Km Of Water Research Articles

Head waves in the Yellow Sea

Signals akin to head waves were obtained from explosive sources during the joint U.S. and China Yellow Sea experiment, conducted in August 1996. The sources were deployed at ranges from 0.7 to 37 km in waters approximately 75 m deep. Data were recorded on a 16-element vertical line array with element spacing of 4 m and the deepest element at depth 66 m. This talk will focus on measurements obtained at source ranges less than 1 km, because the amplitude of the head wave arrivals decreases rapidly with range. The arrival time and power spectra were first used to identify and distinguish ground-, water-, and head-wave arrival types. This latter category was then further studied in the context of pure head waves, noninterfering head waves, and interference head waves. The experimental data are also compared with simulated signals obtained via a Fourier synthesis of a narrow-band complex parabolic equation (PE) field, using the RAM PE code. The overall influence of compressional velocity, velocity gradient, and attenuation in the sediment, and frequency dependency of head wave arrivals are discussed. [Research supported by ONR Ocean Acoustics.]

High Frequency of Micronuclei in Buccal Mucosa of Women Residing Near a Sewage Disposal Drain in Amritsar

Water resources the world over are depleting as well as being contaminated by industrial pollutants sewage waste pesticides and fertilizers. In India 4000 cubic km of water is received annually from precipitation. Only half of it is left on land for use and rest is lost through evaporation and plant transpiration. India has 1100 cu. Km of utilizable surface water sources and nearly 900 cu km of groundwater. The surface water has a continuous exchange with atmosphere of dissolved atmospheric gases and can also get polluted by water run-off industrial discharge and municipal waste. (excerpt)

Mars' upper atmosphere and ionosphere at low, medium, and high solar activities: Implications for evolution of water

Self‐consistent models for 11 neutral and 18 ion species from 80 to 300 km on Mars have been developed by solving the continuity equations including ambipolar diffusion for ions. The models were calculated for the conditions of the HST, FUSE, and Mariner 6 and 7 observations of D, H2, and H, respectively, when the solar activity index was equal to 25, 61, and 88 at Mars orbit, respectively. Special care was taken to simulate the processes of H2 and HD dissociation in the reactions with CO2+, O+, CO+, N2+, N+, Ar+, and O(1D) and by photoelectrons. Thermal and nonthermal escape velocities were used as the upper boundary conditions for H2, H, HD, D, and He. The H2 and HD mixing ratios of 15 ppm and 11 ppb chosen to fit the FUSE and HST observations of H2 and D, respectively, result in (HD/H2)/(HDO/H2O) = 0.4. This value agrees with the depletion of D in H2 because of the smaller HDO photolysis cross section and the preferential condensation of HDO above the condensation level. Therefore the controversial problem of deuterium fractionation is solved throughout the atmosphere. The influx of cometary water was ≈0.5 m planetwide in the last 3.8 billion years. It cannot affect the estimates of more than 30 m of water lost by sputtering and nonthermal and thermal escape and more than 1.3 km of water lost in the reaction with iron with subsequent hydrodynamic escape of H2. The calculated ion density profiles at various solar activity and the column reaction rates provide complete quantitative information for behavior of each ion, its formation, and its loss. The HCO+ ion is abundant in Mars' ionosphere because it is a final product of many reactions of other ions with H2 and does not react with neutral species.

Propagation of muons and taus at high energies

The photonuclear contribution to charged lepton energy loss has been reevaluated taking into account DESY HERA results on real and virtual photon interactions with nucleons. With large ${Q}^{2}$ processes incorporated, the average muon range in rock for energies of ${10}^{9} \mathrm{GeV}$ is reduced by only 5% compared with the standard treatment. We have calculated the tau energy loss for energies up to ${10}^{9} \mathrm{GeV}$ taking into consideration the decay of tau. A Monte Carlo evaluation of tau survival probability and range shows that at energies below ${10}^{7}--{10}^{8} \mathrm{GeV},$ depending on the material, only tau decays are important. At higher energies the tau energy losses are significant, reducing the survival probability of the tau. We show that the average range for tau is shorter than its decay length and reduces to 17 km in water for an incident tau energy of ${10}^{9} \mathrm{GeV},$ as compared with its decay length of 49 km at that energy. In iron, the average tau range is 4.7 km for the same incident energy.

Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece

The Kos Plateau Tuff (KPT) is dominated by a sequence of nonwelded rhyolitic ignimbrites, some of which were deposited more than 60 km from source on islands in the eastern Aegean sea, Greece. Palaeo-shoreline reconstruction at the time of the KPT eruption (161 ka) suggests that northward-directed pyroclastic flows travelled over an extensive landmass, although the proximal zone may have been sea. In comparison, flows directed to the southeast and east most likely crossed wide expanses of sea, depositing ignimbrite on islands and peninsulas. The ignimbrites include basal coarse lithic-rich breccias overlain by thick, massive pumiceous zones and generally have low abundances of fine ash. They are interpreted to have been deposited from pyroclastic flows that were relatively dilute and stratified, with lower, denser, lithic-rich zones and upper, expanded, pumice-rich zones. High-velocity inherited from the intensity of the eruption, combined with turbulence and low bulk density, favoured transport across water. The pyroclastic flows remained hot even after crossing 35 km of the sea. Heat capacity equations indicate that a depth of ~1 cm of water ingested over the outflow path would have cooled the flows by only a few degrees. The ignimbrites deposited prior to the eruption climax have distributions restricted by 300-m-high topographic barriers and they thicken in topographic depressions. These ignimbrites show significant decreases in grain size, proportion of lithic clasts and thickness with distance from source. In comparison, during the eruption climax, the coarsest, most voluminous and widespread KPT ignimbrite was deposited. This ignimbrite has relatively even thickness and uniform componentry and texture regardless of topographic relief or distance from source. Textural analysis indicates that crossing 35 km water to the southeast had little effect on the pyroclastic flow generated during the eruption climax, except for the loss of the coarsest (>20 cm) vent- and conduit-derived lithic clasts. Preferential loss of lithic clasts is related to the mechanism by which they were transported. On land, large, dense clasts could be saltated over the ground surface at the base of the pyroclastic flow. In the case of transport over water, these large, dense clasts sank and were permanently removed from the transport system. Outflow across water had a greater effect on the pre-climax pyroclastic flows. The ignimbrites formed at this stage are significantly thinner and slightly finer grained, and lack basal lithic breccias and lithic clasts >5 cm where the pyroclastic flows crossed the sea, reflecting the lower velocities of the flows. All KPT ignimbrites to the east of the source were deposited from pyroclastic flows that crossed >25 km of water. Ignimbrites in this sector are markedly finer grained, possibly as a result of blocking of coarser pyroclasts by a topographic barrier close to source.

Subduction, platform subsidence, and foreland thrust loading: The late Tertiary development of Taranaki Basin, New Zealand

Borehole, seismic, and gravity data are used to investigate deformation of continental lithosphere at a Miocene collisional zone. Deformation is manifested in the three following principal forms: a long wavelength (>500 km) platform subsidence ascribed to mantle convection; flexural deformation on a scale of 100–200 km due to crustal thrusting at the eastern boundary of the Taranaki Basin; and a ductile thickening, evident on the deep seismic section of Taranaki Basin, that occurs on a scale of ∼10 km. Evidence for flexural deformation principally comes from the deep seismic section that shows a 150‐km wavelength bending of the Moho down toward the major zone of thrusting within the Taranaki Fault Zone. Paleowater depths, however, provide evidence for an initial early Miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading. Instead, we propose that this broad "platform subsidence" was driven by loading from a deep source, probably subduction‐induced flow in the mantle. By ∼22–19 Ma, 1–2 km of water existed over most of the area now occupied by South Taranaki Basin. By ∼19–17 Ma the water depth in the zone east of the Taranaki Basin, the Taranaki Fault Zone, had been replaced by rock due to submarine thrusting and crustal thickening. This build up of submarine topography in the Taranaki Fault Zone constitutes part of the load (25±8 MPa) that created and maintains South Taranaki Basin. Gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading, equivalent to 15±7 MPa over a 50‐km‐wide zone. This intracrustal load is explained as being due to thick‐skinned thrusting bringing denser, lower‐crustal rocks nearer to the surface in the thrust zone. The complete load on the Taranaki foreland is therefore in three parts; the submarine‐topographic load, the intracrustal load, and the loading of infilling sediments.

