A DXVER-OPERATED SUBMERSIBLEDRILL FOR STUDYING THE GEOLOGICAL HISTORY OF CORAL REEFS

Transcription

1 A DXVER-OPERATED SUBMERSIBLEDRILL FOR STUDYING THE GEOLOGICAL HISTORY OF CORAL REEFS Ian G. Macintyre Department of Paleobiology National Museum of Natural History Smithsonian Institution Washington, DC20560 USA In the early 1970s I developed a hydraulically powered portable submersible drill for collecting cores from coral-reef substrates ranging from the exposed reef flat to the deep fore reef. A marked improvement over earlier electric and pneumatic drilling techniques and destructive blasting, this new coring tool enabled scientists to at last obtain detailed information on the growth histoy of modem reef structures and their postdepositional processes. It consisted of an underwater hydraulic wrench fitted with standard drilling..., pipes and barrels, a hydraulic power source at the surface, and a Pum? to circulate water around the txtting edge of the core bit. For optimum results, the operation fequired a three-man erew. In the past two decades, this drill and similar I- I.--- La à L.11.. models nuvc ucm uxu SUUWJUII~ by numerous coral-reef researchers in the United States, Australia, Japan, and Germany. A classic example of the application of this equipment can be seen in a study of the growth histoy of a fringing reef off Galeta Point, Panama. Thirteen closely spaced core holes across this reef, extending from inshore mangroves to the outer fore-reef slope, produced valuable data on reef community succession, rate of framework construction, and postdepositional processes. INTRODUCTION Most of my twenty-six years at the Srnithsonian Institution have been devoted to studying the recent history of coral-reefs, particularly in the Caribbean region. This recent history (the past 18,000 years) is the interval since the earth's last major glaciation. In this recent period, sea level has risen about 100 m, because of the runoff from me1ting ice sheets. In the 1960s, coral-reef geologists knew little about the internal structures of modem reefs. Much of their information came from topographic features and the surface distribution of sediments and biota. Consequently, the recent history of western Atlantic reefs or the factors that influenced their development were poorly understood (Macintyre, 1988). The predominant view at that time dated back to the 1920s, when geologist W. M. Davis (1926) had come to the conclusion that these reefs were "feeble novice" and "island-bordering" structures. By the late 1950s, it was still thought that these reefs had been "reestablished only recently with the gradual warming of post-pleistocene climate to form thin veneers over older relief (Newell, 1959). In other words, modem reefs were considered to be immature, thin growths having inherited topographic relief. Since then, many drilling projects have brought new information to light about reef history. With their help, "we now know that many reefs of the western Atlantic have an impressive record of Holocene accumulation, in terms of both the amount and duration of deposition" (Macintyre, 1988). THEDRILL The basic concept applied in designing the first submersible drill was to fit an underwater

