Inferences on sediment production and transport at ...

Inferences on sediment production and transport at carbonate beaches using larger foraminifera. Johann Hohenegger 1

Abstract In the tropical Central and West Pacific as well as in the East Indian Ocean the symbiont-bearing benthic foraminifera (single celled marine organisms getting size of a few millimeters) are the main producers of calcium-carbonate grains deposited at carbonate beaches. The proportion of larger foraminifera tests on sand grains ranges from 20 to 95 %. This amount depends on the two factors productivity and transport, since the larger foraminifers do not live on or in the sandy bottom near the beaches, but prefer firm substrate in high-energy environments close to the front of coral reefs. Therefore, foraminiferal tests are produced in extreme numbers on the reef crest that is covered by filamentous macroalgae or in the transition zone between the crest and the reef moat. After the release of the test by reproduction or death, empty tests are entrained through the high water energy acting at the reef crest and transported. This transport depends on the direction and intensity of currents connecting the production area (reef crest and transition to the moat) with the deposition area (central moat, lagoon, beach). Devastation of the production area and/or the interruption or diversion of water flow hinders the accumulation of larger foraminifera tests at the beach. Understanding the ecology of larger foraminifera in combination with the transport of empty tests is thus necessary to preserve the equilibrium between deposition and removal of sand at carbonate beaches.

Introduction Foraminifera are single celled organisms living in marine environments except a very few species that inhabit fresh-water. Most of these organisms protect their cells by complex, multi-chambered tests. One group of foraminifera constructs Institut fur Palaontologie, Geozentrum Universitat Wien, Althanstrasse 14, A1090 Wien, Osterreich (Austria); [email protected]

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CARBONATE BEACHES 2000

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test-walls by clueing foreign particles to an organic sheet. These particles are almost fixed using organic or inorganic (calcium-carbonate) cements. The second group of foraminifera, which is more abundant in normal marine environments, secrets their test walls in form of either aragonite or calcite. Since foraminifera inhabit marine environments in high abundance from the surface to the bottom, from the estuarine to the deep-sea and from the pole to the equator, these organisms became important calcium-carbonate producers in marine environments during Earth history. Especially in subtropical and tropical shallow marine environments, where calcium-carbonate production is relieved by high temperature, alkalinity, and raised salinity, the low nutrition content in oligotrophic waters has led to the development of symbiosis with single-celled microalgae in many invertebrate organisms. This is often characterized by the development of huge calcium-carbonate skeletons and shells in invertebrates (e.g. scleractinian corals, giant clams). Gigantism in shells and skeletons is interpreted either as a result of photosynthesis enabling calcification (e.g. Simkiss, 1986) or, vice verse, relieving photosynthesis by providing carbon dioxide for the microalgae through the calcification process (McConnaughey and Whelan, 1997). While the 'normal' test size of foraminifera is between 100 micrometers and I mm, tests can become huge in species living in oligotrophic tropical shallowmarine environments. The test form and size depends on the environment, especially to resist high water energy above the fair weather wave base by fixing to the substrate or to get enough light for symbionts near the base of the euphotic zone. The average size of symbiont-bearing benthic foraminifera in high energy environments is between I and 4 mm, but some species can get extreme size up to 1.5 centimeter. High-energy forms are almost characterized by spherical or lenticular tests, while the second group with large sizes is distinguished by platelike tests. Species with plate-like tests are significant for light depleted water, but they can also live in the shallowest regions under strong hydrodynamics (Soritidae), where they must avoid transportation by fixing through strong pseudopods to flat and smooth, organic or inorganic hard substrates, like seaweeds or boulders. In the Eu- and Sublittoral of oligotrophic tropical environments, the lack of competition with smaller, nutrition dependent foraminifera gives place for extreme abundance of the symbiont-bearing larger foraminifera. Today, the highest diversity of symbiont-bearing benthic foraminifera is found in IndoPacific areas distinguished by oligotrophic conditions (e.g. coral reef environments). Lower diversity is characteristic for the Caribbean and the tropical Atlantic, possibly due to their less oligotrophy. Since all symbiont-bearing larger foraminifers produce calcium carbonate skeletons, their proportion on the global calcium carbonate production is important (Langer et aI., 1997). The high production rate of these organisms is caused, on the one hand, through the short longevity (average 6 months or I year) leading to numerous reproduction phases in comparison to multicellular invertebrates and, on the other hand, extreme density. In combination with their habitat on the reef flat, this explains the