A model for the hydrologic and climatic behavior of water on Mars

Past studies of the climatic behavior of water on Mars have universally assumed that the atmosphere is the sole pathway available for volatile exchange between the planet's crustal and polar reservoirs of H2O. However, if the planetary inventory of outgassed H2O exceeds the pore volume of the cryosphere by more than a few percent, then a subpermafrost groundwater system of global extent will necessarily result. The existence of such a system raises the possibility that subsurface transport may complement long‐term atmospheric exchange. In this paper, the hydrologic response of a water‐rich Mars to climate change and to the physical and thermal evolution of its crust is considered. The analysis assumes that the atmospheric leg of the planet's long‐term hydrologic cycle is reasonably described by current models of insolation‐driven exchange. Under the climatic conditions that have apparently prevailed throughout most of Martian geologic history, the thermal instability of ground ice at low‐ to mid‐latitudes has led to a net atmospheric transport of H2O from the “hot” equatorial region to the colder poles. Theoretical arguments and various lines of morphologic evidence suggest that this poleward flux of H2O has been episodically augmented by additional releases of water resulting from impacts, catastrophic floods, and volcanism. Given an initially ice‐saturated cryosphere, the deposition of material at the poles (or any other location on the planet's surface) will result in a situation where the local equilibrium depth to the melting isotherm has been exceeded, melting ice at the base of the cryosphere until thermodynamic equilibrium is once again established. The downward percolation of basal meltwater into the global aquifer will result in the rise of the local water table in the form of a groundwater mound. Given geologically reasonable values of large‐scale crustal permeability (i.e., ≳ 10−2 darcies), the gradient in hydraulic head created by the presence of the mound could then drive the equatorward flow of a significant volume of groundwater (≳ 108 km3) over the course of Martian geologic history. At temperate and equatorial latitudes, the presence of a geothermal gradient will then result in a net discharge of the system as water vapor is thermally pumped from the higher temperature (higher vapor pressure) depths to the colder (lower vapor pressure) near‐surface crust. By this process, a gradient as small as 15 K km−1 could drive the vertical transport of 1 km of water to the freezing front at the base of the cryosphere every 106–107 years, or the equivalent of ∼102–103 km of water over the course of Martian geologic history. In this manner, much of the H2O that has been lost from the crust by the sublimation of equatorial ground ice, impacts, and catastrophic floods may ultimately be replenished. The validity of this analysis is supported by a detailed review of relevant spacecraft data, discussions of lunar and terrestrial analogs, and the use of well‐established hydrologic models. Among the additional topics discussed are the thermal and hydrologic properties of the crust, the potential distribution of ground ice and groundwater, the thermal evolution of the early cryosphere, the recharge of the valley networks and outflow channels, the polar mass balance, and a review of several important processes that are likely to drive the large‐scale vertical and horizontal transport of H2O beneath the Martian surface. Given a geologically reasonable description of the crust, and an outgassed inventory of water that exceeds the pore volume of the cryosphere by just a few percent, basic physics suggests that the hydrologic model described here will naturally evolve. If so, subsurface transport has likely played an important role in the geomorphic evolution of the Martian surface and the long‐term cycling of H2O between the atmosphere, polar caps, and near‐surface crust.

Southern Segment of the European Geotraverse ? a wide-angle seismic refraction experiment in the Sardinia Channel

The Sardinia Channel dataset was collected as part of the European Geotraverse (EGT)—a 4000 km seismic refraction line running from Northern Norway to the Sahara, designed to investigate the structure of the lithosphere beneath Europe. Wideangle seismic data recorded by ocean bottom seismometers deployed in the Sardinia Channel as part of the Southern Segment of the EGT, together with gravity data, were used to constrain the final crustal model. In the centre of the Channel the crust is identified as thinned continental in nature, with a crystalline thickness of 10 km overlain by 4 km of sediments and 2.5 km of water in the most extended region. High velocities in the lower crust in the central region are thought to represent an area of underplating or intrusion by igneous material caused by extension related to the opening of the Tyrrhenian Sea. The crust overlies an anomalously low velocity upper mantle.

