Sandy Seafloor Research Articles

Studies on the conversion of near-grazing underwater ultrasonic waves in a sandy seafloor

Ultrasonic modeling results are presented supporting the hypothesis given by the author [J. R. Chamuel, IEEE Trans. Ultrason. Ferroelectr. Freq. Control, submitted Letter 10 April 1998] explaining the anomalous ultrasonic slow wave observed in water-saturated sand [N. P. Chotiros, J. Acoust. Soc. Am. 97, 199–214 (1995)] using a layered elastic model. The rapid increase of the shear wave velocity of sand with depth causes the conversion of near-grazing underwater acoustic waves into multiple coupled shear and compressional waves. Physical insight into the physical mechanisms is obtained from the refraction/conversion of broadband transient shear and compressional waves penetrating a layered solid with a low shear wave velocity. Polycarbonate/Plexiglas and wax/polycarbonate layered models were utilized. PP, PS, SP, and SS wave components were detected in the lower solid simulating the multiple features present in the anomalous acoustic sand data. In the effectively layered water/sand model, an underwater acoustic wave incident beyond the compressional wave critical angle is partially converted into a shear wave in the sand which is reconverted into a compressional wave appearing as an anomalous slow wave when detected by a buried receiver. [Work supported by ONR.]

The Helmholtz–Kirchhoff approach to modeling penetration of acoustic waves into rough seabeds

The Helmholtz–Kirchhoff integral is applied to model the penetration of sound waves into sandy seafloors at grazing angles above and below the critical angle. Although the conditions for the validity of the Kirchhoff approximation can be limiting, this approximation should be valid at high frequency for gently undulating seafloor surfaces even at moderate to low grazing angles, providing that the self-shadowing effect is carefully removed. The analytical development of the method is first presented, followed by numerical examples and comparisons with the experimental data of Maguer et al. [SACLANTCEN report, SR-287, April 1998]. The model predicts, in agreement with the 2- to 15-kHz acoustic data, the frequency where the contributions due to roughness effects begin to dominate those due to the evanescent wave. Secondary effects such as Bragg interference patterns and the loss of signal coherence with grazing angle or depth are correctly predicted. The model simulations strongly suggest that roughness of the sediment interface is most likely the cause of anomalous sound penetration into the seabed.

The Geochemistry of Hydrothermal Vent Waters from Milos Island, Hellenic Volcanic Arc

The Hellenic Volcanic Arc is a tectonically and volcanically active part of the earth's crust, situated above the subduction zone of the African plate under the Aegean. The arc extends from the Methana peninsula (NE Peloponese) through the greek islands Milos, Santorini, Kos, Nisiros and Yali to the Bodrum peninsula, Turkey (e.g. Fytikas et at., 1986). Hydrothermal venting has been reported from Nisiros, Yali (Varnavas and Cronan, 1991), Santorini (Varnavas and Cronan, 1988) and Milos (Fytikas, 1989). The island of Milos, at the central west part of the volcanic arc, is dominated by volcanic and voIcaniclastic rocks, with the exception of minor occurrences of metamorphic basement rocks and Neogene sediments outcropping at the south part of the island, underneath the volcanics (Fytikas et al., 1986). Submarine and, later, sub-aerial extrusive sequences formed during a prolonged period of volcanic activity (3.5 to 0.1 Ma, Fytikas, 1989). Geochemically, the Milos volcanics show a calcalkaline character and comprise andesites, dacites and ryolites (Fytikas et al., 1986). Hot water springs and fumaroles occur at several localities around the island. In submarine settings, hot fluids exit through the sandy seafloor, often in the form of a mixed liquid/ gas phase; as a result, venting can be located due to the visually impressive presence of streams of gas bubbles. In the most hydrothermally active south-east part of the island (Palaeohori bay), extensive areas of bacteria mats are associated with submarine hydrothermal brine seeps (Fitzsimons et al., 1997). Field observations and sampling

Nebalia Daytoni N. Sp. a Leptostracan From Southern California (Phyllocarida)

The leptostracan Nebalia daytoni new species, is common of the San Diego coast (Southern California), USA, on the sandy sea floor from 8 to 35 meters in depth. Unlike other California Nebalia it inhabits clean, oligotrophic sands rather than enriched sediments. The species is most easily recognized by its eye which has a flat anterior margin bordered by a superior and inferior protuberance. It is also easily distinguishable from other California Nebalia by its short antennular flagellum.

