Different types of sediment

Different sediment types.

Green Island and surrounding reef. Where is the mud?

There are many different types of sediment on the seabed, on the coral reefs, the coastal ocean, and washing down rivers. The composition and grain size of the sediment is very revealing about what is happening. As far as the GBR is concerned, it is usually only mud that could cause a problem because only particles of mud are small enough to stay suspended in the water for long enough that that they could settle on corals. Moreover, mud can absorb light so a coral living in muddy water may not get sunlight for days or even weeks.

Sand particles are, by definition, much bigger and do not stay in suspension for very long so are not usually a problem except in cyclones when massive underwater sand dunes can move around totally destroying any organisms living on the seabed.[i]

Examination of the composition of the sediment on the GBR readily reveals that rivers have almost no effect. How? The sediment on the GBR reefs is almost entirely sands made from dead coral and other organisms – they are made of calcium carbonate. This is entirely different in chemical composition from the sand, mud or silt that comes from the land and rivers.

Figure: If you took a handful of sediment from this reef (the white sand that has a blue tinge due to the reflection from the blue sky), it would be almost 100% calcium carbonate. There would be almost no quartz, feldspars and other common “terrigenous” grains. The sand is made from broken down coral and other organisms which have a calcium carbonate skeleton. Geologists test how much CaCO3 is in the sand by dissolving it in hydrochloric acid. Anything that is not dissolved is the terrigenous fraction – and it is almost zero. This is proof that the rivers are not impacting the reefs of the GBR.

Simply, the GBR, a long way from the coast, contains almost no sediment that comes from the land.[1]  It may be asked, if there is no sediment from the land on the reef, why are we worried that it is being smothered or affected in any way. To date I have not seen any explanation by scientists arguing that the GBR is being damaged by sediment.

The question of the effect of sediment from rivers still remains for the Mediocre Fringing Reefs (MFR). It may be only one percent of the coral but it is the focus of most of the concern and will be considered in the rest of this chapter.

The composition of the sediment changes completely from the coast where the MFR is located, across the lagoon, to the GBR which is between 30 and 100 km offshore. Close to the coast, especially in northward facing bays that are sheltered from the wind and waves, the sediment is composed almost entirely of brown “terrigenous” mud that has come from the rivers over thousands of years.

On the offshore reefs, the sediment is almost entirely white sand and rubble derived from the dead calcium carbonate skeletons of the coral reef. Some of this coral sand is eroded from the reefs during storms and deposits in the region between the reefs or falls down the continental shelf slope into the deep water of the Coral Sea. On the MFR, the sediment is composed of a mixture of mud transported by waves and currents from the river mouths, and coral sand produced by the coral and other organisms on the fringing reefs.

[1] Note: there is certainly terrigenous sediment (from the land) around the reefs and there are sediment flows past the reefs into the deep water. We know very little about this transport, but it is possible it occurs mainly in cyclonic events.

[i] Larcombe, P. and Ridd, P. (2015). The Sedimentary Geoscience of the Great Barrier Reef Shelf – context for Management of Dredge Material. Brisbane: Queensland Port Association.

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