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1.
Most of the atolls found worldwide are under microtidal regimes, and their circulation mechanisms are widely documented and well known. Here, we describe the flushing mechanisms of a small-sized mesotidal atoll, based on water-level, wave and current data obtained during two different periods (total of 60 d). Rocas is the only atoll in the South Atlantic Ocean and is built primarily of coralline algae. Two reef passages connect the atoll lagoon to the ocean. Synchronous current profilers were deployed at the two reef passages, one inside and one outside the atoll, to characterize the influence of tides and waves on the circulation. Results showed that wind waves drove a setup on the exposed side of the atoll and that currents were predominately downwind, causing outflow at both reef passages. Waves breaking on the windward side supplied water to the atoll causing the lagoon water level to rise above ocean water level, driving the outflow. However, unlike microtidal atolls, at Rocas Atoll the water level drops significantly below the reef rim during low tides. This causes the reef rim to act as a barrier to water pumping into the lagoon by waves, resulting in periodic activation of the wave pumping mechanism throughout a tidal cycle. As result, inflow occurs in the wider passage during 27% of each tidal cycle, starting at low tides and reversing direction during mid-flood tide when the water level exceeded approximately 1.6 m (while overtopping the atoll’s rim). Our findings show that tides play a direct role in driving circulation on a mesotidal atoll, not only by modulating wave setup but also by determining the duration of wave pumping into the lagoon. 相似文献
2.
Foraminifera from the coral-reef lagoons of two atolls are used to investigate the distribution and diversity of assemblages in the central Indian Ocean. Eight assemblages and 270 species of foraminifera are identified. Three assemblages are reefal and dominated by Amphistegina and Calcarina. Only one lagoon assemblage is present in both atolls, which is characterized by abundant Ammonia sp. 1 and smaller miliolid foraminifera. In Ari Atoll, abundant Amphistegina sp. 1, Operculina ammonoides, Amphistegina radiata and Nummulites venosa characterize one lagoon assemblage and abundant Neoeponides bradyi and Textularia cf. T. cushmani characterize the other. In Rasdhoo Atoll, abundant Textularia gr. foliacea and Spiroloculina nummiformis characterize one lagoon assemblage and abundant Textularia cf . T. cushmani and Textularia sp. 5 characterize the other. The assemblages compare well with sedimentological characteristics, which is also reflected in their distributions. Species diversity in the Maldives appears to be higher than the western Indian Ocean region due to the stronger influence of central Indo-Pacific faunas. It is not however, as high as the central Indo-Pacific region, which may be due to lower habitat diversity. 相似文献
3.
Summary The chief mode of carbonate sedimentation on the Belizean atolls Glovers Reef, Lighthouse Reef and Turneffe Islands is the
accumulation of organically-derived particles. Variations in the distribution of the composition and grain-sizes of surface
sediments, collected along transects across the atolls, are environmentally controlled. Two major sediment types may be distinguished.
(1) Reef and fore reef sediments are dominated by fragments of coral, coralline algae and Halimeda. Mean grain-sizes range from 1–2 mm. (2) Back reef sediments contain more mollusk fragments, more fine-grained sediment (<125
μm) and appear to have fewer Halimeda fragments. In addition, sediments from inner platforms and shallow lagoonal parts of Glovers and Lighthouse Reefs comprise
non-skeletal grains, namely fecal pellets. Sediments from lagoonal patch reefs may contain up to 20% coral fragments. Mean
grain-sizes range from 0.1–1 mm and are finest on the inner platform and lagoon floor of the back reef environment. Within
the reef and fore reef environments, it is not possible to distinguish sub-environments on the basis of textural and compositional
differences of the sediments. Sediments from patch reefs contrast with those from back reef lagoons and inner platforms and
are similar in terms of grain-sizes and compositions to reef and fore reef surface sediments.
