Normal High Tide Research Articles

Optimizing the spatial distribution of Southeast Asia mangrove restoration based on zonation, species and carbon projection schemes

Mangroves contain large amounts of carbon that can reduce the effects of global climate change, which is termed as blue carbon and noted as an important ecosystem service. However, mangrove ecosystems have experienced a huge decline due to human activities and any rehabilitation and restoration efforts should be undertaken with careful planning to optimise their ecosystem service provision and to improve the success of these programs. The main objective of this study was to model a scenario specific to Southeast Asian mangrove distribution based on zonation, species, and coastal morphology, as well as the estimation of future aboveground biomass carbon content resulting from multiple scenarios of successful mangrove growth. This study provides the first assessment of the differences in the distribution of carbon content in a range of native mangrove species, as well as the relationship between the number of propagules, carbon content, mangrove species, and the percentage of living mangroves. The potential location for mangrove planting was dominated by zone 4 inundated by spring high tides, with a suitable area of 4,081,325 ha the next largest area was zone 2, inundated by medium high tides, with a potential area of 2,080,475 ha, followed by zone 3, inundated by normal high tides, with an area of 1,685,800 ha. This study is expected to serve as a reference for mangrove ecosystem conservation and optimisation of carbon storage, especially in Southeast Asia.

The King Tide Conundrum

ABSTRACT Roman-Rivera, M.A. and Ellis, J.T., 2018. The king tide conundrum. Over the last decade the concept of “king tides” has become a common colloquial term to describe higher than normal high tides. The terminology originated in 2009, when Australia experienced their highest seasonal tides in almost 20 years. In response, a public engagement program, “The King Tides Project,” was formed that has since spread globally. Discrepancies regarding the definition and frequency of king tides can be identified between the different initiatives. This communication analyzes the multifaceted and contradictory meanings of king tides and concludes by providing a holistic and comprehensive definition.

Small-island communities in the Philippines prefer local measures to relocation in response to sea-level rise

Most adaptation studies suggest that sea-level rise will lead to relocation as flooding worsens. Here we identified and evaluated potential adaptation strategies for adapting to sea-level rise, based on the experiences of four low-lying island communities in central Philippines that have experienced flooding during normal high tides since a 2013 earthquake that induced land subsidence. Coastal surveys, interviews and household questionnaires showed that island residents generally prefer in situ adaptation strategies rather than relocation to the mainland. These results are unexpected, particularly because a relocation programme has been developed by authorities on the mainland. Direct measurements during a flooding event indicate stilted housing as the most effective type of adaptation strategy. Many households have also raised their floors using coral stones, although this might inadvertently increase their vulnerability to typhoons and storm surges in the long-term. It is often assumed that increased flooding due to sea-level rise will lead to mass migration. However, this study shows that residents of island communities in the Philippines prefer to implement local adaptation measures in response to frequent severe flooding rather than relocate.

Oxygen and pressure changes measured in situ during flooding in roots of the Grey Mangrove Avicennia marina (Forssk.) Vierh.

Manometers and oxygen electrodes were attached to the root system of mangroves grown in pots flooded to soil level. Measurements of internal pressure and oxygen concentration were made at the tip and middle of cable roots and on a pneumatophore, in artificial tidal conditions in a constant temperature growth-room. At the end of a period of ‘low tide’ there was a substantial oxygen gradient within the root system, from 6.41 ± 0.26 mol m −3 at the middle of the cable root to 4.81 ± 0.37 mol m −3 at the cable root tip (means and standard errors, n = 8). All oxygen concentrations fell during ‘high tide’ when the whole plant except the shoot was submerged, and the internal gradients gradually diminished, until by the end of a 6-h flooding the concentration throughout was in the range 4.59 ± 0.37 mol m −3 to 2.82 ± 0.41 mol m −3. Concentrations and gradients recovered over a few hours after unflooding. The oxygen concentrations measured in the roots fell among those previously published. Pressure measurements within the root system showed pressures similar to atmospheric or slightly positive (with respect to atmospheric) throughout the root system at ‘low tide’, a rapid decline in pressure reaching −1.0 to −1.7 kPa after flooding, with a slow partial recovery to −0.8 to −0.4 kPa when flooding lasted much longer than normal high tide, and finally a rapid return to atmospheric pressure on unflooding. We suggest that the above-atmospheric pressure at ‘low tide’ is consistent with the possibility of thermo-osmotic pressurisation, and further tracer experiments are needed to test whether this would result in significant gas exchange by mass flow. The low pressure during ‘high tide’ is likely to result from removal of respiratory carbon dioxide from the gas space: it would be likely to result in an influx of air through the exposed shoot, and a brief rapid influx through the pneumatophores on unflooding.