Earth and Mars: Water inventories as clues to accretional histories

The Earth has 2.7 km of water on its surface. Its mantle contains at least 150 ppm water, and probably significantly more depending on the amount of undepleted mantle and subducted crustal water that is present. Geologic evidence suggests that a few hundred meters of water are close to the Martian surface, but evidence from SNC meteorites indicates that the Martian mantle is very dry, containing no more than about 35 ppm water. Part of the difference in water content of the mantles of the two planets is attributed to plate tectonics. However, the Earth's mantle appears to contain at least several times the water content of the Martian mantle, even accounting for plate tectonics. We attribute the difference to two possible causes. The first possibility is that melting of the Earth's surface during accretion, as a result of the development of a steam atmosphere, allowed impact-devolatized water at the surface to dissolve into the Earth's interior. In contrast, because of Mars' smaller size and greater distance from the Sun, the Martian surface may not have melted, so that the devolatilized water could not dissolve into the surface. A second and preferred possibility is that Mars, like the Earth, acquired a late volatile rich veneer, but it did not get folded into the interior as with the Earth, but instead remained as a water-rich veneer. The perception of Mars as having a wet surface, but a dry interior, is consistent with what we know of the geologic history of Mars, which can be viewed as the progressive intrusion and overplating of a water-rich crust by dry, mantle-derived volcanic rocks.

Jökulhlaups in Iceland: prediction, characteristics and simulation

Jökulhlaups drain regularly from six subglacial geothermal areas in Iceland. From Grímsvötn in Vatnajökull, jökulhlaups have occurred at 4 to 6 yearly-intervals since the 1940s with peak discharges of 600 to 10000 m3s−1, durations of 2 to 3 weeks and total volumes of 0.5 to 3.0 km3. Prior to that, about one jökulhlaup occurred per decade, with an estimated discharge of 5 km of water and a peak discharge of approximately 30000 m3s−1. Clarke’s (1982) modification of Nye’s (1976) general model of discharge of jökulhlaups gives, in many respects, satisfactory simulations for jökulhlaups from Grímsvötn the best fit being obtained for Manning roughness coefficients n = 0.08 to 0.09 m−1/3s and a constant lake temperature of 0.2°C (which is the present lake temperature). The rapid ascent of the exceptional jökulhlaup of 1938, which accompanied a volcanic eruption, can only be simulated by a lake temperature of the order of 4°C.Jökulhlaups originating at geothermal areas beneath ice cauldrons located 10 to 15 km northwest of Grímsvötn have a peak discharge of 200 to 1500 m3s−1in 1 to 3 days, with total volume of 50 to 350 × 106m3, and they recede slowly in 1 to 2 weeks. The form of the hydrograph has reversed asymmetry to that of a typical Grímsvötn hydrograph. The reservoir water temperature must be well above the melting point (10 to 20°C) and the flowing water seems not to be confined to a tunnel but to spread out beneath the glacier and later gradually to collect back to conduits.Since the time of the settlement of Iceland (870 AD), at least 80 subglacial volcanic eruptions have been reported, many of them causing tremendous jökulhlaups with dramatic impact on inhabited areas and landforms. The peak discharges of the largest floods (from Katla) have been estimated at the order of 100 000 to 300 000 m3s−1, with durations of 3 to 5 days and total volume of the order of 1 km3. It is now apparent that the potentially largest and most catastrophic jökulhlaups may be caused by eruptions in the voluminous ice-filled calderas in northern Vatnajökull (of Bárdharbunga and Kverkfjöll). They may be the source of prehistoric jökulhlaups, with estimated peak discharge of 400 000 m3s−1.At present, jökulhlaups originate from some 15 marginal ice-dammed lakes in Iceland. Typical values for peak discharges are 1000 to 3000 m3s−1, with durations of 2 to 5 days and total volumes of 2000 × 106m3. Hydrographs for jökulhlaups from marginal lakes have a shape similar to those of the typical Grímsvötn jökulhlaup. Simulations describe reasonably well the ascending phase of the hydrographs assuming a constant lake temperature of about 1°C; but they fail to describe the recession. Some floods from marginal lakes, however, have reached their peaks exceptionally rapidly, in a single day. Such rapid ascent can be simulated by assuming drainage of lake water at 4 to 8°C.An empirical power-law relationship is obtained between peak discharge,Qmax, and total volumeVtof the jökulhlaups from Grímsvötn:Qmax= KVtb, whereQmaxis measured in m3s−1,Vtin 106m3, Κ = 4.15 × 10−3s−1m2and b = 1.84. In general, the jökulhlaups (excepting those caused by eruptions) occur when the lake has risen to a critical level, but before a lake level required for simple flotation of the ice dam is reached. The difference between the hydrostatic water pressure maintained by the lake and the ice overburden pressure of the ice dam is of the order 2 to 6 bar.

