of physical reworking of sediments by currents. In addition to such current reworking, sediments are also mixed and reworked by the activities of burrowing organisms, a process known as bioturbation (Aigner 1985).
The current regime on any specific section of the continental shelf is the product of a combination of different current types, although one type may dominate locally. A widely used classification scheme of shelf types is that which emphasises the role of different currents (Swift 1972, 1976). Thus, sedimentation processes on a section of continental shelf may be dominated by: (1) swell waves and storm currents (80% of the world's continental shelves); (2) tidal currents (17%); or (3) intruding ocean currents (3%).
Currents produced during storm events - either as tropical cyclones or temperate storms -dominate in the erosion and transportation of sediment over 82% of the Australian shelf surface area (Figure 6). The energy expended and the amount of sediment transported during one storm event may equal many months (or years) of non-storm background processes. Even on highly dynamic tidally influenced shelves, the effect of a storm is to initiate sediment movement at even greater water depths and at greater rates in shallower depths than is experienced under normal conditions. Each year, storm-dominated shelves may experience less than one or as many as four or five storm events that cause sediment transporting flows.
Sediments on the southern parts of Australia's continental shelf are arranged in zones parallel to the coast, reflecting the dominance of currents related to ocean swell and storms (Figure 6). Studies which have documented the development of shelf storm deposits include those of the Rottnest Shelf (Collins 1988), the Lacepede Shelf (James et al. 1992) and the New South Wales Shelf (A Davies 1979). In these locations, long-period swell waves cause nearly continuous reworking of sediments on the inner shelf, winnowing away fine-grained sediments and leaving a sorted sandy sediment in the current affected depth zone. Muddy sediments are deposited below this depth; they are accumulating on the middle New South Wales Shelf in 60-130 m depth, and on the higher energy Lacepede Shelf are deposited in water deeper than 140 m.
Tropical cyclones are the cause of storm events in much of northern Australia (Figure 6). They are associated with atmospheric low pressure systems and attain mean wind speeds of at least 63 km/h. The sections of the Australian shelf most frequently affected by cyclones are the North West Shelf with up to 25 cyclones per decade and the Great Barrier Reef with up to 15 cyclones per decade. These shelf areas are not affected greatly by swell generated in a major ocean basin. The swell pattern is represented in significant wave height data obtained from satellites: whereas significant wave heights are less than 1.5 m for 50-70% of the time in northern Australia, they are larger than 3.5 m for 30-50% of the time along much of southern Australia (see Figure 6).
Tropical cyclones induce strong currents that erode and transport sediment over a wide area. Current measurements obtained in the Gulf of Carpentaria during one tropical cyclone, recorded near-bottom currents with hourly average speeds up to six times larger than background current speeds (Church & Forbes 1983). One cyclone in the Great Barrier Reef province is reported to have caused the erosion on the mid-shelf of a sediment layer averaging 6.9 cm thick and to have transported sediment a minimum distance of 15 km (Gagan, Chivas & Herczeg 1990). Modelling studies by Hearn and Holloway (1990) on the North West Shelf have shown that, under the influence of tropical cyclones, strong westward flowing coastal and inner shelf currents are established between the eye of the cyclone and the coast. Such cyclone-induced currents are clearly a significant factor affecting sediment movement on cyclone-dominated shelves, but they may also influence the long-term (net) sediment movement on some otherwise tidally-dominated sections of the shelf (Figure 6, and see below).
Figure 6: Division of the Australian shelf into regions in which sediment transport is caused mainly by tidal currents (17.4% of the shelf area), currents derived from tropical cyclones (53.8%), ocean swell and storm generated currents (28.2%) and intruding ocean currents (0.6%). Contours of tropical cyclone frequency per decade are from Lourensz (1981)and contours for significant wave height percentage exceedance are fron McMillan (1982). Mean spring tidal ranges indicated along the coastline are derived from the Australian National Tide tables. The location and direction of flow of major ocean currents are indicated.
Tidally dominated shelves occur generally where the mean spring tidal range measured along the coast exceeds 4 m (macrotidal). Around Australia, tidal ranges greater than 4 m occur along the north-western coastline between Port Hedland and Darwin and reach a maximum range of about 9.2 m in King Sound (Figure 6). The southern Great Barrier Reef coastline is also macrotidal, with a tidal range of 8.2 m in Broad Sound. In the Fly River estuary (western Gulf of Papua) tidal ranges are 5 m.