Marsh Vertical Accretion in a Southern California Estuary, U.S.A.

Vertical accretion was measured between October 1992 and March 1994 in low and high saltmarsh zones in the north arm of Tijuana estuary from feldspar market horizons and soil corings. Accretion in the Spartina foliosalow marsh (2–8·5 cm) was related almost entirely to episodic storm-induced river flows between January and March 1993, with daily tidal flooding contributing little or no sediment during the subsequent 12-month period of no river flow. Accretion in the Salicornia subterminalishigh marsh was low (≈1–2 mm) throughout the 17-month measuring period. High water levels in the salt marsh associated with the storm flows were enhanced in early January 1993 by the monthly extreme high sea level, when the low and high marshes were flooded about 0·5 m above normal high tide levels. Storm flows in January–March 1993 mobilized about 5 million tonnes of sediment, of which the low salt marsh trapped an estimated 31 941 tonnes, including 971 tonnes of carbon and 77 tonnes of nitrogen. Sediment trapping by the salt marsh during episodic winter floods plays an important role in the long-term maintenance of productivity of Tijuana estuary through nutrient retention and maintenance of marsh surface elevation. The potential exists, however, for predicted accelerated rates of sea-level rise to out-pace marsh surface elevation gain during extended periods of drought (i.e. low sediment inputs) which are not uncommon for this arid region.

Decline in a Population of Spectacled Eiders Nesting on the Yukon-Kuskokwim Delta, Alaska

The number of spectacled eiders (Somateria fischeri) nesting on two study areas near the Kashunuk River, on the central Yukon-Kuskokwim (Y-K) Delta, Alaska, declined by over 75% in the last 20 years. Nesting densities have remained low, but have not significantly declined since 1985. There has been no decrease in the reproductive effort of individual females as indicated by average clutch sizes. There has been a significant decline in the proportion of nests located on islands on one of the two study areas. Nesting success declined significantly during the 1970's. Success was not monitored in recent years, but has likely been low, based on the poor nesting success and declining numbers of cackling Canada geese (Branta canadensis minima) and black brant (B. bernicla nigricans) nesting on the area. Nest predation by arctic foxes (Alopex lagopus) severely limited the productivity of cackling Canada geese, and foxes were likely the major predators of eider nests. Persistent high predation rates may lead to local extirpation in highly philopatric species such as eiders. The spectacled eider (Somateria fischeri) breeds along the coasts of the Bering and Chukchi seas in western and northern Alaska, and northern and eastern Russia (Bellrose 1980). The population on the Yukon-Kuskokwim (Y-K) Delta is in decline (Stehn et al. 1993), and is listed by the U. S. Fish and Wildlife Service (1993) as a threatened species. Evidence for the decline is based on changes in the number of eiders counted during aerial surveys, and current estimates of nesting density (Stehn et al. 1993), relative to an extrapolated estimate derived from the number of eiders nesting on small study sites on the Y-K Delta in the early 1970's (Dau and Kistchinski 1977). Here we present evidence of a long-term decline in the number of nesting eiders based on changes in the number of birds on two study sites monitored for up to 20 years. STUDY AREA AND METHODS The study areas were located in the Yukon Delta National Wildlife Refuge, along the Kashunuk River on the central Y-K Delta, Alaska (61°26'N, 165°27'W), approximately 20 km upriver from the Bering Sea (Fig. 1). Habitat at the sites is described in detail elsewhere (Ely and Raveling 1984, Babcock and Ely 1994). Both study areas were covered with numerous small lakes, ponds, and tidal sloughs and dominated by moist tundra vegetation (mixed sedges and grasses) interspersed with intermediate and upland dry tundra (lichens, mosses, and dwarf shrubs). Portions of the Old Chevak study site were slightly higher in elevation and more inland than the Onumtuk site, and were characterized by more intermediate and upland habitat. Habitat used by nesting spectacled eiders on the Y-K Delta lies within an estimated 9300 km2 of wet sedge/grass meadows categorized as the vegetated intertidal zone (King and Dau 1981). This zone is essentially the lowland terrain between the outer edge of vascular plant growth (i.e. normal high tide line) and the maximum inland extent of storm surges identified by the driftwood line. All of the Onumtuk area, and the southern and eastern portions of the Old Chevak area were within this vegetated intertidal zone. Eider work on the 11.1-km2 Onumtuk study area began in 1969 (Mickelson 1975), and was continued through 1973 (Dau 1974). In 1975, two 2.8-km2 sites were selected within the original study area. Investigations at these plots were conducted by CPD untiI 1980; refuge biologists lead research activities there untiI 1985, when CRE took over the work. The 12-km2 Old Chevak study site is adjacent to the Kashunuk River just north of the Onumtuk plots. The Old Chevak plots were initially searched in 1977 and 1978, when CRE was conducting work on nesting geese (Ely and Raveling 1984). Field methods used here were exactly the same during all years. Although we did