Jökulhlaups in Iceland: prediction, characteristics and simulation

Jökulhlaups drain regularly from six subglacial geothermal areas in Iceland. From Grímsvötn in Vatnajökull, jökulhlaups have occurred at 4 to 6 yearly-intervals since the 1940s with peak discharges of 600 to 10000 m3s−1, durations of 2 to 3 weeks and total volumes of 0.5 to 3.0 km3. Prior to that, about one jökulhlaup occurred per decade, with an estimated discharge of 5 km of water and a peak discharge of approximately 30000 m3s−1. Clarke’s (1982) modification of Nye’s (1976) general model of discharge of jökulhlaups gives, in many respects, satisfactory simulations for jökulhlaups from Grímsvötn the best fit being obtained for Manning roughness coefficients n = 0.08 to 0.09 m−1/3s and a constant lake temperature of 0.2°C (which is the present lake temperature). The rapid ascent of the exceptional jökulhlaup of 1938, which accompanied a volcanic eruption, can only be simulated by a lake temperature of the order of 4°C.Jökulhlaups originating at geothermal areas beneath ice cauldrons located 10 to 15 km northwest of Grímsvötn have a peak discharge of 200 to 1500 m3s−1 in 1 to 3 days, with total volume of 50 to 350 × 106m3, and they recede slowly in 1 to 2 weeks. The form of the hydrograph has reversed asymmetry to that of a typical Grímsvötn hydrograph. The reservoir water temperature must be well above the melting point (10 to 20°C) and the flowing water seems not to be confined to a tunnel but to spread out beneath the glacier and later gradually to collect back to conduits.Since the time of the settlement of Iceland (870 AD), at least 80 subglacial volcanic eruptions have been reported, many of them causing tremendous jökulhlaups with dramatic impact on inhabited areas and landforms. The peak discharges of the largest floods (from Katla) have been estimated at the order of 100 000 to 300 000 m3 s−1, with durations of 3 to 5 days and total volume of the order of 1 km3. It is now apparent that the potentially largest and most catastrophic jökulhlaups may be caused by eruptions in the voluminous ice-filled calderas in northern Vatnajökull (of Bárdharbunga and Kverkfjöll). They may be the source of prehistoric jökulhlaups, with estimated peak discharge of 400 000 m3 s−1.At present, jökulhlaups originate from some 15 marginal ice-dammed lakes in Iceland. Typical values for peak discharges are 1000 to 3000 m3s−1, with durations of 2 to 5 days and total volumes of 2000 × 106 m3. Hydrographs for jökulhlaups from marginal lakes have a shape similar to those of the typical Grímsvötn jökulhlaup. Simulations describe reasonably well the ascending phase of the hydrographs assuming a constant lake temperature of about 1°C; but they fail to describe the recession. Some floods from marginal lakes, however, have reached their peaks exceptionally rapidly, in a single day. Such rapid ascent can be simulated by assuming drainage of lake water at 4 to 8°C.An empirical power-law relationship is obtained between peak discharge, Qmax, and total volume Vt of the jökulhlaups from Grímsvötn: Qmax = KVtb, where Qmax is measured in m3s−1, Vt in 106m3, Κ = 4.15 × 10−3s−1 m2 and b = 1.84. In general, the jökulhlaups (excepting those caused by eruptions) occur when the lake has risen to a critical level, but before a lake level required for simple flotation of the ice dam is reached. The difference between the hydrostatic water pressure maintained by the lake and the ice overburden pressure of the ice dam is of the order 2 to 6 bar.