Non-skeletal grains forming in shallow parts of the back reef in Glovers and Lighthouse Reefs are interpreted to be indurated
by interstitial precipitation of calcium carbonate from warm, supersaturated water flushing the sediment. The lack of hardened
non-skeletal particles in the back reef sediments of Turneffe Islands is most probably due to the abundance of muddy, organic-rich
sediment in the well-protected lagoon. Fine sediment is less permeable and organic films prevent cement overgrowth on particles. 相似文献
4.
The knowledge of Pleistocene reef facies of Belize, Central America, is largely limited to outcrops in the northernmost part
of the country. Otherwise, Pleistocene limestone, which forms the basement of the modern barrier and atoll reefs, occurs in
the subsurface and is to a major extent unstudied. Based on the study of 40 m of core from 25 rotary core holes collected
on central and southern Belize barrier and on atoll reefs, five Pleistocene reef facies are distinguished in the present study.
They include (1) Acropora palmata grainstone, (2) Acropora cervicornis grainstone, (3) biogenic grainstone, (4) mollusk packstone, and (5) mollusk-foram wackestone. Facies 1 and 3 occur on marginal
reefs, facies 2 is found on marginal and lagoonal reefs, and facies 4 and 5 mark lagoon shoals and lagoons, respectively.
Most of the facies have equivalents in the Pleistocene of the wider Caribbean and also in the modern of the study area. Diagenetic
features include dissolution, caliche formation, laminated blocky low-magnesium-calcite and dogtooth spars. Age data from
Pleistocene corals obtained during earlier studies are discussed, and indicate deposition during marine isotope stage 5, between
140–80 ka bp. 相似文献
5.
Summary Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata
are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence
of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian,
stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids,
receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef
occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef
area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China
with regard to the biotic composition. The reef is composed of fasciculate colonies of Smithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The
species of Smithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control: Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereas Smithiphyllum occidentale
Sorauf, 1972 and Smithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like
coral colonies baffled sediments. Algae and stromatoporoids (mainly Stachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods.
The reefs are composed from base to top of five lithofacies types: 1) cryptalgal micrite, 2) peloidal packstone, 3) stromatactis
limestone, 4) coral-bafflestone, and 5) pseudopeloidal packstone. The reef complex can be subdivided into back-reef subfacies,
reef flat and marginal subfacies, and marginal fore-slope subfacies. The Houshan coral-bafflestone reef is not a barrier reef
but a coral patch reef located near the inner margin of a carbonate platform. 相似文献
6.
Henderson Island, in the Pitcairn Group, preserves a Pleistocene atoll physiography with the rim of the raised reef structure, supporting spur and groove topography, enclosing a central lagoon. Excellent preservation of coral reef communities occurs along the ancient atoll rim and within the central lagoon. The previously interpreted depositional nature of the fossil atoll structure is herein corroborated with geomorphologic and stratigraphic evidence from previously un-visited portions of the island. Stratigraphic and lateral facies relationships indicate a physiographic zonation which includes spur and grooves, outer reef flat, lagoon margin, and an interior lagoon with patch reefs. The in situ occurrence and zonation of reef coral communities around the periphery and within the interior of the island appear to reflect the original physiography of the atoll lagoon, with the most pronounced reef development on the SE side of the original atoll. Stratigraphic units which comprise the raised atoll lagoon structure represent different time intervals, so the atoll lagoon structure formed during various sea level fluctuations. The modern atolls of the Pitcairn Group, Oeno and Ducie, provide some comparisons (similarities and differences) with the fossil lagoon on top of Henderson Island. 相似文献
7.