Geomorphological effects of catastrophic flooding on a small subtropical Estuary

AbstractThe Mgeni Estuary is situated on the east coast of South Africa. Tidal salinity changes commonly extend 2.5 km upstream from the mouth. The subtropical climatic regime causes seasonal flooding, the geomorphological effects of which are increased by a steep river gradient. A 120‐150 year flood event during September 1987, with an estimated peak discharge of 10800 cumecs, resulted in the erosion of 1.86 × 106 m3 of sediment from the estuary. The normal high tide volume of the estuary (0.35 × 106 m3) increased to 1.85 × 106 m3 after the flood. The post‐flood intertidal volume (tidal prism) of the estuary was 0.43 × 106 m3 compared to 0.19 × 106 m3 before the event.The nature and extent of erosion in the estuary was controlled by the composition of the estuary banks and bed material. Gravel and sand substrates were preferentially eroded from the estuary bed before the cohesive bank materials were undercut in sections of the estuary. Mud and fine sand were deposited on overbank areas when the flood waned and a lag of coarse sand and gravel was produced on the estuary bed. Approximately 46 per cent of the bedload sediment supplied to the estuary since 1917 was retained in the estuary until the 1987 flood. The remainder was transported through the system and into the Indian Ocean. Results show that catastrophic floods play an important role in sedimentation in small estuaries.

Holocene coastal deformation in Simeulue and Nias, Indonesia

West of Sumatra (Indonesia), the islands of Nias and Simeulue as well as their smaller neighbours display elevated coral reefs and abandoned intertidal platforms. Faunas from three sections indicate a late Cainozoic age for the reefs. Unaltered mollusc shells from nineteen fossil platforms were dated by first-order 14C analysis. The localities sampled were 0.5 to 3.5 m above normal high tide. Apart from one non-finite result, the ages ranged between modern (< 300 yrs and 5800 ± 300 yrs B.P. The resulting average uplift rates are 0.3–1.0 mm/yr. No simple relationship between sample age and height is apparent but the bulk of the ages fall within three periods, 0–800 yrs B.P., 2000–3400 yrs B.P. and 4500–5800 yrs B.P., suggesting that episodes of uplift were separated by periods of quiescence and that they were seismic in origin. The pattern of deposition at the key section on Nias supports this inference. Evidence of recent subsidence in eastern Simeulue shows that uplift was locally accompanied by tilting. The pattern and rate of movement are consistent with the position of the islands on the outer-arc ridge of the Sunda Arc and with the long-term uplift rates derived from palaeontological sources. The “inverse Darwin” coral reef model proposed here for the development of reefs on the flanks of rising Neogene cores may also be applicable elsewhere.