2 Lang and Baldwin (Eds.): Methods and Techips of UnderwaterResearch hydraulic wrench with standard drilling equipment (Macintyre, 1975; 1978). A ring was inserted into the Ackley 23HS-OC wrench (the Ackley firm is now part of Stanley Hydraulic Tools) to lock out the impact mechanism, so as to prevent the carbide or diamond teeth in the drill bit from shattering and being displaced. A custom-made spindle was used to attach the drill pipe and core barrels to the chuck on the wrench. These were standard Acker NWM double-tube core barrels, 54 romin diameter. It was found that carbide drill bits could cut through the soft carbonate of reef-substrates much more rapidly than diamond bits, which are therefore used mainly to drill through the hard pre-holocene bases en which the reefs are established. The first Ackley power unit used to drive the wrench was a dual-drive Webster hydraulic pump, powered by a Triumph 4-cylinder gasoline motor (Fig. 1). This unit had a manifold attachment that allowed both sides of the unit to be accommodated on one pair of hydraulic lines. With a fluid flow of 16gallmin (73cubic dm/rnin), this attachment drove the wrench with a chuck rpm of 600 at optimum efficiency. The main disadvantage of this power unit, particularly in offshore shallow operations, was its operating weight of 817 kg. I later replaced this unit with a much smaller Hydra-Braker power unit that has an operating weight of only 100 kg (Fig. 2). Although this smaller unit has only half the fluid flow of the original unit, it has proven to be adequate for reefdrilling operations. The operator controls the rotation speed of the drill bit by depressing a large trigger. When the trigger is released, drilling stops instantly. Thus, any sudden increase in torque is not a problem for the operator. Rgure 1. Surface drill-team member operating the large Ackley power unit. With a 0.6 mare barrel attached to the wrench, the drilling unit weighs about 68 kg and can be easily handled by two divers for shallow penetration such as the coring of an individual coral head (Fig. 3). A tripod-winch assembly must be used, however, when working with the larger 15 m core barrel and additional drill pipes (Fig. 4). When drilling deeper into the reef, the diver will experience some torque, particularly when the pipes and core barrel are being pulled out of the hole. This problem is easily overcome by bracing one of the wrench handles against one of the tripod legs with a short length of pipe. After every 1.5 mof penetration, the core barrel is retrieved and the core removed from the inner barrel. This part of the operation can be time-consuming at greater depths, since the drill string may become caught in tight sections of the hole. We found that under ideal conditions we could complete a 12 m hole in about two days. The most important factor in all cases is to have a well-trained drill crew.

3 Macintyre: Diver-operated submersiâ bldrill 2. Small Hydra-Braker power unit easily fits into the side of a yacht. Figure 3. Two divers drillin a lar e Pumna sp. coral colony with 0.6 m corebarref Pearl Islands, Pacific coast of Panama (Macintyre, 1978). 4. Divers coring a relict reef KCroix,U.S. Virgin Islands at a depth of 15 meters. One diver is operating the winch on the tripod and the other is controlling the speed of the wrench (drill).

4 Lang and Baldwin (Eds.): Methods and Techniques of Underwater Research For maximum drilling efficiency, the crew should consist of three people: one to operate the wrench and control the speed of the drill and the stability of the drilling; one to operate the winch on the tripod, and also assist with the stability of drilling and recovery of the drill stem; and, one to operate the power unit and the water pump en the surface. A particular advantage of the Ackley wrench is that it emits only a medium-high frequency hum, which is not bothersome to the underwater operator. The drillers need to keep a detailed record of the progress of the drilling operation, noting the lengths of core barrel and drill pipe in the hole, recording intervals of smooth coring and rapid drops, and finally, before core recovery, the length of pipe above the substrate surface. This information is important for locating voids in the reef structure and the position of cores for radiocarbon dating. It should be noted that core recovery will always be less than 100 percent, except in areas where the pavement is well lithified. Recovery from the open framework of the branching coral Acropora palmata, for example, is usually around 30 percent. Intervals of sand and fine branching corals such as Acropma -cornis are only recovered in traces and can be detected by a rapid drop of the drill stem. Other researchers have improved upon my drilling techniques in recent years. A wire-line system now in use allows the drillers to recover cores without pulling the drill stem after each core run. The retractable inner tube containing the core is recovered on a wire-line cable (E.A. Shinn and D.K. Hubbard, pers. comm 1996). This device saves time, particularly in the drilling of deeper holes. Smaller, more efficient, and cheaper hydraulic motors are also being used to operate the drilling equipment. In addition, Hubbard is developing a system that can operate unattended. It could be very useful in depths of more than 15 m A SAMPLE STUDY Thediver-operateid submersible drill made it possible to conduct one of the most detailed studies of?nob.a. no-cimnnal...by.,"..-.,..". a ". ". * n~ctnrv J of a Caribbean reef ever undertake] I: This was a fringing reef established off a promontory of the Panan Mmian coast. Thirteisn holes were idrilled across this reef, extending from the inshore mangroves to thi e fore-reef slope at depths of 11 rn (Macintyre and Glynn, 1976). The cores retrieved during this op ex,ation indicated thi it Galeta Reef i.sat least 14 m thick and is established on L11c clvalu1ml auilalc ~f LI1c ILUUULC LYU~C~W GfltUTI FOlrrnation, which consists of a calcareous argillaceous siltstone (Macintyre and Glynn, 1976)... I & '.".":.I "...c.."... 4 L -:<l,a1.. njt:'. Six distinct reef fades were identified in the cores (Fig. 5). The unit at the base and outer edge of the framework is the "Fore-Reef Coral-Heads Facies" ai~d consists dominantly of massive corals, including Dichocoenia stokesi, Siderastrea siderea, Step}, lanocoenia michelinii, Diploria strigosa, Porites astreoides, and Montastraea annularis. Many of thes e corals are extensively bored and infilled with a microcrystalline magnesium calcite submarine cemer it (Fig. 6) (Macintyre, 1977). The dominant facies of the Galeta Reef is the "Reef Acropora palmafa Facies," which forms most of the structural framework. This unit is up to 11m thick and consists of fresh and well-preserved A. palmafa. The corallite growth patterns in most of the corals indicate that they have not been overturned and are probably in growth position. These corals generally have light coatings of encrusting biota and microcrystalline magnesium calcite submarine cement (Fig. 7) (Macintyre, 1977). For the most part, the overlying "Reef-FIa' t Rubble Facies" consists of water-worn and extensively bored and encrusted coral rubble composed of c variety of corals, including Diploria clivosa, Agaricia agaricit es, Millepora complanata, Siderastrea siderea, Acropora palmata, Porites astreoides, Porites furcata, and Diploria strigosa. Submarine ceme nt was poorly-developed in these corals, which occur in a Halimeda-rich sand matrix. By contrast, the shallow-water "Fore-Reef Pavement Facies" is extensively cemented and consists mainly of a shallow-water coral-reef assemblage of Millepora complanata, Agaricia agaricit es, Porites astreoides, and crustose coralline algae. This well-indurated pavement limestone (Macintyre