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dominant role of larger foraminifera in the production and composition of carbonate beach sands, especiaIly in the Indo-Pacific. Larger foraminifera as important producers of Indo-Pacific carbonate beach sand

Today, larger foraminifera living in shaIlow water can be divided in two groups. Beside moleculargenetic and phylogenetic aspects, both groups differ in the chemistry of their secreted test walls. The first group is characterized by highmagnesian calcite tests, while the second group builds the walls with lowmagnesian calcite. Differences in the crystallography between these calcium carbonate modifications are not only responsible for the transparency of test walls, but also for test strength. The optical axes of the tiny rhombohedral low-magnesium calcite crystallites are oriented almost perpendicular to the test surface, thus relieving light penetration. These transparent tests are called 'hyaline'. The direct contact of the crystal surfaces between the crystallites leads to compact ultrastructures and, in combination with the low solubility according to the regular arrangement of calcium atoms, impedes diagenetic alteration and mechanical destruction. Additionally, the test walls become thicker during growth, since the wall of the last chamber do not only bound the chamber volume, but also covers the older test parts. This construction is called a lameIlar test wall. High-magnesian calcite crystals are pseudo-hexagonal and the optical axes coincide with the main axes. The extremely tiny crystals (mean length = 2 micrometers, mean diameter = 0.3 micrometer) show random orientation. Incident light is broken and totally reflected. Smooth surfaces strengthen this reflection and give these tests a white appearance, called 'porcellaneous' tests. Consequently, the foraminiferal ceIl is protected from strong irradiation even through thin test walls. The random orientation of the tiny crystals weakens test strength against mechanical destruction in empty tests. Additionally, the empty space between crystals, fiIled during life by organic substance, relieves solubility. The latter process is intensified within a single crystal through the irregular arrangement of calcium atoms due to their stochastic replacement by the smaIler magnesium atoms. The main foraminiferal producers of calcium carbonate with porcellaneous tests are the two recent species of the genus Marginopora. Specimens are characterized by a cyclic arrangement of chambers, leading to plate-like tests. Due to the nonlameIlar test walls, the test center containing the embryonic part wiIl not become thicker during growth, resulting in biconcave tests. Specimens can grow up to a maximum size of 1.5 centimeters. They live in less energy environments on the reef flat (channels and grooves), the moat and in the shallowest part of lagoons. Individuals are either totally attached by one surface to the sea grass that grows near the reef front, or they are less strongly attached to coral rubble and macro-algae, thus protected against transport by the structured