Underground Vertical Distributions of Macrofauna and Root in a Mangrove Forest of Southern Thailand

In a mangrove forest of southern Thailand, underground vertical distributions of macrofauna and root biomass were investigated to a depth of 1 m. While smaller roots were distributed more or less evenly, larger roots were restricted to shallower layers. Animals were more abundant in shallower layers, but some of them penetrated into the deepest. Relation between the distribution of animals and that of larger roots is discussed. Cross-shore and for along-shore distributions of the benthic macrofauna in mangrove swamp have been studied quantitatively in various parts of the world (e.g. Warner, 1969; Day, 1974; Sasekumar, 1974; Frith et al., 1976; Shokita et al., 1983; Shihe & Fuxue, 1985). But the quantitative data on the underground vertical distribution of it have not so far been presented. One of the reason is probably that matted roots of mangrove trees make it difficult to dig the substrate deeper. In the temperate salt marsh habitat, the root mat density of the plants has been known to play an important role in controlling the distribution of fiddler crabs (Ringold, 1979; Bertness & Miller, 1984). Likewise, the underground root system of mangrove forest is assumed to influence more or less the distribution of the burrowing animals there. In this paper, we report the vertical distribution of benthic macrofauna in relation to the root biomass in a mangrove forest of southern Thailand. Study site and methods The study site was located in a mangrove forest at Hatsaikhao village near Ranong (9°58'N, Publ. Seto Mar. Biol. Lab., 32 (4/6), 329-333, 1987. (Article 12) 330 K. WADA, A. KOMIYAMA & K. OGINO 98°38'E), southern Thailand. In December 1982, a plot of 15m X 20m was established about 50 m landward from the seaward fringe of the forest. The plot area was not flooded at normal high tides and the substratum was sandy mud, with scattered puddles in places. The temperature and salinity of the water in a puddle measured at 10:00 on 1 Dec. were 27.0°C and 27.28%0, respectively. The dominant tree species in the plot was Rhizophora apiculata Blume with only a few smaller trees of Bruguiera gymnorrhiza (L.) Lam. and B. cylindrica (L.) Blume. The maximum diameter and height of the trees were 44.5 em and 37.2 m, respectively. The basal area totalled 31.3 m2/ha and the stand density was 733 treesjha. Shrub measurements were 31500/ha in the density, 7.5 m2jha in basal area, 2.4 em in mean diameter and 3.1 min mean height. A trench (0.2 m X 15.5 m) was laid out in the plot between two big trees of R. apiculata (diameter: 44.5 and 43.6 em). Fresh weight of prop roots/pneumatophores above the ground of the trench totalled 63.8 kg. The water table of the trench was 28-30 em deep and the temperature and salinity of the underground water were 25.0°C and 22.98%a, respectively (10: 10 on 9 Dec.). The trench was subdivided into 31 compartments, each 0.5 m long and each compartment was further subdivided vertically into 10 layers of soil blocks to a depth of 1.0 m. The 310 soil blocks, each 0.2 m (width) X 0.5 m (length) X 0.1 m (depth), were cut by a hand saw from the smoothed wall of the trench. Each soil block was sieved by a net with I mm opening to collect living roots and animals. Living roots were sorted into eight diameter classes ( 50 mm) and weighed fresh. Animals were fixed in 10% sea-water formalin and the number of individuals of each species was recorded for each soil block.