A series of surveys were carried out to characterize the physical and biological parameters of the Millennium Atoll lagoon during a research expedition in April of 2009. Millennium is a remote coral atoll in the Central Pacific belonging to the Republic of Kiribati, and a member of the Southern Line Islands chain. The atoll is among the few remaining coral reef ecosystems that are relatively pristine. The lagoon is highly enclosed, and was characterized by reticulate patch and line reefs throughout the center of the lagoon as well as perimeter reefs around the rim of the atoll. The depth reached a maximum of 33.3 m in the central region of the lagoon, and averaged between 8.8 and 13.7 m in most of the pools. The deepest areas were found to harbor large platforms of Favia matthaii, which presumably provided a base upon which the dominant corals ( Acropora spp.) grew to form the reticulate reef structure. The benthic algal communities consisted mainly of crustose coralline algae (CCA), microfilamentous turf algae and isolated patches of Halimeda spp. and Caulerpa spp. Fish species richness in the lagoon was half of that observed on the adjacent fore reef. The lagoon is likely an important nursery habitat for a number of important fisheries species including the blacktip reef shark and Napoleon wrasse, which are heavily exploited elsewhere around the world but were common in the lagoon at Millennium. The lagoon also supports an abundance of giant clams ( Tridacna maxima). Millennium lagoon provides an excellent reference of a relatively undisturbed coral atoll. As with most coral reefs around the world, the lagoon communities of Millennium may be threatened by climate change and associated warming, acidification and sea level rise, as well as sporadic local resource exploitation which is difficult to monitor and enforce because of the atoll''s remote location. While the remote nature of Millennium has allowed it to remain one of the few nearly pristine coral reef ecosystems in the world, it is imperative that this ecosystem receives protection so that it may survive for future generations. 相似文献
8.
Detailed measurements of water levels, and tide and wave-induced currents were undertaken to examine physical processes and
their relationship with morphology in the Cocos (Keeling) Islands, a medium sized atoll in the Indian Ocean. Results indicate
that the atoll structure controls both lagoon circulation and the spatial pattern of energy distribution. Lagoon circulation
is tide dominated (currents 16–31 cms -1) with flushing (2–5 days) of the lagoon occurring through the deep leeward passages. Wave- and tide-driven unidirectional
flows through shallow passages (26–65 cms -1) are important mechanisms of ocean to lagoon water exchange and contribute up to 24% of the lagoon neap tide prism. Reef
flats are dominated by wave energy (maximum velocity 140 cms -1, east) with measurements of the attenuation of wave energy between reef flats and shallow lagoon (80–90%) conforming to measurements
from fringing and barrier reefs. Spectral analysis shows that the characteristics of wave energy vary on different sectors
of the atoll, with gravity wave energy dominating the east, and infragravity wave energy dominating the southern reef flat
and passages. Wave setup at the reef crest is considered to be responsible for an identified 0.1 m higher water level in the
southern as opposed to eastern and northern atoll, which promotes higher reef flat growth. Transmission of gravity waves across
reef flats requires threshold water depths of 0.65 (east) and 0.70 m (south). The higher southern reef is an effective filter
of gravity wave energy for most tidal elevations. Differences in the type and magnitude of physical processes within the atoll
are discussed with relation to geomorphic development on Cocos.
Accepted: 28 February 1998 相似文献
9.
