Lady Elliot Earthcache
Lady Elliot Island is the most southerly reef of the Great Barrier Reef province. The island is located approximate 80 km from the Australian mainland. The island has an area of 0.54 km2 and has an average height of 4.5 m above sea level. The island is a vegetated shingle cay [1].
A cay is a small, low elevation island composed of coral reef detritus of rubble or shingle and/or sand sized materials that have accumulated on the reef top surface. Shingle is relatively homogeneous in composition (mainly Acropora clasts) but varies in size and shape, whereas sand cay sediments are mainly well-sorted medium to coarse sand. Shingle cays may become cemented and relatively stable, but their exposed position inhibits plant colonization [2]. Therefore, vegetated shingle cays are uncommon. These cays typically develop near the windward margins of larger reef flats or centrally on smaller ones exposed to high energy [1].
Formation
Cays may develop on the lee side of reefs at the point of wave convergence [3]. The island growth will not commence until the reef platform is sufficiently established by coral growth and sedimentation to provide a foundation close to sea level [4]. Gradually, layers of deposited sediment build up on the reef surface. A supply of biogenic sediments is fundamental for cay formation, but most reef tops have partial veneers of sand and shingle and there are many with diffuse sediment sheets that remain unconcentrated. The most critical factor for cay formation is the centripetal pattern of sediment movement produced by waves and currents in response to reef shape; centripetal sediment transport delivers sediment to a focal point or depositional node where it may accumulate [1].
Figure 2: Geomorphology of Lady Elliot Reef and Island from Chivas, et al.
A shallow platform reef has existed at Lady Elliot for at least 6500 years and the start of the island growth by shingle progradation probably began at Lady Elliot shortly before 3200 years ago and proceeded to the present at a rather uniform lateral rate. Ocean currents transport loose sediment across the surface of a reef to the convergence where the sediment load is released. Once the island growth was initiated on Lady Elliot, the shingle ridges appear to have accreted concentrically (Figures 2 and 3), with fairly constant ridge heights (Figure 4). Growth to windward and to leeward has occurred at rather similar uniform rates, resulting in the concentric shape of the cay.
Figure 3: Satellite view of Lady Elliot Island. The yellow lines indicate the surveyed sections shown in Figure 4.
Figure 4: Section views from Chivas, et al. The vertical height indicates the elevation above the tidal datum.
Composition
Lady Elliot Island is a near concentric accumulation of prograded single ridges composed of coral, Tridacna shells (a genus of large saltwater clams) and bioclastic sand bound together with a reddish-brown to cream coloured guano-derived phosphatic cement. Phosphatization may have occurred continuously or episodically at any time throughout the last 3200 years. Guano has provided the source of phosphate for cementation of the shingle ridges. The youngest cemented ridge top is less than 770 years, whereas at the base of uncemented ridges, phosphatization is a modern (~1950 A.D.) process. Similarly, low-level beach rock has continued to form within the last 25 years [4].
A significant period of instability occurred less than 500 years ago during which marine erosion truncated the southwestern extremity of Lady Elliot Island, formed an eastern "spit" and stranded high-level beach rock.
Cemented and now-eroding cay rock occurs as pavements or platforms on the eastern and southwestern extremities of the island. The southwestern platform is stepped, with a lower level further offshore which may be a wavecut feature. The eastern tongue of eroding cemented rubble lacks the phosphatic cement of the southwestern cay-rock and the inner shingle ridges.
Near the southwestern extremity of Lady Elliot Island (section C, Figure 4), a Tridacna from the lower cay-rock platform has an age of 1890 years; two from the upper platform have ages of 1470 and 470 years. The step in the platform, which is parallel to the present shoreline, may be a wave-cut feature and/or the line of an original shingle ridge. The cay rock is phosphate cemented shingle debris and is thus compositionally similar to the cemented ridges elsewhere on the island. It is clear that the island was once more elongated towards the southeast and has been truncated by later erosion. On the southwestern tip of the island it is apparent, even from the map (Figure 3), that older easterly trending ridges are truncated perpendicularly by the present; coastline and two modern unconsolidated storm ridges. Storm ridges occur when storm associated high tides and waves build gravel beach ramparts as high as 6 m above sea level over tide-flooded backshore surfaces [5].
The eroding cemented rubble on the eastern extremity of the island contains no phosphate cement. The shingle fragments are bound by algal cement and the mound is presently undergoing erosion. This accumulation is not unlike a spit that has formed from debris transported northwards by the prevailing winds from the southeastern reef flat. Two cemented Tridacna from the rubble have ages of 950 and 610 years, indicating accumulation in part since the last 600 years. This date (610 ± 80) is indistinguishable from the maximum age (470 ± 80) of erosion at the southwestern tip of the island. It is possible that erosion in the southwest was broadly contemporaneous with deposition of debris as a diffuse spit on the eastern point of the island.
The oldest phosphate-cemented Tridacna from the core of the island has an age of 3200 years. It is not possible to determine when phosphatic cementation of the inner ridges occurred. Avian occupation and phosphatization may have been continuous from 3000 years ago until last century. However, a shorter period or periods, for example with a duration(s) of a few hundred years at any time in the past 3000 years could produce the cemented ridges.
