Coral Boulders Research Articles

Identification of 1771 Meiwa Tsunami deposits using a combination of radiocarbon dating and oxygen isotope microprofiling of emerged massive Porites boulders

The Meiwa Tsunami, one of the largest tsunamis recorded in historical documents in Japan struck Ishigaki Island and neighboring islands of the Ryukyu Arc in April 1771 AD, killing more than 12 000 people. An enormous number of massive Porites coral boulders are scattered on the shore and in the reef moat of eastern Ishigaki Island. Although these boulders likely were cast ashore by the Meiwa Tsunami, a detailed examination has not yet been conducted. When the marine reservoir effect is taken into account, one of mode values of calibrated radiocarbon dates possibly corresponds to the time of the 1771 event. However, the range of calibrated radiocarbon ages indicates that the transport of the boulders cannot be ascribed solely to the tsunami. Oxygen isotope microprofiling, which indicates sea-surface temperature variation, was therefore conducted to further investigate the mechanism of transport. The results suggest that the skeletal growth of most coral colonies was interrupted in summer or autumn; hence, tropical storms and typhoons are also very likely to be agents of transport. Thus, by combining radiocarbon dating with oxygen isotope microprofiling to investigate Porites coral boulders, it is possible to separate paleotsunami boulders from those transported by storm events as far as tsunamis occurred during the non-storm season.

Extremely high-energy wave deposits inside the Great Barrier Reef, Australia: determining the cause—tsunami or tropical cyclone

A hydrodynamic approach is used to determine whether tsunami- or cyclone-generated waves were responsible for the deposition of fields of well-imbricated rock boulders (up to 290 tonnes) along the coast of Cairns inside the Great Barrier Reef, Australia. Calculations of the overturning moments show that only tsunami are capable of moving such large boulders in this environment. It is hypothesised that large tsunami (> 11 m) have been able to penetrate the Great Barrier Reef through wide (5–10 km) 50–70 m deep passages between individual reefs. Three such passages each approximately 35 km apart and oriented in the same direction exist in the Cairns region. It is possible that these passages have funnelled and amplified palaeotsunamis. The preferential location of eroded coral boulders up to 3 m in length on reef flats alongside these passages and their absence on other reefs throughout the region provide further evidence that extremely high-energy waves have been able to penetrate the Great Barrier Reef into the inner channel adjacent to the mainland. Carbon-14 ages of the coral boulders on these reef flats matches closely the ages of coral fragments pinned below the very large rock boulders along the coast. These ages suggest that the Cairns region has experienced large tsunami twice over the last millennium.

Antimicrobial activity of tropical and subtropical sponges

Extracts of 19 of 24 species of sponge collected from Queensland. Australia, inhibited the growth of test microorganisms in in-vitro assays. A similar result has been obtained by earlier workers for extracts of species of sponge obtained from temperate waters. Some of the extracts, including extracts of Thorecta vasiforis, Arenochalina mirabilis and Acanthella kleutha, showed activity against bacterial strains that was comparable with that exhibited by penicillin G and streptomycin against these strains. Gram-positive bacteria were expecially sensitive to many extracts. Little activity was exhibited by any extract against four species of fungi tested. Some of the extracts were markedly toxic to one or more of the test organisms (a fish, a crustacean and a hydroid) used, but no clear pattern linking toxicity to these organisms with antimicrobial properties of the extracts emerged. In general, there bas a negative correlation between antimicrobial activity and surface-fouling, raising the possibility of using freedom from surface-fouling as an indicator of antimicrobial activity. Four of five encrusting species from the undersides of coral boulders showed antimicrobial activity. This study confirmed the value of using methanol-toluene extracts in field-based screening programmes, but emphasised the need to use additional extracting media such as methylene chloride in order to augment the number of antimicrobial compounds detected. There are indications that antimicrobial activity may be widespread in the order Dictyoceratida, in the order Verongida and in the order Axinellida, but sporadic in other orders of Porifera.

An intertidal Rhabdopleura (Hemichordata, Pterobranchia) from Fiji

Rhabdopleura has been discovered living in coral rubble on reefs in Fiji. The habitat is unusual, being the underside of coral boulders in the intertidal zone. Some features of the environment and the fauna associated with the Rhabdopleura are briefly described. The Rhabdopleura zooids exhibit other modes of tube building besides the regular cylindrical horizontal and erect tubes. The colony ramifies through interstices in the dead coral and the zooids can line larger cavities within the coral with coenecial tissue. It is probably these cavities, together with the overall porosity of coral, that retain enough water to keep the zooids alive while the intertidal reef flat is exposed to the air. Within the depths of the coral the black stolons are frequently naked. The species is probably R. normani Allman.

Distribution, burrowing, and growth rates of the clam Tridacna crocea on interior reef flats : Formation of structures resembling micro atolls.

Larvae of the burrowing clam Tridacna crocea (Tridacnidae) settle preferentially on top of detached coral heads lying on the surface of the interior reef flat in the Great Barrier Reef province. This species burrows as it grows, eroding the central top surfaces of coral boulders, producing structures that superficially resemble micro-atolls. Storm surges roll these coral heads onto the now flattened surface, killing the live population of clams, and exposing the fresh underside for unimpeded larval settlement. As these clams grow and burrow into the substratum, the coral head becomes progressively flattened and finally breaks apart. Field observations and growthring data documented growth rate; growth rates plus burrow volumes were converted to annual sediment production. At average population densities approximately 140 gm/m2/yr of coral are eroded. Concomitant with erosion is a calcium carbonate increase in the shell of these clams amounting to 60gm/m2/yr. Assuming a stable population structure, with annual mortality equal to annual estimated growth, total sediment production is 200 gm/m2/yr. Clams are usually aggregated at higher densities, however, with numbers regularly exceeding 100 clams/m2. Consequently maximum sediment production rate locally is often 4,500 gm/m2/yr.

Recent Carbonates of Isla Mujeres? Quintana Roo: ABSTRACT

Isla Mujeres, off the northeast tip of Yucatan, consists of oolitic dunerock. The Caribbean coast is subjected to heavy waves, and the sediments are coarse, well sorted, highly polished and rounded; coralline algae, carbonate rock fragments, Halimeda and coral are abundant. The lee (western) beach is protected; sands are finer, well sorted, dull, and angular with Halimeda greatly predominant. Straits separating Isla Mujeres from Yucatan are swept by strong northerly currents: sediments are poorly sorted, negative-skewed, and consist of a bimodal mixture of (1) dominant small, polished oolites, and (2) subordinate coarse oolitic rock fragments with some skeletal grains. Sorting of beach sediments is a sinusoidal function of mean grain size; best sorting is at 2^phgr, -3^phgr, and -8^phgr. Sorting values (^sgr) of 0.3-0.6^phgr are characteristic of all beaches regardless of grain size, from fine sand to coral boulders, regardless of composition and regardless of wave energy of the coast. Protected bay sediments are muddy and poorly sorted (immature); sediments from current-swept straits are winnowed, but poorly sorted (submature); sediments of lee beaches are well sorted but dull and angular (mature); and sediments of beaches exposed to high waves are well sorted, rounded and highly polished (super-mature). End_of_Article - Last_Page 357------------