Epibenthic macrofauna communities (species composition and densities of the dominant species) were investigated at Taiaro
Atoll, Tuamotu Archipelago, 22 years after a previous survey. This small atoll is completely closed, has no permanent functional
hoa, and is not affected by direct anthropogenic disturbances since it is a Biosphere Reserve. Ten species were identified
in 1994 (5molluscs, 4 corals, and 1 echinoderm). With the 14 species identified in 1972 (12 molluscs, 1 coral, and 1 echinoderm),
a total of 17 species (12 molluscs, 4 corals, and 1 echinoderm) has been recorded for the lagoon. There has been a shift in
dominance away from the bivalve Crassostrea cucullata (in 1972) to Pinctada maculata (in 1994). A high mortality of the epibenthic
macrofauna affected the central part of the lagoon shortly before1972 and reached the inner reef flat afterwards. The distance
of Taiaro from sources of colonizers, its small size, and the isolation of its lagoonal waters posing a physical barrier to
colonization by organisms and leading to harsh environmental conditions (e.g. very high salinities: 42.5–43 psu), are likely
to be the major causes of the exceptionally low diversity observed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
The skeletal composition of 273 sediment samples, collected within 14615 km 2 of lagoon habitat in New Caledonia (Ouvea and Chesterfield atolls and eastern and northern lagoons of the main island), was analyzed. Major constituents were molluscs (bivalves and gastropods), foraminifers, and Halimeda plates. The quantitative examination showed that, even in a pure coralline structure such as the two atolls studied, coral debris and calcareous algae, potentially produced within the barrier reef, never constituted a dominant element in the lagoonal sediments. Distribution of coral debris showed that coral is significant only close to the barrier reef (i.e. passes and back-reef slope). From the point of view of sedimentology, this suggests that the major role of the barrier reef is to provide a physical barrier that allows the development and preservation of lagoon sediments. Sedimentation within the lagoon of grains coarser than 63 µm is the result of in situ organic production combined with low hydrodynamic control. 相似文献
11.
Nui Atoll, an isolated atoll in Tuvalu, has a total vascular flora of 86 species, including a large number of introduced species. If only the 44 species considered indigenous are examined, there exists a strong linear relationship (r 2= 0.94) between the total number of species and the logarithm of island area, over the entire wide range of island sizes from 0.01 ha to 138 ha. This relationship is strong for both three species and herb species, but weak for shrub species for which there is a limited pool of potential colonists. This relationship is stronger than on other Pacific atolls and does not support the existence of a ‘small island effect’. The strong linear relationship on Nui is attributed to uniformity of reef island substrate, distribution and isodimensionality of reef islands, stability of islands and infrequency of catastrophic events and the absence of large monospecific stands, especially of shrub species, characteristic of drier atolls. 相似文献
12.
Cores from ten holes, drilled to a maximum depth of 30 m, on Tarawa atoll in the central Pacific have been utilised in a study of the Holocene development of the atoll. Four dominant lithologies, in descending order, are cay rock, unconsolidated sediment, corals and leached limestone. Petrographic and radiometric age analyses indicate that the Holocene reef has developed on a previous (last interglacial) reef; the latter shows the effects of both vadose and phreatic freshwater diagenesis. Hydrological investigations beneath the present islands indicate the presence of freshwater lenses up to 29 m thick; the modern lenses are unrelated to freshwater diagenetic imprints preserved within the limestones. Vertical accretion rates of 5–8 m/1000 years for the Holocene reef section on Tarawa are significantly higher than rates measured for other Pacific atolls. The dated coral sequences suggest a more rapid rate of sea level rise during the early Holocene, and a relatively earlier stabilisation of sea level than has been suggested previously. 相似文献
13.
Palmyra Atoll has four partially isolated lagoons up to 50 m in depth, each with complex and variable bottom topographies.
Measurements of depth, temperature, salinity, turbidity and dissolved oxygen (DO) revealed a well-mixed shallow surface layer
(0–10 m depth) and below that pronounced stratification of DO in the absence of a pycnocline. Turbidity increased in a step-like
manner at ~25 m depth, at the oxycline. For all deep sections of the lagoon (>30 m), DO declined uniformly to 0% saturation.
As determined from filtration, mass of particulates was independent of depth. Surface mixing and deep-water stratification
are both stable at different temporal scales, including day versus night, daily, weekly and annually. We suggest that lagoon
circulation is represented by a shallow, westward-moving surface layer of well-to-partially mixed water with high DO and low
turbidity, underlain by a relatively static and temporally stable layer with low to zero DO and elevated turbidity. This is
the first report of such conditions within a deep lagoon system, and only the second report of anoxic conditions in any such
system. In deep-water, stable euxinic conditions reflect bottom topography, with dysoxic and anoxic water being constrained
within silled basins. The occurrence and depth of large volumes of sediment-laden and dysoxic/anoxic water need to be considered
in management proposals designed to increase water flow through the lagoon. These novel water column conditions most probably
arose as a consequence of military construction work, consistent with published reports of profound changes to the atoll during
1940–1945. If so, they highlight the need to better understand the possible consequences of cutting channels and modification
of lagoon flow at many atolls across the central Pacific Ocean. 相似文献
14.