5 Macintyre: Diver-operated submersible drill 1) has lost most of its original skeletal framework because of boring, filling, and - ) (Macintyre, 1977). oles 3 7 T 1 1 Back-reef sediments Reef-flat rubble [Yl Reef Acropora palmata Gath formation 30! 7^1 Fore-reef coral heads Fore-reef talus Peat /' I I I I Distance in meters Figure 5. Cross sections across Galeta Reef, Panama, showing the locations of thirteen core holes and the distribution of subsurface reef fades (after Macintyre and Glynn, 1976). I I Figure 6. Core from "Fore-Reef Coral-Heads Facies." This core consists of Di loria strigow bored by bivalves and sponges and extensively infiiled and encrusted with microcryst&ne magnesium calcite submarine cement (Macintyre, 1983). There are two non-framework reef facies in Galeta Reef. One is the "Fore-Reef Talus Facies," which forms a sediment apron at the front of this reef consisting mostly of gravel-size skeletal debris in a sand and mud matrix. The other is the "Back-Reef Sediments Facies," which consists of rubble, including Acropora cervicornis, in a matrix of sand and mud. This sediment facies grades into a calcareous mangrove peat in the upper 1 m of the innermost corehole (No. 9).

6 Lang and Baldwin (Eds.): Methods and Techniques of Underwater Research Figure 7. Fades." Core from "Reef Acropora palma fa Microcrystalline magnesium calcite,=,,h,,,~4"~ --b U-LJU-IaL Luai.3 --4". a l:-l.& ii~iii^-un~reu crust ---- L of crustose coralline algae at the top of the coral and netrates the un rotected base (Macintyre and gym,1976). ~caf e is OS cm. Figure 8. A densely cemented core from the Tore-Reef Pavement Fades." Only remnants of the ori "nal framework of Agaricia uguricites, Millepora comphta, and crustose coralline a1 ae can be seen in this extensively bored, infilled, and lithified section of the reef (Macintyre and Glynn, 1976). 172

7 Macintyre: Diver-operated submersible drill By relating the distribution of these reef fades and the position of eighteen radiocarbon dates to a minimum sea-level curve for the tropical western Atlantic (Lighty et al., 1982) it was possible to postulate the various stages of growth of this fringing reef (Fig. 9) (Macintyre and Glynn, 1976). It was CORE g HOLES*! Figure 9. Stagesof growth ofgaleta Reefover the past 7,000 years based on the distribution of reef fades and the position of ei hteen radiocarbon dates and their relationshi to a minimum sea-level curve (Lighty et al., 1982)Er the tropical western Atlantic (Macintyre, 1983).

