Enewetak Research Articles

Clinal Morphological Variation along a Depth Gradient in the Living Scleractinian Reef Coral Favia pallida: Effects on Perceived Evolutionary Tempos in the Fossil Record

The Holocene reef-building coral Favia pallida was sampled at 4.5 m depth increments (to 40 m) from two reefs on Enewetak Atoll to examine intraspecific environmental effects. An exposed outer reef was massive and wall-like, whereas a sheltered lagoonal reef grew as a slender pinnacle. Corallite diameter and growth rate, two attributes retrievable in fossil corals, were measured with data partitioned into shallow ( 29 m) subsets. Highly significant differences between depth zone populations were found for both corallite diameters and growth rates in analyses of individual and combined reef data sets. Canonical variates analyses (CVA) separated populations from depth zones along single, highly significant, functions. Centroids and 95% confidence intervals, calculated from CVA scores of colonies in each population, are widely separated for the lagoon reef and combined data sets. Conversely, populations from shallow and intermediate depths on the outer reef display overlapping confidence bars indicative of more gradational morphologic changes. When CV's were used to classify specimens to groups, misassignments of intermediate depth specimens to shallow or deep-water populations underscored the gradational nature of the environment. Completely intergrading populations of Favia pallida collected from different depths can be morphologically separatedmore » into statistically distinct groupings. A stratigraphic succession of such morphotypes might be interpreted as abruptly appearing separate species if sampling were not as uniform, systematic, and detailed as was possible on modern reefs. Analyses of evolutionary patterns must carefully assess potential effects of clinal variation if past evolutionary patterns are to be interpreted correctly.« less

The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater: 3. Lead

Atmospheric aerosols collected at Enewetak Atoll in the tropical North Pacific were exposed to seawater in laboratory experiments to assess the impact of atmospheric aerosols on lead chemistry in surface seawater. The net atmospheric flux of soluble lead to the ocean is between 16 and 32 pmol cm−2 yr−1 at Enewetak. The stable lead isotopic composition of soluble aerosol lead indicates that it is of anthropogenic origin. Anthropogenic aerosol lead from Central and North America appears to be less soluble and/or to dissolve less rapidly than that from Asia. Dissolved organic matter and possibly lower pH appear to increase the nonaluminosilicate aerosol lead solubility and/or dissolution rate. The isotopic composition of lead in air, seawater and dry deposition suggests that after deposition in the ocean, nonaluminosilicate paniculate lead can be reinjected into the atmosphere during sea salt aerosol production.

Eocene to early Miocene growth of Enewetak Atoll: Insight from strontium-isotope data

The margin of Enewetak Atoll contains 1,260 to 1,405 m of Cenozoic carbonates above a volcanic basement. Previous biostratigraphic studies on two deep wells (E1 and F1) yielded conflicting results on the ages of Tertiary strata on Enewetak, and on the presence of Oligocene strata. Strontium-isotope ratios support relatively continuous carbonate sedimentation from late Eocene to early Miocene (23 Ma). Carbonate deposition on the atoll margin started with rapid aggradation during much of the late Eocene followed by gradual seaward progradation of depositional facies during the latest Eocene, Oligocene, and early Miocene. The only distinct break in sedimentation from late Eocene to early Miocene was observed at ∼850 m in the E1 well. Here an unconformity formed by subaerial exposure separates backreef, upper Eocene grainstones from lower Oligocene carbonates. No contemporaneous subaerial exposure surface and unconformity were observed in the F1 well because correlative rocks were deposited in a slope environment. Extensive diagenesis occurred in upper Eocene, Oligocene, and lower Miocene strata at the atoll margin where sea water circulated through in response to thermal convection and tidal pumping. The main diagenesc processes are (1) aragonite dissolution, (2) radiaxial calcite cementation, (3) compaction in slope deposits, and (4) dolomitization. Aragonite dissolution is pervasive below 375 m in the F1 well. Radiaxial calcite cement is common at 375-825 m in the F1 well. Compaction-related fracturing and pressure solution are common in poorly cemented packstones and grainstones deposited in a slope environment and now buried more than 1,100 m deep. Dolomite is common in slope carbonates at ∼1,320 m in the F1 well and in reefal carbonates at 1,245 m in the E1 well. Stable carbon- and oxygen-isotope data support formation of radiaxial calcite and dolomite in cool sea water. Radiaxial calcite cement and dolomite have distinctly higher 87Sr/86Sr ratios than does adjacent depositional carbonate. Strontium-isotope ratios of radiaxial calcite cements indicate precipitation at burial depths of 100-350 m. The absence of meteoric cements and association of radiaxial calcite cement with aragonite dissolution suggest that aragonite dissolution also occurred in moderately deep sea water. Strontium-isotope ratios in dolomites indicate dolomitization by sea water at burial depths greater than 950 m. Marine diagenesis is dependent on the saturation state of sea water relative to aragonite, calcite, and dolomite. Shallow sea water, supersaturated with respect to aragonite and high-magnesium calcite (HMC), precipitates aragonite and HMC. Moderately deep sea water (undersaturated with respect to aragonite and HMC, super-saturated with respect to low-magnesium calcite—LMC) dissolves aragonite, converts HMC to LMC, and precipitates LMC (including radiaxial calcite). Deep sea water (under-saturated with respect to calcite, supersaturated with respect to dolomite) dissolves calcite and precipitates dolomite. The intensity of marine diagenesis depends on the amount of sea water circulating through the platform or atoll margin.