Epibenthic macrofauna communities (corals, molluscs, echinoderms, and macroalgae) were investigated at Hikueru Atoll, Tuamotu
Archipelago. The very low diversity and abundance that we observed (1 coral genus, and 2 mollusc, 1 echinoderm, and 17 macroalgal
species) is likely to be the result of a mass mortality event that occurred 18 months earlier. Hydrological surveys suggest
that this high mortality was the result of a major dystrophic crisis. Other atoll lagoons in the Tuamotu Archipelago have
also been affected by high mortalities in the past, but Hikueru appears to have been the most frequently and intensely affected.
The present study confirms that mass mortalities associated with phytoplankton blooms may be one of the major disturbances
affecting coral reef ecosystems.
Accepted: 14 May 2000 相似文献
15.
Christmas (Kiritimati) Island is an unusually large coral atoll, of which a large proportion of the surface is presently
subaerial. Extensive outcrops of in situ branching Acropora corals, together with Porites microatolls, Tridacna, and other shallow marine biota, indicate that the present low-lying area of interconnecting lakes in the island interior
formed as a reticulate lagoon. Radiocarbon dating indicates that these lagoonal reefs flourished between 4500 and 1500 radiocarbon
years BP, and surveying confirms that sea level was 0.5–1.0 m above present at that time, with subaerial exposure resulting
from Late Holocene emergence. Boreholes undertaken for a water resources survey of the island penetrated near-surface Pleistocene
limestones on the northern, southern, and eastern sides of the island. These are highly weathered and fractured, and although
aragonitic clasts are preserved, U-series dating indicates a Middle Pleistocene or older age. At one location flanking the
Bay of Wrecks, an outcrop of limestone, with an erosional notch, 1–2 m above present sea level, yielded a U-series age of
130 ka, and is interpreted as Last Interglacial in age. In contrast to previous interpretations which have suggested that
Christmas Island comprised an atoll superstructure that is entirely Holocene, or the layer-cake interpretation appropriate
for many mid-ocean atolls, Christmas Island appears to have had a form similar to its present in the Middle Pleistocene or
earlier. It has undergone karstification during lowstands. Interglacials, particularly the Last Interglacial and the Holocene,
appear to have resulted in only a minor veneer of coral over older limestone surfaces. Christmas Island is considered characteristic
of an atoll that has not experienced significant subsidence through the Late Quaternary.
Accepted: 15 May 1998 相似文献
16.
Benthic cover, current strengths, and fish abundance and diversity were examined on 150 lagoonal patch reefs and mapped to
determine their distribution, inter-relationships, and relationship to the fisheries closure in Glovers Reef Atoll. Current
strength was highest at both the northern and southern ends of the atoll and largely controlled by local wind and weakly by
tidal forcing. Benthic functional group distributions varied throughout the atoll and had distinct areas of dominance. In
contrast, dominance of coral species was weaker, reflecting the lost cover and zonation of Acropora, Porites, and Montastraea that were reported in the 1970s. Hard and soft corals dominated the windward rim, while the central and leeward lagoon had
lower current strengths and sea grass and fleshy green algae were relatively more abundant. Brown erect algae were relatively
more common in the north and calcifying green and red algae the southern ends of the atoll. Only Montastraea- Agaricia agaricites distributions were similar to reports from the 1970s with high relative dominance in the southern and northeast atoll. The
central-northern zone, which was described as an Acropora zone in the 1970s, was not recognizable, and Porites porites, P. astreoides, Millepora alcicornis, and Favia fragum were the most abundant species during this survey . Hard and soft coral cover abundance declined away from the reef rim and tidal channels and was associated with fast seawater
turnover and high surgeonfish abundance. Consequently, the windward rim area has retained the most original and persistent
hard-soft coral and surgeonfish community and is considered a priority for future management, if the goal is to protect coral
from fishing impacts. 相似文献
17.