Effects of Storms on Rice Rats Inhabiting Coastal Marshes

During 1979, three storms with winds in excess of 73 km/hr and tides up to 1.5 m above normal struck a section of the Mississippi coast where a population study of the rice rat, Oryzomys palusrris, was in progress. There were no noticeable effects on population levels, and many individual animals survived the storms. INTRODUCTION The rice rat, Oryzomys palustris, is a semiaquatic mammal that occurs in wetland habitats throughout the southeastern United States. Throughout most of its range, 0. palustris is the most abundant small mammal in coastal marshes. It is also the most common mammal on many offshore islands in the northern Gulf of Mexico (Negus et al. 1961; personal observation). The tidal marsh and insular habitats of rice rats are occasionally subjected to devastating storms; and yet the rats appear either to survive these storms or to rapidly recolonize immediately following them. MATERIALS AND METHODS Since January 1979 I have been monitoring a rice rat population on the northern shore of St. Louis Bay in Harrison County, Mississippi. Two study grids about 1 km apart have been live-trapped quarterly. These grids have trapping stations placed at 15-m intervals. A 300-mm square floating platform that rises and falls with the tides is placed at each station. Each sampling session consists of one day of prebaiting the platforms with a mixture of oatflakes, peanut butter and sardines followed by four or five days of live-trapping. One Sherman live-trap is used on each platform. All animals captured are weighed, sexed, checked for reproductive condition, marked by toe clipping, and released. The westemmost grid is 12 stations long and 4 stations deep. Its long axis parallels shore. Water depth on its southern edge is about 400 mm at normal high tide. The northern edge parallels the marshforest ecotone and it is dry at normal high tide. The other grid consists of 100 stations in a 10 x 10 pattern. It extends from the forest-marsh ecotone for 135 m out into the marsh. Although the grid is about 0.5 km from shore, the southem half is flooded to a depth of about 300 mm daily by overflow from tidal creeks in the area. In the spring and summer of 1979 three storms had an impact on the area. A storm in mid-April produced a tide of 1 m above normal and winds gusting to about 40 knots. Manuscript received July 29, 1981; accepted September 30,1981. Hurricane Bob struck the area on July 1 1 with a storm tide of 1.5 m and winds up to 64 knots. On August 12, the western edge of Hurricane Frederick passed through with 74-knot winds but no storm tides, as the winds were from the north. The April storm and Hurricane Bob completely inundated both grids with a tidh surge that swept away all the trapping platforms and deposited them in the forest, 50-100 m to the north. The duration of these storm tides was 2-4 hrs.

Tree and Shrub Establishment and Survival on Sandy Dredged Material

Tree and Shrub Establishment and Survival on Sandy Dredged Material

Bethany Falls Limestone (Missourian) Sedimentation and Diagenesis, Missouri and Kansas: ABSTRACT

The Bethany Falls Limestone is a 15-30-ft-thick shelf deposit within lower Missourian rocks of the Mid-Continent region. In outcrop the lower Bethany Falls is typically an open-marine, faunally diversified, carbonate wackestone. The upper Bethany Falls consists of a wide variety of shallower water carbonate mudstone, wackestone, and grainstone. Cross-bedded oolitic grainstone and faunally restricted mudstone are perhaps the most distinctive rocks in the upper Bethany Falls. Cross-bed dip azimuths in the oolitic facies provide evidence that deposition resulted from 2 dominant current systems, one basically longshore drift toward the northwest and another essentially onshore tidal flow toward the north-northeast. The sequence of open-marine wackestone succeeded by shallower water limestone resulted from rapid transgression. As progradation built the sea floor into the zone of agitation, oolitic sand developed. These bars restricted circulation in many interbar and back-bar areas which became sites of the faunally restricted carbonate mud. Carbonate deposition ceased when progradation built to normal high tide. Color mottling, ubiquitous in most upper Bethany Falls mudstone and wackestone, is a diagenetic effect related to selective bleaching of the original medium-gray or grayish-tan color. The bleaching apparently follows areas of initially high permeability. The controlling paths are commonly burrows or churned areas, crumbly vugs resulting from bioturbation and other processes, and microfractures formed by slumping and shrinkage. Grain diminution or removal commonly accompanies the bleaching, and many of the bleached areas exhibit microslump and convergence of laminae. End_of_Article - Last_Page 869------------