Modern stromatolite reefs fringing a brackish coastline, Chetumal Bay, Belize

Reef-forming stromatolites have been discovered along the windward shoreline of Chetumal Bay, Belize, just south of the mouth of the Rio Hondo. The reefs and surrounding sediment are formed by the precipitation of submicrocrystalline calcite upon the sheaths of filamentous cyanobacteria, principally Scytonema , under a seasonally fluctuating, generally brackish salinity regime (0‰10‰). Well-cemented, wave-resistant buttresses of coalesced stromatolite heads form arcuate or club-shaped reefs up to 42 m long and 1.5 m in relief that are partially emergent during low tide. Oncolitic rubble fields are present between well-developed reefs along the 1.5 km trend, which parallels the mangrove coastline 40-100 m offshore. The mode of reef growth, as illustrated by surface relief and internal structure, changes with increasing water depth and energy, proximity to bottom sediments, and dominant cyanobacterial taxa. Sediment trapping and binding by cyanobacteria are of limited importance to reef growth, and occur only where stromatolite heads or oncolites are in direct contact with the sandy sea floor. Radiocarbon-dated mangrove peat at the base of the reef suggests that it began to form about 2300 yr B.P., as shoreline encrustations that were stranded offshore following storm-induced retreat of the mangrove coast.

Direct observations of larval dispersal in the colonial ascidian Podoclavella moluccensis Sluiter: evidence for closed populations

The colonial ascidian Podoclavella moluccensis Sluiter (Aplousobranchia, Polycitoridae) occurs on discrete patches of substrata, separated by uninhabitable sandy seafloor. The potential for exchange between these patches was determined by continuously observing individual larvae from their release from adult colonies until their settlement. Of 270 larvae, 100 were successfully followed to settlement and 80% settled within 2.5 m of their parent colonies. The average distance moved by larvae was 2.2 m after swimming for < 2 min. The distance over which larvae dispersed was a positive function of the time that they spent swimming, but was influenced by current speed and the behaviour of larvae. We conclude that populations of P. moluccensis have extremely limited potential for gene flow and may be regarded as effectively closed systems.

Occurrence of a Pseudophragmina (Proporocyclina) Zaragosensis Bank in Upper Wilcox (Lower Eocene), Pointe Coupee Parish, Louisiana: ABSTRACT

ABSTRACT Rock-forming buildups of larger foraminifera, while not unknown in Louisiana, are rare, and are less common in the Gulf Coast than in Eocene strata from the Pyrenean and Caribbean regions. In strata from the Upper Wilcox Group (lower Eocene) in Pointe Coupee Parish, Louisiana, we have identified a thin, very rich faunal deposit, or bank, made up almost exclusively of the tests of the larger foraminifera species, Pseudophragmina (Proporocyclina) zaragosensis. This is the first report of P. (P.) zaragosensis from the Wilcox Group of Louisiana. The bank fauna colonized the sandy sea floor on the continental shelf, flourished briefly, and then gradually declined, probably in response to increasing water depth, caused either by compaction-induced subsidence, a local or regional rise in relative sea level or both. This bank indicates that during late Wilcox time, warm shallow continental shelf conditions prevailed in this region.

Studies of Biomass Yield From a Near-Shore Macroalgal Test Farm

This paper reports the results of studies in 1981 and 1982 of marine biomass yield from a near-shore, 0.48 hectare test farm at Ellwood, California. The experimental planting was made to ascertain whether or not the plants would maintain a high yield after repeated cutting, and to see what effect planting density has on yield. The test farm was planted with 722 plants and operated in all seasons of the year. Quarterly harvests were made from plants planted at densities of one plant per m2, one plant per 4 m2 and one plant per 16 m2. Eleven metric tons of biomass were planted initially on a sandy sea floor. This produced a total of 50 wet metric tons of harvested biomass in 12 months.