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surface of these firm substrates. The two species, which are restricted to the tropical East Indian and Central to West Pacific Ocean shows a clear geographical separation. Marginopora kudakajimaensis can be found north of the equator, while M vertebralis (Figure 1) dominates in the southern hemisphere. Coral reefs from the Philippines through the Carolines to the Marianas Islands represent the transitional regions and are inhabited by both species. Two families of larger foraminifera with hyaline tests are found in coral reef environments. Within the Amphisteginidae that are characterized by lenticular smooth tests, Amphistegina lobi/era is restricted to the reef crest and the uppermost part of the reef slope. This species shows a wide geographical distribution over the whole tropical Indo-Pacific and is an important lessepsian migrant in the Mediterranean. In the central part of the Indo-Pacific region, the amphisteginids are in competition with the second group of larger foraminifers possessing hyaline tests, the Calcarinidae. Representatives of this family are adapted to extreme hydrodynamic conditions since they can use their strong spines as anchors fixing the tests with protoplasmatic sheets to macroalgae or hatching together. Therefore, they inhabit algal turfs in the highest energy environments on the reef flat. The strong spines give the impression of little stars, and the predominance of such tests in carbonate sands resulted in the name 'star sand foraminifera'. Four species of this family are extremely abundant in the West Pacific and Indonesian archipelago. Calcarina hispida and C. gaudichaudii (Figure 1) are distinguished by a nearly planspiral arrangement of chambers possessing strong spines. Baculogypsina sphaerulata and Schlumbergerella floresiana develops spherical tests by adding lateral chamberlets to a central layer of chambers (Figure 1). Calcarina gaudichaudii is found in the northern West and Central Pacific, where it dominates in reef crest environments. Its close relative C. hispida is distributed over the whole tropical West and Central Pacific, but prefers less energy environments compared to its relative. North of the equator, C. hispida is in competition with C. gaudichaudii and thus restricted to less energy environments like algal turf in the shallow moat or coral rubble on the reef slope. This species becomes the dominant star-sand foraminifer in shallow algal turfs of the Southern Pacific, especially the Great Barrier Reef Province. Baculogypsina sphaerulata can be found in similar abundance like C. gaudichaudii in the same regions north of the equator, and it becomes less abundant in the South Pacific. This species is totally replaced on reef flats of the Indonesian archipelago by the larger and evolutionary progressive relative Schlumbergerellafloresiana. Proportion of larger foraminifera in beach sands

The most detailed studies on the composition of beach sands in the Indo-Pacific were made by Yamanouchi at the Ryukyu archipelago (summarized in Yamanouchi 1998). Concentrating on the very coarse sand fraction ( = -1 to 0, sand-grains between 1 and 2 mm), which is the most frequent size class in beach


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Figure 1. Symbiont-bearing benthic foraminifera on Pacific and Indonesian coral reefs that are important for beach sand production.

grain size

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~ § J1L :

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Baculogypslno

Figure 3. Particle size and components of sands on the reef and beach at the Kasari Reef in Amarni-O Island (source Yamanouchi, 1990).


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/ /

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coral fragments

Calcarina Baculogypslna

shell fragments

Itthlc fragments

Figure 2. Components of the 'very coarse' sand fraction of beach sands from several islands of the Ryukyu archipelago (source Yarnanouchi, 1998).

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sands, this fraction contains a lot of Calcarina gaudichaudii and Baculogypsina sphaerulata tests (Figure 2) beside coral and mollusk shell fragments. The diagrams in Figure 2 demonstrate the high proportion in beach sand of both star sand foraminifera with a range from 20 % at Ishigaki-Island to 66 % at Amami-O Island. This confirms their important role in the production of beach sands beside coral and mollusks. There is no significant geographical trend in the proportion between both calcarinids. While C. gaudichaudii seems to dominate at some islands, especially in the southern Ryukuys, the proportions of B. sphaerulata are higher in the northern part. But the dominance of C. gaudichaudii at Amami-O Island and its high proportion at Tokuno and Okinaberu Island contradicts this trend. Therefore, differences in abundance of both species may not depend on the geographical scale mainly influenced by temperature differences, but on smaller scales like the morphology of the reef and their position to currents and preferred wind direction. Frequencies and proportions between both calcarinids also differ regarding the composition within separate grain size classes and the position of samples on the fringing reef (Figure 3). A slight shift from 'very coarse' to 'coarse-grained' sand is typical from the outer reef flat to the beach. In the 'granules' fraction ( = -2 to -I) that is an important class in the outer reef flat sands, only a few C. gaudichaudii tests can be found since this class represents the upper limit for test size in living C. gaudichaudii. The main proportion of this species is represented in the 'very coarse' grain size fraction, becoming low in the 'coarse' grain size class ( = 0 to I). In the latter fraction, the amount of B. sphaerulata is small, but higher than C. gaudichaudii. With the diminution of grain size towards the beach, the proportion of foraminiferal tests increases. At the beach, this results in a dominance of C. gaudichaudii in the 'very coarse' sand class, while B. sphaentiata takes highest proportion in the 'coarse' sand fraction. The proportion of foraminifera in sands at a fringing reef of the South Pacific was also investigated (Yamano et al. 2000). Similar to the investigation by Yamanouchi (1980), sediment samples were taken from the algal turf behind the coral zone, from the reef flat that is covered by Halimeda, and from the sandy beach of Green Island, Great Barrier Reef. As explained above, the characteristic foraminiferal component in the southern Pacific is Calcarina hispida that replaces the larger C. gaudichaudii on reef flats of the South Pacific, accompanied by a much smaller proportion of B. sphaerulata compared to the North Pacific. Nevertheless, the proportion of total foraminiferal tests is high with ranges from 20 to 50 % and remains more or less constant from the outer reef flat to the beach. A strong dominance of foraminifera in the composition of beach sands can be found in coral reef regions of the southern Indonesian archipelago (e.g. Bali, Flores, Timor). More than 90 % of the grains are represented by rounded, equally sized tests of Schlumbergella jloresiana (Figure 4) that is a close relative to B. sphaerulata. Therefore, the main producers of the white carbonate sand in the tourist regions of Bali are foraminifera.


