The first reef drilling commenced between 1891-1898 on Funafuti Atoll. A Royal Society Expedition used coal mining equipment to drill down to 340 m in an effort to test Darwin’s subsidence theory for the origin of Coral Reefs (Darwin, 1842; Bonney, 1904; Hopley and Davies, submitted for publication). Subsequent reef drilling on Southwest Pacific Atolls, to test atomic weapons during World War Two, lead to significant gains in knowledge of atoll structure and evolution (Revelle, 1654; Schlanger, 1963; Ladd, 1973). The first reef drilling within the Great Barrier Reef (GBR) province took place in 1926 on Michaelmas Cay and subsequently on Heron Island in 1937 down to 183 m and 223 m respectively (Hill, 1973). Indeed reef drilling has enabled the testing of various hypotheses of reef development (Darwin, 1842; Davies, 1973; Purdy, 1974). The first drilling operations used large heavy cumbersome drilling platforms and rigs. The expansion of reef drilling studies came with the invention of smaller light weight drilling rigs which can investigate a wider range of reef sites, are more economical and speed up the drilling process (Macintyre, 1975; 1978; Hopley, 1982; Davies et al., 1977; Thom, 1978; Thom et al., 1978; Shinn et al., 1982; Hopley and Davies, submitted for publication). These modern drilling rigs lead to an explosion of reef drilling investigations world wide and resulted in the drilling of 161 holes into 48 reefs within the GBR alone since 1973. In the course of this project I have compiled a dataset of radiometrically dated coral cores from over 308 drill holes from
more than 133 reefs worldwide to calculate accretion rates and a global mass balance of coral CaCO3 (Figure 2.1; Chapter 5).
Figure 2.1 Cartoon illustrating the estimation of accretion rates from radiometric dating of coral drill cores for Chapter 5. The dotted lines linking surfaces of similar ages are called isochrons.
2.2.1 Reef Drilling at Rodrigues, Lizard Island and MacGillivray Reef
Reef drilling was undertaken to investigate the Holocene evolution of the fringing reefs of Rodrigues and Lizard Island and the platform reef of MacGillivray Reef. The drilling equipment was loaded onto a small aluminum dinghy driven to each drill location on a falling tide. The dinghy was anchored and crew waited until water depth would allow safe and efficient drilling (≤ 0.5 m). Reef cores were obtained using a hydraulic rotary drill with diamond and tungsten drill heads (5 cm diametre for Rodrigues and 3.5 cm diametre for Lizard Island and MacGillivray Reef cores) powered by a petrol generator (Plates 2.1, 2.2 and 2.3).
Plate 2.2 Drilling into a thrown up coral head to access the reef margin below, near the leeward sand cay on MacGillirvay Reef (14˚39.916’S, 145˚29.310’E). Photo taken from support dingy harbouring
The drill string was constructed of 1 and 1.5 m long rods and the whole system was flushed with seawater to aid drilling using a petrol generated water pump. Cored material was recovered by manually removing the entire drill stream from the core hole (Plate 2.3). Unlike many modern reef drill rigs (Braithwaite et al., 2000), the drilling equipment used at Rodrigues, Lizard Island and MacGillivray Reefs does not employ a scaffold and is therefore faster to deploy, operate and less expensive.
ORIGINAL PAPER
Plate 2.3 Reef drilling on Rodrigues showing drill string being manually removed from core hole to retrieve coral core.
Occasionally the drill would drop unhindered through voids in the reef, the approximate depth range of these voids was recorded. In some cases drilling was impeded or had to be abandoned because large volumes of sand made progress extremely slow or impossible. Drill hole Mac 1 was initially drilled during the first drilling field season to MacGillivray Reef in January 2003. This core site was revisited during the second field season in September 2003, during that time it had completely filled up with sand which first had to be pumped out using the water pump and hose (Plate 2.2). Unconsolidated material is not usually retained by reef drilling because water flushes the entire system to aid drill progress. It is therefore necessary to keep a log of drill bit behaviour, which, with experience, can be used to interpret the type of material the drill head encounters. Each piece of drilled core recovered was labeled to record orientation and position
within the section (reef sequence, Plate 2.4). Drilling was abandoned when progress was repeatedly impeded by sand or by some impenetrable surface, for example, granite basement. At both Lizard Island and MacGillivray Reef, drilling was interpreted to have reached granite basement based on drill bit refusal characteristics (Chapter 3).
Plate 2.4 Drill core recovered from MacGillivray Reef leeward margin (Mac1). Cored material is labelled; for example, 1-9, number 1 refers to drill core name (e.g. Mac 1), number 9 refers to number of times the drill string was removed to extract coral core. The numbers and arrows labelling the coral indicate the sequence of coral pieces within each cored section. Increasing numbers indicate increasing depth from the top. Arrows indicate the orientation of the coral as it was retrieved from the core barrel. The orientation and order of the coral section retrieved assists in determining whether or not the coral was recovered in growth position, this is especially important for selecting material for radiometric dating and calculating accretion rates.
2.2.2 Thrown-up Coral Block
At MacGillivray Reef large thrown-up coral blocks were drilled through to reach the underlying reef because the water level here is too high during most of the tidal cycle to allow effective and safe drilling on the leeward margin. Drilling through thrown up coral blocks proved to be a very successful tactic allowing uninterrupted drilling for up to 5 hours at Mac 4 (Plate 2.5; Chapter 3) where a total of 8.5 m of core was recovered. It was important to log the contact between the thrown up coral block and the reef flat surface. This contact was actually quite obvious because (i) a large gap existed between the coral block and reef surface (0.5 m), (ii) encrusting foraminifera and detritus had built up at the contact area and (iii) the orientation of the corallites changed
markedly within the cored material. The recovered cores were cut longitudinally, logged, photographed, thin sections made and petrology carried out (Plate 2.4). Core logging involved recording detailed descriptions of flora and fauna to establish any changes in facies composition as the reef developed.
Plate 2.5. Large thrown up bombie (14˚38.95’S, 145˚29.25’E) on leeward margin of MacGillivray Reef and site of drill core Mac 4 at low tide.
2.2.3 Age Reversals
Occasionally age reversals appear in reef stratigraphies. There are various mechanisms which can lead to age reversals appearing in reef stratigraphy including;
1. The growth configuration of corals is not only vertical, as such a single slice through a coral colony might reveal older growth on top of younger growth. 2. Coral growth or deposition of younger material beneath overhangs.
3. Older fragments of corals being re-worked to a deeper but younger growth horizon for example from reef crest to reef slope.
4. Older coral fragments being re-worked to a younger growth horizon, for example, thrown-up coral blocks onto reef flats during storms and cyclones (Plate 2.5).
But the most common source of these reversals is incorrect interpretation of coral growth position (Kleypas, 1991; Graham, 1993). Meticulous recording of the position of each piece of coral material recovered by coring and examination of material in thin section is used to determine the orientation of cored coral material and aid the appropriate selection of cored material for radiometric dating (Chapters 3 and 4).