Geology crucial to understand the reef

Introducing the marine geologists

Two of the most influential scientist of the GBR.

Everybody has heard of marine biologists but few know about marine geologists. Rather than scuba diving among the beautiful corals, fish and turtles, as biologists do, geologists drill holes in the seabed and collect mud. Their work is not photogenic. School children do not dream of being marine geologists so they have always been a relatively rare species. They were nevertheless common enough on the GBR until the end of the twentieth century. Their numbers have since declined drastically. At the same time the number of biologists has exploded. If the science of the GBR was an ecosystem, then this one has become unbalanced.

The geologists, in particular, have been largely responsible for demonstrating that mud from farms is not killing the GBR. They look at how the sediment moves, where it comes from, where it ends up, and over what periods of time it takes.  

I first started working with geologists in the Marine Geophysical Laboratory at James Cook University, which I joined in 1989. The geologists included Professor Bob Carter and many other New Zealanders. They mapped and drilled holes in the seabed, and used seismic surveys to map the sediment types and the history of the formation of the coral reefs and the coastline. As a group they revolutionised knowledge and understanding of the reef.

They found that the sea level had fallen over the last five thousand years[1] and that this had killed huge amounts of coral on the GBR. They worked from the river mouths to the very deep water of the Coral Sea (more than a kilometre deep) that is outside the GBR (Figure 3.6). They found huge quantities of mud in this deep water that was caused by erosion of the coast on a biblical scale that occurred when the sea-levels started to rise 18 000 years ago.[2] They worked out how the reefs and coastlines changed as sea-levels fell and rose over the last few hundred thousand years.  They found that the mouths of some of the biggest rivers had moved by tens of kilometres roughly every thousand years.[3] They mapped the ancient channels of these rivers, now covered by the ocean, when the sea level was lower and the GBR was dry land.  They found the remains of old mangrove swamps that existed thousands of years ago when the sea-level was ten metres lower.  They were documenting monumental changes to the GBR system that put contemporary minor fluctuations in perspective. This is the vital, indeed, the indispensable contribution of geologists.

Figure 5.1

Top. Satellite image of a river plume from the Burdekin River during a flood (see Orpin and Ridd, 2012). The brown area at the river mouth is muddy water. The plume moves to the north-west, clings to the coast, and turns green as the sediment settles out and phytoplankton grows on the nutrients released from the water. The GBR reefs are further offshore, well outside the river plume, but the plume is covering a few small fringing reefs on the islands and coast. Instruments measuring sediment concentrations at Horseshoe Bay on this day showed very low concentrations (less than 1 NTU[*]).

Bottom Satellite image 12 October 2006. This image, taken during a strong wind event during the dry season, clearly shows the turbid coastal boundary region which extends beyond Magnetic Island. Measuring instruments at Horseshoe Bay on this day showed high turbidity (over 10 NTU). This is very typical – strong winds cause waves that suspends the mud near the coast. The suspension by waves is far more important than the river plumes.

Geologists have made a great contribution to our understanding on how humans have, or have not, affected the GBR. Professor Bob Henderson showed, by looking at skeletons of ancient starfish that are buried on the reefs, that plagues of starfish are not a new phenomenon. Such work often contradicted the conventional wisdom that the GBR was in peril. Particularly tellingly, they provided valuable counter argument to doom-scenarios proposed by biologists and demonstrated why study of the GBR must be multidisciplinary. Biologists are often completely unaware of the geological history of the GBR. Geologists work on the history of changes evident in the sediment and do not automatically ascribe changes to human influences as biologists seem to do almost automatically. 

As a physicist I watched and worked with the geologists, and learned. Unfortunately, after the early 2000s the number of geologists working on the GBR dropped dramatically. The counterbalance was lost and the contribution from geologists became unwelcome and unfunded. Alas, much of their work is now studiously ignored and almost forgotten.