8 Lang and Baldwin (Eds.): Methods a d Techniques of Underwater Research concluded that Galeta Reef established on the erosional surface of the middle Miocene Gatun Formation about 7,000 years ago. Throughout most of its development, it was a typical Caribbean Acropore palmate-dominated fringing reef, with a mixed coral-head community on its outer slope, similar to reefs described off Jamaica (Goreau, 1959) and off Florida (Shinn, 1963). As the reef kept pace with the rising sea level, it formed a thick structure that masked the relief of the erosional surface on which it was established and began to construct its own topographic relief. When the rise in sea level started to subside, about 3,000 to 4,000 years ago, vertical development of the reef became restricted. At the same time, a loose sediment apron formed in the fore reef, and it restricted lateral reef growth. With time, the Galeta structure became an emergent reef with a well-cemented shallow fore-reef pavement and an extensive coral-bolder reef flat. The encroachment of mangroves over the back-reef sediments, absence of present-day active framework construction, and thick talus deposits on the fore-reef slope all indicate that this fringing reef has passed the climax of its development. The cores from this region have also revealed that extensive submarine cementation occurs in modem reefs under conditions of high agitation, slow accumulation, and little or no sediment accumulation (Macintyre, 1977). Thus, the shallow fore-reef pavement is slowly forming under conditions of high wave energy and is extensively lithified. By contrast, the Acropora palmata framework formed under heavy wave conditions but grew so rapidly that submarine lithification was limited to thin coatings. CONCLUSION A series of core holes can be drilled along transects across modem reefs relatively inexpensively by a three-man dive team operating a submersible hydraulic drill. The resulting cores can provide valuable information on the depositional history of a reef and on the postdepositional processes that have occurred within the reef framework. ACKNOWLEDGMENTS Special thanks to William T. Boykins for his help with photographs and Mary E. Parrish for her assistance with graphics. Contribution No. 503 of the Smithsonian's Caribbean Coral Reef Ecosystems Program LITERATURECITED Davis, W.M The Lesser Anti1les:American Geographical Society Publication 2,. 207 p. Goreau, T.F The ecology of Jamaican coral reefs 1: Species composition and zonation. Ecology 40: Lighty, R.B., I.G. Macintyre and R. Stuckenrath Acropora palmata reef framework: a reliable indicator of sea level in the western Atlantic for the past 10,000 years. Coral Reefs 1: Macintyre, I.G A diver-operated hydraulic drill for coring submerged substrates. Atoll Research Bull. 185: Macintyre, I.G A hand-operated submersible drill for coring reef substrata. In: Stoddart, D.R. and R.E. Johannes (eds.). Coral Reefs: Research Methods. UNESCO Monographs on Oceanographic Methodology 5: Macintyre, I.G Distribution of submarine cements in a modem Caribbean fringing reef, Galeta Point, Panama. J. Sed. Petrol. 47: Macintyre, I.G Modem coral reefs of the western Atlantic: a geological perspective. Am. Assoc. Petrol. Geol. Bull. 72: Macintyre, I.G. and P.W. Glynn Evolution of modem Caribbean fringing reef, Galeta Point, Panama. Am. Assoc. Petrol. Geol. Bull Macintp, I.G. and J.F. Marshall Submarine lithification in coral reefs: some facts and misconceptions. Proceedings 6th International Coral Reef Symposium (Australia) 1: Newell, N.D The coral reefs. Nat. Hist. 68: Shinn, E.A Spur and groove formation on the Florida reef tract. J. Sed. Petrol. 33: