Terence P. Scoffin

The Trapping and Binding of Subtidal Carbonate Sediments by Marine Vegetation in Bimini Lagoon, Bahamas

ABSTRACT In the shallow water lagoon of Bimini, Bahamas, the following plants are sufficiently abundant to influence sedimentation locally.--mangroves (Rhizophora mangle), marine grass (Thalassia testudinum), macroscopic green algae (Penicillus, Batophora, Halimeda, Rhipocephalus and Udotea) and microscopic red, green and blue-green algae forming surface mats of intertwining filaments (Laurencia, Enteromorpha, Lyngbya and (?)Schizothrix). Plants were observed under conditions of natural tidal currents and artificial unidirectional currents produced in an underwater flume and measurements were made of the abilities of the plants to trap and bind the carbonate sediment. The density of plant growth is crucial in the redu tion of current strength at the sediment-water interface. The most effective baffles are Rhizophora roots exposed above the sediment, dense Thalassia blades and Thalassia blades with dense epiphytic algae, Laurencia intricata and Polysiphonia havanensis. All three types can reduce the velocity of water from a speed sufficiently high to transport loose sand grains along the bottom in clear areas (30 cm/sec) to zero at the sediment-water interface in the vegetated areas. The strongest binders of sediment are the roots of Rhizophora and Thalassia. These two hardy plants trap and bind sediment for a sufficient time to produce an accumulation higher than in nearby areas without dense mangroves or grass. Macroscopic green algae growth is not suf iciently dense and the holdfasts too weak to appreciably affect the accumulation of sediment although they provide a degree of stabilization to the substrate. Algal mats trap sediment chiefly by adhesion of grains to the sticky filaments. Their ability to resist erosion by unidirectional currents varies considerably depending on mat type, smoothness of surface and continuity of the cover. The intact areas of dense Enteromorpha mat can withstand currents five times stronger than those that erode loose unbound sand grains. Premature erosion of mats by currents occurs at breaks in the mat surface caused by the burrowing or browsing action of animals. Algal mats were found to be ephemeral features and consequently do not build up thick accumulations of sediment as do dense grass and m ngroves. The thickest accumulations of sediment in the lagoon correlate with deepest bedrock surfaces. The distribution of many plants in the lagoon is directly or indirectly controlled by the depth to bedrock; for example, mangroves on bedrock highs, marine grass in sediment-filled depressions.

The geological effects of hurricanes on coral reefs and the interpretation of storm deposits

Hurricanes occur in belts 7° to 25° north and south of the equator. Reefs growing in these belts suffer periodic damage from hurricane-generated waves and storm surge. Corals down to 20m depth may be broken and removed, branching colonies being much more susceptible to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to link with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and grooves, on the intertidal reef flat as storm ridges of shingle or boulders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-reef lagoons, as drapes of carbonate sand and mud in deep off-reef locations in the fore-reef and lagoonal areas. In addition to the coarse debris deposited, other features may aid the recognition of former hurricane events, including the assemblage of reef biota, its species composition and the structure of the skeletons; graded internal sediments in framework cavities; characteristic sequences of encrusting organisms; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000–4000 y BP in certain parts of the world most likely relate to a slight fall in relative sea level rather than an increase in storminess during that period. A higher frequency of storms need not result in more reef flat storm deposits. The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previous major storm influences the amount of coral debris created; the length of time after the hurricane, and before a subsequent storm influences the degree of stabilization of reef-top storm deposits and hence their chances of preservation.

Patterns and possible environmental controls of skeletogenesis of Porites lutea, South Thailand

Eleven fringing reef sites were investigated over a distance of about 50 km in the Phuket Region. There is a wide range in exposure to wave energy, and also water turbidity across the area. Annual increments of growth of shallow-water reef-front colonies of Porites lutea were calculated for the period November 1984 November 1986 using seasonal fluorescent banding (revealed with ultraviolet light) and Alizarin staining. Measurement of linear extension rate, skeletal bulk density, calcification rate, polyp numbers per unit area and colony surface morphology were made and compared. Linear extension rate and skeletal bulk density are inversely related within and between reef sites. Linear extension rate decreases and bulk density increases along a gradient of increasing hydraulic energy of the setting. Calcification (the product of linear extension rate and bulk density), although varying slightly from site to site, does not appear to relate to any obvious environmental inshore-offshore gradient. Skeletal bulk density is the most sensitive discriminator between reef sites, and we suggest that hydraulic energy of the setting is the main control on these spatial variations in skeletogenesis.