The role of thermal vapor diffusion in the subsurface hydrologic evolution of Mars

Various lines of geomorphic evidence suggest that large bodies of groundwater were present on Mars throughout much of its early geologic history. In this paper the hydrologic response of groundwater to the thermal evolution of the early martian crust is considered. When a temperature gradient is present in a moist porous medium, it gives rise to a vapor pressure gradient that drives the diffusion of water vapor from regions of high to low temperature. By this process, a geothermal gradient as small as 15 K km−1 could drive the vertical transport of 1 km of water to the freezing front at the base of the martian cryosphere every 106–107 years, or the equivalent of ∼10²–10³ km of water over the course of martian geologic history. Models of the thermal history of Mars suggest that this thermally‐driven vapor flux may have been as much as 3–5 times greater in the past. The magnitude of this transport suggests that the process of geothermally‐induced vapor diffusion may have played a critical role in the initial emplacement of ground ice and the subsequent geomorphic and geochemical evolution of the martian crust.

Reproductive Performance of Female Key Deer

The reproductive performance of the endangered Key deer (Odocoileus virginianus clavium) is a potential limiting factor in its conservation. We analyzed data collected by necropsy of 142 female Key deer from 1968 through 1989 to provide a better understanding of their reproduction. A breeding season of about 6 months was longer than for more northerly herds of white-tailed deer. Productivity of Key deer was low (0.76 fetuses/F -1 yr of age at breeding) and fetal sex ratio (74% M) was high for the species. Rates of reproductive activity (% pregnant or lactating) for females at age of breeding were 4% for fawns, 58% for yearlings, 61% for ages 2-4 years, and 90% for females -5 years of age. Eight (17%) of 48 pregnant females carried twins, and the remainder carried single fetuses. We hypothesize that poor reproductive performance of female Key deer is due to a nutrient deficiency or that it evolved as an adaptation to an insular habitat. J. WILDL. MANAGE. 55(3):386-390 The endangered Key deer of the lower Florida Keys are smallest in mass of the subspecies of North American white-tailed deer (Hardin et al. 1984) and are isolated from mainland Florida by 60 km of water. Prior to protection from hunting, the population of Key deer declined to a low of 25-80 in 1951-52 (Dickson 1955). Klimstra et al. (1978), using spotlight surveys, estimated that the population increased to 300-400 in the early 1970's. The U.S. Fish and Wildlife Service (1985) reported a decline in population to 250-300 by 1984. Low reproductive output apparently contributed to the slow recovery of the Key deer population from the low of the early 1950's (Hardin 1974). Currently, development encroaching on habitat (U.S. Fish Wildl. Serv. 1985) and mortality due to collisions with automobiles result in a precarious status for the Key deer population. Nutrition, one of the most important factors affecting productivity of white-tailed deer (Verme 1967, McCullough 1979), affects both ovulation rate and the proportion of females that become pregnant (Sadleir 1987). An imbalanced ratio of calcium (Ca) : phosphorus (P) or deficiencies of cobalt (Co), iron (Fe), or copper (Cu) probably contribute to low productivity of whitetailed deer from the southeastern United States (Smith et al. 1956, Harlow 1972, Smith and Hunter 1978). The diet of Key deer may be deficient in P (Widowski 1977) or other nutrients, forcing females <4 years old (that have not reached peak mass) to concentrate energy and nutrients on body growth rather than reproduction. We analyzed long-term data collected from mortalities of female Key deer to indirectly test this hypothesis. Numerous individuals of the National Key Deer Refuge and the Cooperative Wildlife Research Laboratory at Southern Illinois University, Carbondale contributed to the collection and examination of carcasses. We thank M. L. Folk, J. W. Hardin, J. L. Roseberry, T. G. Scott, T. J. Wilmers, and A. Woolf for reviewing a draft f the manuscript. The Key Deer Refuge pr vided use of computer facilities. This project was funded by a grant from the Richard King Mellon Foundation.