Free-living corals: distributions according to plant cover, sediments, hydrodynamics, depth and biological factors

in 1974, 8 free-living coral species were found to inhabit the sandy sea floor adjacent to Lizard Island, Great Barrier Reef (14°40′S; 145°78′E). They fall into two groups which colonize two dissimilar sediment types. Plant cover increases with depth and, because of its effect on sediment characteristics, is thought to be a significant factor affecting coral distributions. The shallower coarse to medium grain sediments (0.5 to 0.125 mm) are mainly colonized by Heteropsammia cochlea, Heterocyathus aequicostatus, Diaseris distorta, and to a lesser extent by Cycloseris cyclolites. The deeper sediments are made up of a biogenically derived coarse fraction (larger than 0.5 mm) combined with an equally high proportion of fine-grade material. Corals typically found on these sediments are: Trachyphyllia geoffroyi, Catalaphyllia jardinei, Cynarina lacrymalis, and Cycloseris patelliformis. The content of non-carbonate material in the sediments reflects the hydrodynamics of the area and hence the degree of sedimentation, i.e., traction, saltation, or suspension loads, the corals have to cope with. Depth of occurrence was found to predict local coral distributions but was not applicable to other regions. Other factors which are discussed in relation to coral distributions include: coral mobility, coral shape, the effect of an obligate sipunculan associate, Aspidosiphon jukesii, in Heteropsammia cochlea and Heterocyathus aequicostatus, and settlement requirements.

The algal and seagrass ecology of Waterloo Bay, South Australia

The subtidal algal and seagrass communities of Waterloo Bay on the west coast of Eyre Peninsula, South Australia, are described. Their distribution, together with that of the dominant or common species, is related to substrate, degree of water movement and to depth (i.e. to light). Observations were mostly limited to the month of October from 1969 to 1974. Notes are also given on intertidal communities of the platforms near and just inside the heads of the Bay. The Bay is more or less circular and some 2.5 km 2 in area, partly separated from the open sea by a submerged reef. Depths in the centre of the Bay are about 10 m, and the bar reef slopes off to a sandy sea-floor at about 27 m deep. Sea conditions are rough (i.e. high energy) at the entrance to the Bay and on the rock platforms there, and decrease to moderate within the Bay. Sea temperatures range from 14–15°C inter to 18–20°C in summer. By means of Scuba diving, an underwater sled and numerous spot dives (involving 71 samples for detailed analysis), six algal communities and five seagrass communities were recognized and mapped. Communities on upward facing substrate were studied, with some attention being given to algae on vertical surfaces. Communities were recognized by and named after their dominants, the latter being distinguished by abundance, size and biomass. Some communities are sharply separated from others, largely due to change in substrate, while others intergrade more gradually as water movement or depth changes. Most of the algal communities within the Bay are dominated by the larger brown algae, especially species of Cystophora, Ecklonia and Scaberia. On sandy substrate, the seagrass communities are dominated by Heterozostera tasmanica, Amphibolis antarctica or by species of Posidonia. Considerable changes (probably due to storm damage) in the seagrass beds were evident between 1974 and 1979, and suggestions are made with regard to succession in these seagrass communities. Detailed analyses of distributions of species support the recognition and mapping of communities. Changes in biomass, coverage, species richness, percentage similarity and dominance within communities were related to the degree of water movement and to depth. The algal biomass in Waterloo Bay is comparable with that in shallow water of temperate coasts elsewhere in the world, and somewhat greater than that at depths of 10–20 m in either temperate or tropical waters. Coverage of the algal and seagrass communities is high, and in no areas of the Bay are animals dominant. On the outer reef slope, algal communities are still dominant at about 27 m deep, below which the bottom is sandy. Rates of change in algal communities along environmental gradients were generally low within the restricted depth ranges in the Bay, and highest near the extremes of the water movement gradient. Species richness of the upper stratum is greatest in shallow water at intermediate water movement, and richness of the middle stratum is the reverse, being greatest at the extremes of this gradient. On the depth gradient, species richness is highest at 5–10 m deep. Dominance is a major factor determining structure of the communities, and strong dominance of a few species with high upper stratum coverage leads tolow middle stratum coverage of few species. Most subtidal communities have two or more co-dominants and there is little evidence for ‘competitive exclusion’ resulting in a single dominant in these communities. The peaks and limits of individual species were scattered along the gradients with extensive overlaps. Clustering of the distributional limits corresponded to changes between communities. Most common middle and lower stratum species occur over a relatively wide range of conditions, without strong peaks of abundance. With increasing slope, upper stratum dominance declines and species richness of the middle stratum increases. In some areas, sand cover or abrasion occurs and here only sand tolerant species occur. Grazing effects are not prominent and overgrazing (largely due to molluscs) was only observed on areas of the submerged reef at the entrance of the Bay. The algal flora of Waterloo Bay is rich, and 364 species are listed in the Appendix.

CUTTING OF SANDY SEA FLOOR AND ITS APPLICATION

CUTTING OF SANDY SEA FLOOR AND ITS APPLICATION