The Composition, Structure and Erodability of Subtidal Mats, Abaco, Bahamas

ABSTRACT The composition and microstructure of widespread subtidal biological mats binding sandy carbonate sediments in the Rock Harbour Cays vicinity of Little Bahama Bank were examined in detail; these mats were subjected to in situ flume experiments. The mats consist of various assemblages of green algae, red algae, blue-green algae, diatoms and animal-built sand grain tubes. Green algae, red algae and/or sand grain tubes provide a rigid open network into which grains infiltrate and are trapped. The mucilaginous secretions of both blue-green algae and diatoms in association with the fine filaments of blue-green algae bind the grains to each other and to the mat network. On the basis of composition and microstructure, three basic mat types were recognized: a fibrous, rigid, Cladoph ropsis mat; a thin, gelatinous, Lyngbya mat; a cohesive, aggregated, Schizothrix mat. The erosion by artificial currents of initially undisturbed mats was studied in the field using an underwater flume, and the complex manner in which the mats disintegrated was recorded. The surface sediment from each mat area was then treated with NaOCl to remove the organic matter and erosion tests repeated in a tank in the laboratory with the same apparatus. The natural, in situ, mat-bound sediment could withstand current velocities at least twice as high and, in some cases, as much as five times as high as those that eroded the treated, unbound sediment. The intact mat surface could withstand direct current velocities three to nine times as high as the maximum tidal currents 13 cm/sec) recorded in the mat environment. Each mat type eroded in a characteristic manner and sequence dependent upon the mat composition and microstructure. This breakdown process differed markedly from the erosion behavior of loose sediment. Observations indicate that grain size, sorting, packing, structure and sediment surface morphology are influenced by mat formation. This study demonstrates the need for close consideration of interfacial biological communities when examining depositional and erosional processes at the sediment-water interface, or when making interpretations from the products of these processes in ancient rocks.

Taphonomy of coral reefs: a review

This review summarises the major factors that affect the post-mortem history of skeletons in a coral reef environment. Skeletal material is traced from life, through death, breakdown, transport, burial and diagenesis to its final fossil form. The fact that most reef sediments are of skeletal composition poses problems of concentration or dilution of individual grain types in taphonomic analysis of reefs. Rates of supply of grains vary, not only with organism abundance and skeletal growth rates, but also with rates of physical and biological breakdown to transportable sediment. Physical and organic processes affect sedimentary structures and textures by mixing or segregating skeletal grains, though biogenic processes normally dominate in the protected setting of reef lagoons. The soft and hard substrates associated with reefs present different media for calcium carbonate accumulation and post-depositional disturbance, for example, loose sediments suffer bioturbation and rocks surfaces suffer bioerosion. The wide range of durability of skeletons and their susceptibility to diagenesis contribute further to the complexities of the preservation of coral reefs.

The Effects of Callianassa Bioturbation on the Preservation of Carbonate Grains in Davies Reef Lagoon, Great Barrier Reef, Australia

ABSTRACT Coring, air-life excavation, and tracer-sediment experiments in the lagoon-floor sediments of Davies Reef revealed that callianassid shrimp had produced, during their feeding, a 5-60-cm-thick surface layer of moderately to poorly sorted gravel-free sediment, above very poorly sorted gravelrich sediment. The subsurface gravel is epilithic in origin close to reef framework but consists predominantly of infaunal molluscs through most of the lagoon. These infaunal skeletons tend to be buried rapidly in a well-preserved state escaping the microboring that attacks all grains on the surface. Two geologically significant points emerge; first, the surface layer will not get preserved in its present form and any analyses of surface samples can give a false impression of the accumulating sedimen pile and the prevailing hydraulic regime, and second, the combination of continuous fine-sediment recycling and surface microboring can lead to a bias in the fossil-reefal limestone record, with epilithic organisms seemingly subordinate to infaunal molluscs.

SEDIMENTARY ENVIRONMENTS OF THE CENTRAL REGION OF THE GREAT BARRIER-REEF OF AUSTRALIA

The sediments and calcareous organisms on the outer reefal shelf of the Central Region of the Great Barrier Reef were collected and observed by SCUBA diving and research vessel techniques (including underwater television) to understand the production and processes of deposition of the sediment. The carbonate grains are mainly sand and gravel size and solely of skeletal origin. Over the whole area the major CaCO3 producers, in order of decreasing importance are: benthic foraminiferans (chiefly Operculina, Amphistegina, Marginopora, Alveolinella and Cycloclypeus), the calcareous green alga Halimeda, molluscs and corals. Coral abundance is high only close to reefs and submerged rocky substrates. Benthic foraminiferal sands dominate the inter-reef areas i.e. the bulk of the shelf, and Halimeda gravels form an outer shelf band between 60 and 100 m depths. Seven distinct facies are recognised after quantitative analyses of the sediments. These are: A. Shelf edge slope (>120 m depth); B. Shelf edge (with rocky outcrops); C. Outer shelf with high Halimeda (>40%); D. Inter-reef I; E. Inter-reef II ( 100 m depth but >2% pelagics); F. Lee-ward reef talus wedge (<2 km from sea level reefs); G. Lagoonal.

