May 1999 Newsletter

STUDY FINDS MID-SHELF AND OUTER REEFS NOT AFFECTED BY NORTH-MOVING SEDIMENT

Fears that sediment from rivers is harming corals on middle and outer reefs may be unwarranted according to geologists at James Cook University.

New results from an extensive coastal sediment sampling survey show that northward and shore-directed sediment transport occurs along the inner continental shelf and this protects most mid to outer reefs from potentially harmful sediment. Earlier work has shown that sediment remains close to shore, however the new results show net long-term northward moving sediment occurs along the coastline under the influence of the south-east trade winds. This northward sediment movement is interrupted primarily by prominent headlands and northward facing bays, giving rise to six off-shore sedimentological zones.

Research by marine geologist Alexis Lambeck at James Cook University, who is completing an MSc on aspects of sediment distribution in the Great Barrier Reef lagoon with support from the CRC Reef Research Centre Ltd, has demonstrated for the first time that northward directed transport dominates the continental shelf.

South-east trade winds prevail in the region from April to October but also occur at other times during the year. These winds seldom exceed 30 knots and average 15-20 knots. Earlier research conducted by the marine sedimentology group at James Cook University suggests that wave and wind-driven currents from these SE trade winds form a major long-term regional influence on off-shore sedimentation patterns. Once terrigenous material is suspended in sea water, tides together with wind and wave-driven currents are dominant in sediment transport.

The study samples along 1500 kilometres of north Queensland coast between Cape York and Bowen were collected using a van Veen grab sampler from 10 metres water depth at five nautical mile intervals. The 10 metre isobath was chosen because this depth approximates the middle of the coastal sediment wedge in the central GBR. This is a seaward tapering prism of land-derived sediment that extends from the coast out to a water depth of about 10-20 m (depending on location).

Sediment samples were filtered through a 125 micron sieve and petrographic thin sections were prepared using the coarser fraction. The coarse fraction was placed in an ice cube tray and impregnated with epoxy resin and very thinly sliced (each slide 30 microns thick) using standard rock techniques. The composition of each sample was determined by identifying (using a petrographic microscope) and counting three hundred grains per slide.

Figure 1: Coastal sediment composition zones click to enlarge

Six large-scale northward directed cycles of compositional change were identified (see figure 1, the boundaries of these zones corresponding with major headlands or bays). The mineral abundance, particularly quartz, exhibit a number of depletion cycles, each covering about 50 nautical miles along the coast, in which there is a rapid or sudden enrichment from a point source followed by a systematic northwards depletion of the consistent grains (figure 1). The six different compositional zones extend from (1) Point Abbot to Halifax Bay,

The zones represent pockets of sediment moving northward along the coast from source regions. Little or no mixing appears to occur between zones. The zone boundaries commonly correspond with areas of little or no sediment and inshore reefs. In essence, the zones define a series of seaward and northward tapering sediment bodies separated by zones of sediment starvation.

In some of the more sheltered bays, for example Cleveland Bay and Princess Charlotte Bay, compositional variations reflect limited local transport and mixing over distances less than 8 km. For this reason these areas commonly preserve storm ridges, cheniers and products of major cyclones.

Alexis is currently writing the data up into a couple of papers and a thesis. He will also present some of his findings at a James Cook University sediment research workshop in June.

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