This paper describes the results of a field survey designed to test the prediction that the density of benthic juveniles of
shallow-reef fishes is greater on wind-wave “exposed” sectors of a pair of isolated oceanic atolls (Kure, Pearl and Hermes)
at the far northwestern end of the Hawaiian Islands, an archipelago in which east-northeasterly trade winds dominate onshore
water flow and transport by surface currents. The densities of recruits (juveniles ≤5 cm total length) were higher overall
on windward versus leeward sectors of carbonate rock-rubble back reefs at both atolls, and the pattern was stronger for smaller
(likely younger, more recently settled) recruits of four of the five most abundant species and the remainder pooled as an
“Other” taxon. The windward-leeward disparity was four-fold greater at Pearl Hermes (the atoll with a three-fold longer perimeter)
than at Kure. Resident predator biomass also was correlated with recruit densities, but habitat (benthic substratum) effects
were generally weak. The distribution and abundance of recruits and juveniles of the primarily endemic reef fishes on shallow
back reefs at these atolls appear partly influenced by relative rates of water flow over windward vs. leeward sectors of barrier
reef and by the size, shape, and orientation of habitat parcels that filter out postlarval fishes with relatively weak swimming
capabilities like labroids. Whole-reef geomorphology as well as fine-scale habitat heterogeneity and rugosity should be considered
among the suite of many factors used to interpret observed spatial patterns of post-settlement juvenile fish distribution
at atolls and perhaps some other tropical reefs.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
19.
Measurements of interstitial waters in deep wells (0–500 m) in Mururoa and in shallow bores (0–35 m) in Tikehau (French Polynesia) have shown high contents of dissolved nutrients and relatively low salinities, implying a deep oceanic origin. Studies of specific thermal fields within these atolls and carbonate platforms have led to models of deep oceanic water circulation by convection resulting from upward geothermal heat flow. Cold, low-salinity, low-pH, high-CO 2, nutrient-rich deep oceanic water pervades the permeable atoll, where it loses density by heating related to geothermal heat flow, subsequently rising to seep out through the outer rim. New nutrients are thus continuously provided, enabling the reef-building community (algae+corals) to thrive in optimal conditions for photosynthesis and carbonate precipitation. This geothermal endo-upwelling concept is defined and its role in atoll trophic networks is emphasized as a necessary and sufficient process for reef net production and organic matter exportation. The endo-upwelled flow can then be viewed as a key factor for internal diagenesis, as early cementation of reef framework and dolomitization of deep limestone. Generalization of this interstitial, deep seawater circulation is discussed and linked to previously described thermal convection models in Florida or raised atolls. 相似文献
20.
The fully enclosed Taiaro lagoon is hypersaline (42.5 psu) and non-tidal; constant salinity and water level result from strong
evaporation balanced by low percolation through the lagoon floor. Seawater can flow over the atoll rim during exceptionally
high seas and may then replenish lagoonal communities with propagules of oceanic origin. The distinctive water chemistry of
the lagoon suggests a possible way of identifying these immigrants. We established this potential by analysing stable isotopes
of carbon and oxygen in the recent growth layers of otoliths of two adult reef fishes, Chaetodon ulietensis and Acanthurus triostegus, collected from both sides of the atoll rim. Fish from the two locations were discriminated by their isotopic signatures,
suggesting that analysis of the microchemical signatures deposited during the larval development could be used in future work
to determine which individuals and species complete their life-cycles in this unusual lagoon.
Accepted: 28 August 1997 相似文献
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