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The rounded tests of S. jloresiana in beach sands indicate mechanical destruction, e.g. abrasion through neighboring particles in high-energy environments. Especially the oscillatory movement of beach sands through waves leads to abrasion and well-rounded tests. This is confirmed by the investigation of abrasion in other ca1carinids that are distinguished by strong spines. Three attrition grades of Calcarina galldichaudii spines were used by Yamanouchi (1998) for getting a qualitative measure of test exposure to water movement. Investigating various sands on reef flats in the Ryukyus, the proportion of strongly abraded forms increases from the reef edge to the beach, where wellrounded, spherical tests (grade 3) predominate and give evidence for the intensive water movement through breaking waves. Since all symbiont-bearing benthic foraminifera on the reef flat avoid to live on instable sand, their tests as an important component of the beach sand cannot originate in this environment, but are transported from the areas of production to these deposition regions. Inferences about transport intensity and direction are possible by comparing production rates and living habitats of the larger foraminifera on the reef flat with their proportion in beach sands. This is extremely important for the production and preservation of carbonate beach sands in the tropical West Pacific and East Indian Ocean. Distribution, production and transport of larger foraminifera

The distribution of living larger foraminifera on the reef flat and moat depends on the factors water energy, substrate and competition with corals. The last factor is important, since no living larger foraminifer can be found on coral tissues. To resist against entrainment and transportation, the larger foraminifera are fixed to firm substrate that itself will not easily be transported by high water energy, like granules or small boulders. Therefore, larger foraminifers are fixed in the shallow subtidal to seaweeds or larger inorganic gravel, which often possess a structured surface like broken corals. The availability of a proper substrate, differences in hydrodynamics and irradiation are responsible for the distribution and production of larger foraminifers. Since species differ in these ecological demands, their distributions are restricted according to the combination of the factors explained above. The distribution pattern in combination with the differing transport intensity and direction by wave or tidal currents explains the strong variation in foraminiferal proportions of carbonate beach sands. An example can be given for the sand of Sesoko Beach (Figures 5, 6) based on the work of Yamanouchi (1993, 1998) and Hohenegger (1994). The proportion of C. galldichaudii characterized by strongly abraded tests is high (> 30 %) in beach sand (Figures 2, 5). Baculogypsina sphaerlilata is the second important representative (5 - 10 %). Both species live fixed to smaller macroalgae in shallow pools on the reef crest, but are extremely rare in this environment in front of Sesoko-Beach. Therefore, a direct transport from the reef crest west of Sesoko Beach to the shore is impossible. Current measurements in the moat