Source of the Australasian Tektite Strewn Field: A possible off‐shore impact site

Although there is a preponderance of evidence that tektites were formed by asteroid impacts on the Earth, no source crater has been found for the largest and youngest of the strewn fields‐the Australasian strewn field. We have examined a combined Seasat/Geos 3 altimeter data set of sea surface heights in the northern portion of the Australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites. A large negative anomaly, which we call the Qui Nhon Slope Anomaly, is a sea surface depression of approximately 1.5 meters over an area of 100 km diameter, centered at 13° 47′N latitude and 110° 37′ E longitude. It corresponds to a gravity anomaly of about −50 mgal. A surface‐ship gravimeter track across most of the structure confirms the satellite measurements. This site is on the continental slope in 1 to 3 km of water. A sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly. The center of the structure is 175 km offshore and is less than 300 km from the Muong Nong substrewn field and the prolific Dalat site, perhaps the largest concentration of tektites within the Australasian field. Due to its location, size, and magnitude we propose this anomaly may be due to the impact structure that produced the Australasian strewn field.

Observations of deep‐ocean ambient noise using a seismometer array

An array of 12 ocean‐bottom seismometers (OBS) was deployed this past March in the deep ocean near San Diego. The array had a maximum extent of 150 m and a minimum interelement spacing of 8 m. The accuracy of the relative sensor positions was as good as 2 m in some cases. The seismometers were deployed in 3.8 km of water with a partially maneuverable vehicle at the end of a wire guided by a transponder net. The OBS capsules contained a vertical and two horizontal seismometers and a hydrophone. Three of the instruments had a low‐frequency hydrophone of the Cox design. Three‐min windows were recorded at intervals of 3 or 6 h for up to 30 days. Spectral levels have been calculated between 0.01 and 30 Hz. A power level drop of up to 40 dB below the microseism peak at frequencies below 0.1 Hz was observed. Similar spectra have been reported by Webb and Cox [J. Geophys. Res. 91, 7343–7358 (1986)]. At the time of the abstract deadline, correlation was being calculated as a function of sensor separation to determine the correlation length of the ambient noise.

Island and Mainland Pollination Ecology of Centrosema Virginianum and Opuntia Stricta

(1) In 1981 and 1982, male and female reproductive success of mainland and nearand distant-island populations of Centrosema virginianum and Opuntia stricta growing on two island and one mainland beach communities on the western coast of Florida were compared. (2) Pollen dispersal to neighbouring plants (which was used as an estimate of male reproductive success) was significantly reduced in both island populations of Centrosema relative to the mainland population. Pollen dispersal of Opuntia was significantly lower at the far island than at the near island. (3) Female reproductive success (estimated by fruit set) was significantly lower in Centrosema on the distant island than on the near island or mainland during 1981. Fruit set in Opuntia did not differ between the near and far island; however, seeds per fruit were significantly lower on the far island both years. (4) Field experiments indicated significant pollinator limitation of fruit set for Centrosema growing on the far island in 1981, but not in 1982. On the far island in 1983, there was no significant difference in the number of seeds per fruit in naturally-pollinated and hand-crossed Opuntia flowers. (5) The differences between island and mainland pollinator communities and reproductive success that were found in plant communities separated by less than 10 km of water, suggest that pollinator limitation may be an important selective force on plant communities of distant, oceanic islands.

Mars: A water-rich planet?