Tidal currents are also an important sand transporting agent in mesotidal (2-4 m tidal range) areas such as Torres Strait, Moreton Bay (Queensland) and Bass Strait. Tidal currents are able to dominate sand transport in microtidal areas (tidal range less than 2 m) in restricted cases where coastal geometry affords shelter from ocean generated swell and wind-driven currents. Such is the case in many bays (eg Shark Bay), the approaches to some major ports (eg Port Phillip Bay) and in partly enclosed gulfs (eg Spencer Gulf, Gulf St. Vincent and the Gulf of Carpentaria). Tidal currents are accelerated as they flow over and between shelf edge barrier reefs; thus sediment transport is affected by tides along the shelf edge over much of the Great Barrier Reef (Figure 6). In total, sediment transport is dominated by tidal currents on about 17.4% of the Australian continental shelf.
Tidally dominated shelves exhibit discrete zones of seabed scouring and erosion. Such are associated with an area where tidal currents reach a maximum speed (Figure 7). Seabed sediments are arranged in a divergent pattern demonstrated by an increasing supply of sand of decreasing grain size as the distance away from the scour zone increases (Johnson et al. 1982). Such diverging bedload transport patterns are known as bedload parting zones. Examples of areas where tidal currents have produced such bedload parting zone facies are found in the Torres Strait and Gulf of Carpentaria regions (Figure 8). In these areas, tidal currents are accelerated as they flow through constricted channels located between islands and reefs (Harris 1988). The zones of maximum tidal current speed are sometimes related to tidal amphidromic points. These are a type of standing wave node in which sea level change is small but current speeds are large; two such amphidromic points are found in the Gulf of Carpentaria (Figure 8). Sedimentary facies are arranged as shown in Figure 7 with respect to tidal current maxima associated with the Torres Strait, Groote Eylandt and Mornington Island areas.
Sediment transport and dispersal controlled by intrusive ocean currents affects only a little more than 1% of the Australian continental shelf (Figure 6). The only location where such currents are known to dominate sediment movement around Australia is the shelf offshore from Fraser Island, where the southward-flowing East Australian Current intrudes. Sidescan sonar and seabed photographs obtained during the 1980 cruise of the German research vessel Sonne to the northern New South Wales - southern Queensland region, show the development of large submarine dunes at depths as great as 80 m, but more typically in the 40-50 m depth range. Dune morphologies indicate a general southward transport of sand, thought to be related to the effect of the East Australian Current. The Leeuwin Current may have a similar effect over parts of the Western Australia shelf, but further research is needed to confirm this.
An understanding of the impact of humans on sedimentation patterns is of considerable interest in the coastal zone, where various harbour and beach engineering projects have altered such things as natural sediment transport patterns and caused coastal erosion etc. (see Bird, this volume). However, few studies in Australia have considered the effect human activities have had on shelf sedimentation patterns. The use of continental shelf sediments as a record of pre-historical environmental conditions may prove to be a tool of fundamental importance to organisations charged with monitoring the effects of human-induced environmental change. Such changes include those related to:
(1) river discharge of sediment (in terms of total load, sediment composition and nutrients) as it is affected by mining, deforestation and agriculture; and
(2) the nature and mass of biogenic carbonate sediment input as it is related to oceanographic factors (eg temperature, turbidity and nutrients).
Figure 7: Idealised diagrams showing: (1) the spatial distribution of sedimentary facies related to an area of accelerated tidal flow (bedload parting zone) located in a constricted channel, with characteristic maximum surface current speeds; and a cross section showing bedforms and relative thicknesses and the content of deposits (after Allen 1970).
Examples of the forecasting use of sediments are shown below in corals, Fly River delta deposits and sediment accumulation over long time periods.
Corals exhibit seasonal density banding that can be observed in X-ray photographs. The banding is apparently related to seasonal variations in coral growth similar to tree rings. The exact cause of the bands is unclear, although seasonal variation in water temperature, light intensity, nutrient availability and/or water turbidity have been suggested as possible factors. The spacing and character of coral bands, including isotopic and trace element signatures, are potentially useful as environmental indicators (Barnes & Lough 1989).