Anatomy of a Modern Carbonate Tidal-flat, Andros Island, Bahamas

ABSTRACT The carbonate tidal-flat complex of northwestern Andros Island consists of three geomorphic areas: (1) adjacent marine belt, (2) channeled belt, and (3) marsh. The various sedimentary accumulations within these belts are controlled by tide levels. Three tidal zones are recognized: (1) subtidal zone, consisting of sediments deposited below low tide (adjacent marine, channels, ponds); (2) intertidal zone, consisting of sediments deposited between normal low and normal high tide (intertidal-flats); and (3) supratidal zone, consisting of sediments deposited above normal high tide but within the range of spring and storm tides (beach ridges, levees, marsh). The complex may be thought of as a river delta turned inside out. Most sediments that make up this delta have been transported via c annel systems from the adjacent marine zone. Early diagenesis in the form of reduction, oxidation, dolomite formation, and carbonate cementation is also controlled by tide level. Strong odor of H2S in gray subtidal sediments suggests reduction, whereas the absence of this odor in the cream or light tan intertidal and supratidal sediments suggests oxidation. Dolomite formation and carbonate cementation are restricted to the distinctively laminated, algal-coated, and mud-cracked supratidal sediment. Stratigraphically, the channeled tidal-flat complex on northwest Andros Island is transgressive, whereas the nonchanneled flats on the southwestern coast are thought to be regressive. Alternation of transgressive and regressive tidal-flat sedimentation during subsidence or eustatic sea level rise could produce stratigraphic pinch-outs that may also become stratigraphic traps. Although such stratigraphic traps would parallel land, preserved porous channel deposits may produce perpendicular trends within them.

Practical Significance of Birdseye Structures in Carbonate Rocks

ABSTRACT Two kinds of birdseye structures are recognized in Recent and ancient carbonate rocks: (1) planar isolated vugs 1 to 3 mm high by several millimeters in width and (2) isolated more or less bubblelike vugs 1 to 3 mm in diameter. Both types are generally filled with calcite or anhydrite. Study of more than 100 cores and samples of Recent carbonate sediments showed that birdseye voids are preserved in supratidal sediments (sediments deposited above normal high tide level), sometimes in intertidal sediments (sediments deposited between normal high and normal low tide), and never in subtidal sediments (sediments permanently below water). Both kinds of birdseyes are particularly abundant in supratidal dolomitic sediments. Laboratory experiments and observations in limestone quarry tailings suggest that bubblelike vugs were made by gas bubbles and that planar vugs were made by shrinkage resulting from desiccation of exposed sediments. Birdseye vugs may be valuable indicators of depositional environment, especially when they occur in carbonate rocks containing no depositional structures.

Selective Dolomitization of Recent Sedimentary Structures

ABSTRACT Relatively non-dolomitic limestone nodules which float in lithified dolomitic sediment are forming on a supratidal mud flat in the lower Florida Keys. Storm tides periodically deposit layers of lime mud above normal high tide level which dry and crack to form typical mud-crack polygons. These polygons erode into flattened nodules that subsequently become buried in relatively more porous and permeable sediment. Magnesium-enriched brines concentrated through evaporation are more readily transmitted through the more permeable sediment than are the less permeable nodules. Selective dolomitization of the more permeable sediment produces features bearing a strong similarity to sedimentary structures commonly found in ancient dolomitic rocks.