Two of the most significant geologists who worked on the movement of sediment and its effect on the GBR were Piers Larcombe and Ken Woolfe. Piers came from the Devon, UK, and joined the Marine Geophysical Laboratory in 1989. Ken Woolfe joined early in the 1990s. Originally from Cornwall, like most of our laboratory, he was educated in New Zealand.

By the late 1990s, Larcombe and Woolfe concluded that extra sediment from rivers was almost irrelevant to the GBR.[4] Hitherto a firm believer that the increased discharge of sediment from rivers since European settlement must be affecting the GBR, I was shocked by this proposition. To draw this conclusion Larcombe and Woolfe were using geological information about the sediment together with a little data of sediment concentrations from a fringing reef (Paluma Shoals) collected using instruments that I had helped to develop originally for the dredging studies. They, and other geologists, pointed out that

  • There was almost no mud from rivers found on the reefs of GBR so they cannot have been affected by rivers (Maxwell and Swinchatt,1970[5]; Hopley et al., 2007[6]).
  • For the Mediocre Fringing Reefs there was often a considerable amount of sediment from rivers on and around the reefs but the geological data showed that these reefs had always been muddy. Unlike the GBR reefs which are built on the rubble of ancient dead coral, the Mediocre Fringing Reefs are built on a mixture of ancient dead coral mixed with a lot of mud and silt. Over the years, other geologists and geomorphologists such as Scott Smithers, also from JCU, and Chris Perry from Exeter University in Devon UK, have reinforced this data.[7]
  • Waves resuspending the mud was the dominant mechanism that caused the mud to become stirred up and into suspension.
  • There is so much sediment around these reefs, which has deposited over thousands of years, that the little extra that has come down the rivers in the last hundred years is negligible.

They thus concluded that the Mediocre Fringing reefs, like the GBR, were not significantly affected by mud coming recently from rivers.

[*] NTU is a unit of turbidity or water cloudiness. High sediment concentrations increase turbidity. In conditions with strong waves, turbidity can easily exceed 100 NTU close inshore.

[1] Larcombe, P., Carter, R.M., Dye, J., Gagan, M.K. and Johnson, D.P. (1995). New evidence for episodic post-glacial sea-level rise, central Great Barrier Reef, Australia. Marine Geology, 127(1–4), pp.1–44.

[2] Dunbar, G.B. and Dickens, G.R. (2003). Massive siliciclastic discharge to slopes of the Great Barrier Reef Platform during sea-level transgression: constraints from sediment cores between 15°S and 16°S latitude and possible explanations. Sedimentary Geology, 162(1–2), pp.141–158.

[3] Fielding, C.R., Trueman, J.D. and Alexander, J. (2006). Holocene Depositional History of the Burdekin River Delta of Northeastern Australia: A Model for a Low-Accommodation, Highstand Delta. Journal of Sedimentary Research, 76(3), pp.411–428.

[4] Larcombe, P. and Woolfe, K.J. (1999). Increased sediment supply to the Great Barrier Reef will not increase sediment accumulation at most coral reefs. Coral Reefs, 18(2), pp.163–169.

[5] Maxwell, W.G.H. and Swinchatt, J.P. (1970). Great Barrier Reef: Regional Variation in a Terrigenous-Carbonate Province. Geological Society of America Bulletin, 81(3), pp.691–724.

[6] Hopley, D., Smithers, S.G. and Parnell, K.E. (2007). The geomorphology of the Great Barrier Reef : development, diversity, and change. Cambridge: Cambridge University Press.

[7] Perry, C.T., Smithers, S.G., Gulliver, P. and Browne, N.K. (2012). Evidence of very rapid reef accretion and reef growth under high turbidity and terrigenous sedimentation. Geology, 40(8), pp.719–722.

Orpin, A.R. and Ridd, P.V. (2012). Exposure of inshore corals to suspended sediments due to wave-resuspension and river plumes in the central Great Barrier Reef: A reappraisal. Continental Shelf Research, 47, pp.55–67.

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