RHODOLITHS AND CORALLITHS OF MURI LAGOON, RAROTONGA, COOK-ISLANDS

Free-living massive and branching spheroidal growths (about 5 cm diameter) of calcareous red algae (rhodoliths) and corals (coralliths) occur in abundance on the sea bed of shallow Muri Lagoon on Rarotongas reef flat. The rhodoliths are composed of one or more species of Neogoniolithon, Lithophyllum, Tenarea, and Porolithon; the coralliths are Pavona varians (Verrill) and Porites lutea (Milne-Edwards and Haime). Muri Lagoon is the only area on Rarotongas reef flat that is sheltered by reef islands from ocean waves. The tidal currents, which are predominantly unidirectional in Muri Lagoon, are concentrated by the reef islands into channels through which sand and gravel sediment is regularly transported. However, these prevailing currents do not normally roll the rhodoliths and coralliths. The results of field experiments on the pick-up velocity of the various types of spheroidal structure, combined with observations on growth histories of massive coralliths as revealed by the non-concentric nature of skeletal density banding, indicate that the rhodoliths and coralliths may remain static for periods up to several months yet maintain a complete envelope of living tissue. This downward survival may depend on the strong currents. Not only is the water flushing through the upper millimetre or so of the sediment substrate, but it is also capable of moving the sand and gravel grains which laterally support the rhodoliths and coralliths so that no one point of a spheroidal structure is in direct contact with the substrate for a fatal length of time. Massive rhodoliths have a high preservation potential as discrete spheroidal structures; in contrast, branching rhodoliths and coralliths are prone to fragmentation, and massive coralliths grow into stable microatolls. We conclude that a similar assemblage of rhodoliths, coralliths and microatolls in the fossil record may be indicative of the former existence of contemporary reef flat islands.

Fluorescent and skeletal density banding in Porites lutea from Papua New Guinea and Indonesia

Shallow water Porites lutea corals were collected along two transects normal to mainland shorelines, parallel to gradients in water quality: one, 7 km long, near Motupore Island in South Papua New Guinea, the other, 70 km long, from Jakarta Bay along the Pulau Seribu chain in the Java Sea. The corals were slabbed and studies were made of skeletal density bands as revealed by X-ray photography and fluorescent bands as revealed by ultraviolet light. Water quality measurements and rain-fall data were assembled for the two areas and related to skeletal banding patterns. For both areas, with increasing distance form mainland there is a decrease in overall brightness of fluorescence in corals and an increase in the contrast between bright and dull fluorescent bands. Fluorescence is bright, but seasonal banding is obscure in corals within about 2 km of stream mouths at Motopure and about 5 km of the coast in Jakarta Bay; this suggests that, despite low freshwater run-off during dry seasons, there are sufficient organic compounds which cause fluorescence in coral skeletons, to swamp seasonal effects. During the wet seasons, deluges of freshwater consequent on mainland rainfall of greater than about 150 mm/ month extend at least 7 km offshore in the Motupore area and perhaps tens of kilometres into Java Sea, giving distinctive bright and dull fluorescent banding in off-shore corals. The fluorescent banding pattern within corals on the Motupore reefs is similar in most corals along the transect and it correlates well with the Port Moresby monthly rainfall data. This relationship suggests that the same body (or bodies) of freshwater affect all reefs of the area during the wet season. The fluorescent banding in Java Sea corals does not show a precise correlation with either mainland or island monthly rainfall data; indeed the pattern of fluorescent banding on Pulau Seribu can only be matched in corals from reefs less than about 25 km apart. This suggests that in this area discrete water bodies carrying the relevant organic acids for coral fluorescence affect the fringing reefs on the chain of islands. Comparisons of fluorescent and density banding have revealed that for these areas, in general, periods of high freshwater run-off are times of deposition of less dense skeleton in Porites lutea corals.

The environments of production and deposition of calcareous sediments on the shelf west of Scotland

The dominant calcareous organisms and sediment characteristics are described for eight different physical settings on the shelf west of Scotland, each having a different depth, substrate and degree of hydrodynamic exposure. The principal sites of carbonate production are on shallow rocky substrates where barnacles, molluscs, echinoderms and serpulids are the dominant calcareous organisms. In sheltered shallow sandy zones, molluscs, echinoderms and benthic foraminiferans are the active producers, though the sediments are commonly barnacle-rich. Where tidal currents are enhanced between islands and the waves are suppressed, calcareous red algae (Phymatolithon calcareum) and mussel shells build localised banks. In deep, open-shelf water molluscs are the major skeletal contributor to the sediment, though on rocky sea beds bryozoans, serpulids and echinoderms are important. The major sites of deposition are where persistent hydro- (and aero-) dynamic conditions sweep together grains from active production sites (e.g., sand ribbons or beaches and dunes adjacent to shallow rocky platforms) or in sinks where the physiographic configuration favours the deposition and retention of locally produced sediment or sediment derived from suspension. The well-sorted, cross-bedded, beach and dune sands commonly contain > 75% CaCO3. In sheltered depressions, bioturbated muds accumulate with up to 30% calcite silt, which is probably the breakdown product of barnacles and benthic foraminiferans.