Penecontemporary Dolomite in the Persian Gulf: ABSTRACT

Carbonate sediments dominate the shallow waters along the arid southwest side of the Persian Gulf. In the more protected parts of the west coast of the Qatar Peninsula, the processes of near-shore sedimentation have created lagoons and embayments with high chlorinity (30-35 g/l) and reduced tidal range; they are separated from the normal Gulf waters (22-24 g/l) with their average 4-foot tides, by many miles of sea less than 2 fathoms deep. The lowest parts of the lagoonal shores are fringed by salt flats--sebkhas--varying in width from a few tens of yards to several miles. The sebkhas pass seaward into the intertidal zone, commonly via an intermediate algal flat. This is just covered by normal high tides, but only with favourable winds can very occasional spring tides reach far onto the sebkha surface which is a few inches higher. Sedimentation is gradually filling the lagoons by the seaward advance of the environmental belts, so that sebkha sediment overlies stromatolitic algal laminae, and these are underlain by intertidal mud-pellet sands resting on lagoon muds. The chlorinity of the pore waters increases landward and upward in response to surface evaporation losses. It increases rapidly within the algal flat (50-130 g/l), where small selenite crystals form beneath the higher, landward parts. Together with the continuing precipitation of aragonite, this causes an increase in the Mg/Ca ratio of the pore waters from the normal marine value of 3 in the lagoon to more than 10 at the sebkha edge. Within the sebkha, the ratio falls gradually to below 5, while the chlorinity continues to rise slowly to more than 150 g/l. The water table is close to the sebkha surface, and, beneath the uppermost layer subject to large daily temperature changes, the wet sediment reaches well over 40°C. in summer time. Its pH is low (around 6.7) and decreases down ard. These warm magnesium-rich brines cause diagenetic changes in the aragonite sebkha sediment. Dolomite appears. It occurs as a stiff, sticky, tan or tan-gray mud composed of rhombs 1-5 microns in size. Associated with it are turbid flattened crystals of gypsum up to 5 inches across, enclosing, displacing and sometimes replacing aragonite sediment. Depositional textures tend to become obscured, but both macro- and microscopic evidence of relic structures and the changing chemistry of the pore waters make it clear that both the dolomite and the associated coarse platy gypsum are replacing aragonite. They increase in abundance away from the lagoon until they make up the bulk of the sebkha sediment. The dolomite normally appears a few inches beneath the surface, increases rapidly, and almos disappears again in a more irregular fashion within a depth of 2-4 feet. Carbon-14 determination on two dolomite samples collected within 9-18 inches of a sebkha surface gave End_Page 532------------------------------ ages of 2,670 and 3,310 years, confirming that the dolomitization is a penetemporary phenomenon related to the present sedimentary environment. End_of_Article - Last_Page 533------------

Geologic Record of Hurricanes: ABSTRACT

The passage of Hurricane Donna (September 9-10, 1960) across south Florida, an area where details of the pre-storm sea floor were well known, enabled the recognition of widespread storm effects. The recognition of these storm effects led to the conclusion that a significant part of marine sedimentation records the geologic work of large storms. Specific storm effects were as follows: (1) formation of large quantities of boulder-size rubble by the action of surf on corals at the platform edge; (2) transportation of all sizes of material away from the open sea toward the platform interior; (3) stranding of layers of carbonate mud over wide areas on the supratidal flat (above the normal high tide line). The amount of boulder-size rubble formed by hurricane surf on platform-edge reefs far exceeds the amount produced by day-to-day processes of death and deterioration. Each large storm adds an increment to the building of the rubble accumulations. The large extent of supratidal flats is due to the ability of storm tides to strand sediment over large areas, the inaccessibility of the supratidal flats to processes that could rework its sediment into adjacent marine environments, and the supply of the supratidal flat sediments at the expense of adjacent marine sediments that compete with it for a place in the geologic record. These factors help explain the large amount of supratidal sediments in some ancient rocks. Interesting negative aspects of hurricane's effects were that: (1) mound-type accumulations of muddy sediment were not eroded by storm wave or tidal currents, and (2) the sediment-laden waters resulting from the stirring of bottom sediments into suspension by the storm waves and currents did not give rise to effective turbidity currents. End_of_Article - Last_Page 349------------

Prehistoric Fishing Methods in South Africa

In the course of many years of archaeological research in South Africa, cut off by the length of the continent from the European scene, various local problems, unrelated to those of Europe, necessarily arise. One of the most persistent has been that of the origin and age of coastal fishing enclosures whose distribution extends sporadically from the mouth of the Berg river in St. Helena Bay (northwest of Cape Town) to Kosi Bay on the Zululand coast, and perhaps even further in either direction. The known coastal range is well over 1000 miles. Any complete solution of the various problems raised must necessarily be shelved until petrol supplies permit the careful excavation of a series of surrounding midden deposits.The enclosures are known locally as vywers (pronounced much as the English ‘favours’) or sometimes as viskraals, literally dams, or fish-corrals, and we can speak of them in English as tidal fish-traps. Happily their use continues, in a somewhat sporadic way, at several of the sites, and we can still study the methods used. In brief, the enclosure is built up as a dry-stone wall to such a height that normal high tides will cover the vywer and permit fish to swim freely over the enclosed area. As the tide sinks, the water runs out through the interstices between the stones, while the fish are trapped and can be clubbed and caught by hand. It is a simple application of the system used for netting fish, though here the dry-stone walling provides the imbrications.