Mars had outgassed at least 0.5 to 1 km of water, 10 to 20 bar of CO 2, and 0.1 to 0.3 bar of N 2. The volatiles that have been retained are mostly in the cratered uplands. Terrain softening, fretted channels, debris flows, and closed depressions indicate that at least the upper 2 km of the cratered uplands at high latitudes (>30°) contain ice in amounts that exceed the porosity, estimated to be 10–20%. Theoretical studies, and lack of these features in the cratered uplands at low latitudes, suggest that the upper 1 km of the uplands at low latitudes is ice poor. However, valley networks indicate that water was present near the surface early in the planet's history, although in amounts smaller than at high latitudes. The entire upper 1 km, planetwide is estimated to have contained 75–125 m of water at the end of heavy bombardment. The largest sink for water is the megaregolith below 1 km. Episodic eruption of water from the deep megaregolith cut many of the large outflow channels. From the volume of water needed to cut the circum-Chryse channels, and assuming uniform planetwide distribution of water, the deep megaregolith is estimated to have contained at least 350 m of water at the end of heavy bombardment, thereby giving a total minimum inventory of 424–475 m planetwide. Most of the water lost from the low-latitude uplands by diffusion and in cutting the valley networks is now believed to be in the polar layered terrains. Most of the water involved in cutting the outflow channels is in the low-lying northern plains where a variety of features that have been attributed to ground ice is present. A large fraction of the planet's surface has been overplated with water-poor volcanics, of which we have samples in the SNC meteorites. The younger volcanics have reacted extensively with the old volatile-rich basement. Some of the CO 2 and N 2 outgassed was lost during heavy bombardment by impact erosion of the atmosphere and other processes. The remaining was fixed carbonates and folded deep into the megaregolith during heavy bombardment.

Genetic similarity between natural populations of Drosophila ananassae from Kerala and Andaman and Nicobar Islands

In order to study the degree of genetic differentiation in natural populations of Drosophila ananassae, the mean genetic identity has been computed on the basis of differences in the gene arrangement frequencies. The estimates of genetic identity suggest that the populations from Keral, South India are genetically similar to the populations from the Andaman and Nicobar Islands though they are isolated by nearly 2 000 km of water. However, the South Indian populations are genetically more differentiated than the North Indian populations.

A seismic refraction experiment in the Tyrrhenian Sea

Summary. We present the results of two 100 km long reversed seismic refraction lines shot in the Tyrrhenian Sea, using explosive sources and oceanbottom receivers. The results have been interpreted using ray-tracing through laterally varying structures. The more easterly line, T2, was shot in the central part of the Tyrrhenian abyssal plain, in a water depth of 3.5 km. The crust here is shown to be oceanic in character, but to exhibit significant lateral heterogeneity. The upper part of the crust (layer 2) contains a steep velocity gradient from about 4 to over 6 km s-l at a depth of typically 2 km. The lower part of the crust (layer 3) contains a smaller velocity gradient, from 6.6 to 6.9 km s-l at the base of the crust. Total crustal thickness is between 6 and 8 km. The other line, T1, carried out in the Cornaglia Basin in 2.7 km of water, reveals a more complicated velocity structure. We conclude that its southern half is underlain by oceanic crust, similar to that observed at T2. However, its northern half appears to be underlain by crust of similar thickness, but in which seismic velocities increase slowly with depth from 4.9 km s-l immediately below the superficial sediments to 6.3 km s-l at the Moho. We interpret this latter structure as thinned continental crust. Surprisingly neither the seafloor topography nor the depth to Moho reflect the transition from oceanic to continental crust. We infer from these results that although the southern and eastern parts of the Tyrrhenian basin were formed primarily by seafloor spreading, much of the basin north of about 40N and probably including some areas with water depths in excess of 3 km, is underlain by thinned continental crust. The evolution of the northern part of the Tyrrhenian basin must therefore have occurred chiefly through a process of continental extension, which in some areas has reduced crustal thickness to as little as 6-8 km. South of 40N.

Birds Injurious to Crops in West Africa

Abstract A list of birds injurious to crops in the sahelian, Guinean and humid zones of West Africa has been compiled from a survey of 51 publications. In all, 36 species have been found feeding on crops; taking into account the amount of specifically attributed damage and the number of citations, the most important are Quelea quelea (U, Ploceus cucullatus (Müller), Quelea erythrops (Hartlaub), Passer luteus (Lichtenstein) and Ploceus melanocephalus (L). Calculations based on 32 studies reporting damage in 626 plots suggest an average loss of 6.9% of the cereal harvest; the most serious damage is to millet in the sahelian zone (24.6%) and rice in the humid zone (18.8%). Damage is least in November and December. Fields within 10 km of water are more frequently damaged than those further away.