Man's Effect on the Island
Mining of guano on Lady Elliot Island between 1863 and 1873 led to almost total removal of the original vegetation and modification of the topography. In the central western part of the island some of the shingle ridges were removed; in the southwestern portion the topography appears to have been enhanced by excavations in the swales [4]. After extraction from the island, the guano was dried, broken down and collected into sheds before being loaded onto barges. A system of tramways, sheds and moorings for the guano barges was constructed. The impact of guano mining on the island was severe; a layer more than 2.5 metres thick was removed from the surface of the island and, more than a century later, Hopley wrote that little of the original vegetation remained. Heatwole, similarly, found that the environment of Lady Elliot Island had been significantly disturbed; he stated that most of the vegetation and surface material had been removed by the industry, and 'old diggings, tramways, washing mounds and wells' were still detectable. Indeed, Heatwole concluded, 'Lady Elliot Island's prime ecological value is as a reminder of how destructive uncontrolled human activities can be to a coral cay, and of how prolonged those effects can be'.
After the operation at Lady Elliot Island ceased in 1873, a break occurred in guano mining. However, a decade of further, intensive guano mining took place from 1890-1900 [6]. The guano at Lady Elliot was stripped rapidly and severe geomorphological and ecological transformations occurred. Hopley argued that, at islands where the geomorphological impacts of guano mining were severe - especially at Raine, Lady Elliot and North West Islands - their recovery may take hundreds of years, if in fact those impacts are not irreversible. Coral mining occurred in at least twelve islands (but not necessarily Lady Elliot Island) between 1900 and 1940 with the result that thousands of tons of coral were removed from some reefs and pulverised to produce agricultural and industrial lime. The lime was used for the sugar cane farms on the adjacent coastal land. Coral was mined from accessible coral reefs and cays and burnt as a cheap and chemically pure source of lime.
In 1928, Bedford reported that the descendents of domesticated fowls were found on islands that had been worked for guano, since chickens were kept by the miners as a source of food. Chivas et al. [4] report that in 1986, there was no large bird population on the island.
From Mining to Ecology
Humans have a major effect on cays, through direct destruction of vegetation and fauna, construction of buildings, mining guano, altering patterns of erosion, planting gardens and coconut groves, and introduction, intentional or otherwise, of weeds. Lady Elliott Island on the Great Barrier Reef is an example of both good and bad effects of humans. It was first nearly denuded by guano miners who destroyed the vegetation and stripped the soil. Later, it was restored to nearly its former condition by replanting native vegetation as part of the development of an ecofriendly resort [7].
References
The following journal articles and book sections were referenced for this earthcache description. All of these references are available online, but you must have a subscription to the journals in which they are published. You do not need to read the articles to be able to log this earthcache. However, if you have access to the journals through a university or library, I highly recommend taking a look at the full articles before you travel to Lady Elliot Island as you will learn a lot more about the environment you will be visiting.
[1] Smithers, S. and Hopley, D., "Coral Cay Classification and Evolution," in Encyclopedia of Modern Coral Reefs : Structure, Form and Process, D. Hopley, Ed. Dordrecht: Springer, 2011, pp. 237-253.
[2] Smithers, S., "Great Barrier Reef Islands, Geology " in Encyclopedia of Islands, R. G. Gillespie and D. A. Clague, Eds. Berkeley: University of California Press, 2009, pp. 386-388.
[3] Parnell, K. E., Physical Process Studies in the Great Barrier Reef Marine Park, Progress in Physical Geography, 1988,12(2), 209-236.
[4] Chivas, A., Chappell, J., Polach, H., Pillans, B., and Flood, P., Radiocarbon Evidence for the Timing and Rate of Island Development, Beach-Rock Formation and Phosphatization at Lady Elliot Island, Queensland, Australia, Marine Geology, 1986,69(3–4), 273-287.
[5] Otvos, E. G., Beach Ridges — Definitions and Significance, Geomorphology, 2000,32(1–2), 83-108.
[6] Daley, B. E. N. and Griggs, P., Mining the Reefs and Cays: Coral, Guano and Rock Phosphate Extraction in the Great Barrier Reef, Australia, 1844-1940, Environment and History, 2006,12(4), 395-433.
[7] Heatwole, H., "Coral Cays, Vegetational Succession," in Encyclopedia of Modern Coral Reefs : Structure, Form and Process, D. Hopley, Ed. Dordrecht: Springer, 2011, pp. 256-261.
To log this EarthCache
To log this earthcache, please answer the following questions. Some of the questions will require that you walk to various points on Lady Elliot Island to make observations.
- Go to the Reef Education Centre in the main building. How many years does step 2 of the formation of the cay take?
- Proceed to S24 06.693 E152 42.735. Describe the beach and in your own words. Specifically, describe the beach formation and what geological processes have caused the formation. One word answers will not be accepted.
- Proceed to S24 06.987 E152 42.848. Describe the beach and in your own words, describe the beach formation and what geological processes have caused the formation. One word answers will not be accepted.
- Proceed to S24 06.954 E152 42.667. Describe what you see in your own words. From the description above, you should have an idea of what you are looking at when you are at these coordinates. Answers of only a few words will not be accepted.
- Proceed to S24 06.991 E152 42.756 . What geological process is occurring here? See the nearby sign #6 for an indication of the answer that is required.
- What is bioclastic sand and did you see any? Please describe what you saw.
- What is the source of the phosphate that caused the cementation?
Please email the answers to me via my profile and please do not post the answers in you log. I will respond to your email to let you know if your answers are correct.