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demonstrate a flow direction parallel to the shore from the northeast to the south (Yamanouchi 1993; Figure 5). Investigations on living foraminifera by Hohenegger (1994) show the restriction of C. gaudichaudii and B. sphaerulata to the reef crest regions in the north of Sesoko Island. These are the production areas, where both star sand foraminifera live in high abundance. After loosing their attachment to the filamentous macroalgae by reproduction or death, empty tests are transported at low tide by shore parallel currents to the southern part and accumulated on the beach by waves during high tide. Transport is relieved in alI foraminifera by their lower density in comparison with compact carbonate particles of the same size and volume. This is caused by the test construction, where chambers of empty tests are filIed with seawater. The test densities range from 1.8 g mm-3 in small calcarinids to 1.4 g mm- 3 in larger forms, in comparison to 2.71 g mm-3 for pure calcite. Calcium carbonate productivity for 6 species of symbiont-bearing benthic foraminifera was calculated at six stations along one transect in the NNW of Sesoko Island (Figures 5, 6; compare Hallock 1981). Productivity of C. gaudichaudii is high (500g and 900g m-2 yr- 1) in two pools located in the front and center ofthe crest and decreases to the reef front and the moat. Baculogypsina sphaerulata is distinguished by lower production rates in both pools (100g and 250g m-2 yr- 1) compared to its relative. But this high production is restricted to pools that are always filled with water even at the lowest spring tide. 40 to 80 % of pool areas are covered with filamentous algae of the genera Jania, Laurencia, Hypnea, and Gelidiella. The higher part of the crest (60 to 70 % in the western compared to 30 to 40 % in the northern part) is exposed to air during low tide. Both calcarinid species do not live on the exposed crest, which reduces the average carbonate production rate on the reef crest north of Sesoko to 400g m-2 yr- 1 for C. gaudichaudii and to 100g m-2 yr- 1 for B. sphaerulata. The broad reef crests (> 200m) in the southern Ryukyus are less exposed than crests in the central Ryukyus and are totally covered by filamentous macroalgae. Living Calcarina gaudichaudii and B. sphaerulata shows similar abundance on the large crest NW ofKabira Bay, Ishigaki Island, where the mean production of each species is 600g m-2 yr- 1, resulting in an average carbonate production of foraminifera (including C. hispida and A. lobifera) of 1,500g m-2 yr- 1 These tests are the main components of the beach sand. A similar percentage of coverage with microalgae is typical for reef crests at the southern beaches of Bali, Indonesia. These areas are densely settled by Schlumbergerella floresiana and the estimated average carbonate productivity (1,600g m-2 yr- 1) is the highest of all larger foraminifers in reef environments. Calcium-carbonate production of the second group of larger foraminifera with porcellaneous tests (Jv1arginopora) is also important, since they can get high densities in protected areas within or behind the reef crest. Extreme production rates (5,000g m- 2 yr- l ) were experienced for M kudakjimaensis at Myako-Island


environments. Especially the oscillatory movement of beach sands through waves leads to abrasion and well-rounded tests. This is confirmed by the investigation of abrasion in other calcarinids that are distinguished by strong spines. Three attrition grades of Calcarina gaudichaudii spines were used by Yarnanouchi (1998) for getting a qualitative measure of test exposure to water movement. Investigating various sands on reef flats in the Ryukyus, the proportion of strongly abraded forms increases from the reef edge to the beach, where wellrounded, spherical tests (grade 3) predominate and give evidence for the intensive water movement through breaking waves.

Figure 4. Beach sand from the Nusa Dua Beach, Bali, Indonesia, consisting of well rounded Schlumbergerellafloresiana tests. Since all symbiont-bearing benthic foraminifera on the reef flat avoid to live on instable sand, their tests as an important component of the beach sand cannot originate in this environment, but are transported from the areas of production to these deposition regions. Inferences about transport intensity and direction are possible by comparing living habitats and production rates of the larger foraminifera on the reef flat with their proportion in beach sands. This is extremely important for the production and preservation of carbonate beach sands in the tropical West Pacific and East Indian Ocean.

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