Correlation of Sea Level Falls Interpreted from Atoll Stratigraphy with Turbidites in Adjacent Basins: ABSTRACT

Past sea levels can be derived from any atoll subsurface sediments deposited at or near sea level by determining the ages of deposition and correcting the present depths to the sediments for subsidence of the underlying edifice since the times of deposition. A sea level curve constructed by this method consists of discontinuous segments, each corresponding to a period of rising relative sea level and deposition of a discrete sedimentary package. Discontinuities in the sea level curve derived by this method correspond to relative sea level falls and stratigraphic hiatuses in the atoll subsurface. During intervals of relative sea level fall an atoll emerges to become a high limestone island. Sea level may fluctuate several times during a period of atoll emergence to become a high limestone island. Sea level may fluctuate several times during a period of atoll emergence without depositing sediments on top of the atoll. Furthermore, subaerial erosion may remove a substantial part of the depositional record of previous sea level fluctuations. For these reasons the authors must look to the adjacent basins to complement the incomplete record of sea level change recorded beneath atolls. During lowstands of sea level, faunas originally deposited near sea level onmore » an atoll may be eroded and redeposited as turbidites in deep adjacent basins. Three such turbidites penetrated during deep-sea drilling at Sites 462 and 315 in the central Pacific correlate well with a late Tertiary sea level curve based on biostratigraphic ages and {sup 87}Sr/{sup 86}Sr chronostratigraphy for core from Enewetak Atoll in the northern Marshall Islands. Further drilling of the archipelagic aprons adjacent to atolls will improve the sea level history that may be inferred from atoll stratigraphy.« less

Mid-chain ketocarboxylic acids in the remote marine atmosphere: Distribution patterns and possible formation mechanisms

In remote marine aerosol samples collected from the North Pacific ocean, Enewetak Atoll, American Samoa, and New Zealand, series of mid-chain ketocarboxylic acids in the range of C6-C18 were detected. All the positional isomers, except for the 2-oxo and 3-oxo species, were detected for major ketoacid families (e.g. C9, C11 and C13). Higher ketoacid concentrations (up to 19 ng/m3) were obtained in the northern North Pacific aerosol samples, which generally showed an odd carbon-numbered predominance with 5-oxoundecanoic acid being the major species. By contrast, lower concentrations were obtained in the lower-latitude or subtropical aerosol samples, where even carbon-numbered ketoacids were relatively abundant.

Isotopic and Chemical Signatures of Eustasy: The Record at Enewetak Atoll: ABSTRACT

Isotopic and Chemical Signatures of Eustasy: The Record at Enewetak Atoll: ABSTRACT

Post-Miocene diagenetic and eustatic history of Enewetak Atoll: Model and data comparison

The post-Miocene diagenetic and eustatic history of Enewetak Atoll was investigated using a one-dimensional forward model. Comparison of model and data suggests that the post-Miocene history of Enewetak Atoll was dominated by multiple episodes of meteoric phreatic diagenesis attendant with high-frequency (10{sup 4} to 10{sup 5} yr) fluctuations in sea level and a subsidence rate of 39.0 m/m.y. Sensitivity testing indicates that subaerial erosion results in the preservation of additional subaerial unconformities because stratigraphic shortening permits a succeeding sea-level rise to flood the exposure surface and deposit sediment, whereas without subaerial erosion this sea-level rise would be recorded as a paleophreatic lens. Model results indicate that less then 10% of lapsed time is recorded by sediment deposition during periods of high-frequency changes in sea level. Incompleteness of the stratigraphic record suggest that magnetostratigraphy may give erroneous ages for shallow-marine carbonate sequences deposited during times of high-frequency changes in sea level and frequent magnetic polarity reversals.

Meteoric diagenesis, marine diagenesis, and microporosity in Pleistocene and Oligocene limestones, Enewetak Atoll, Marshall Islands

Microporosity occurs in Pleistocene and Oligocene limestones cored on Enewetak Atoll. In cores of Pleistocene limestone (10–80 m deep), mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution (chalkification) leaving a porous meshwork of aragonite needles. Microporous aragonite is most abundant in 5–10 m thick intervals below zones of intense diagenesis associated with paleo-freshwater lenses. Extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses. Microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite. Most calcitized aragonite (neomorphic calcite) formed by precipitation of sparry calcite in (over) microporous aragonite. Aragonitic fossils were not converted to microporous, microcrystalline calcite. Therefore, data from the Pleistocene of Enewetak suggest that widespread, microporous, microcrystalline calcite does not form directly from pure aragonite, at least not in open meteoric systems or mixing zones. Microporous micrite occurs in Oligocene wackestones 930 m deep deposited in a slope environment. The microporous micrite is characterized by equant and prismatic crystals of low-magnesium calcite (LMC). Equant crystals are anhedral to euhedral and 2–10 μm in diameter. Prismatic calcite crystals are subhedral, 5–10 μm wide, and 20–50 μm long. Stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high-magnesium calcite (HMC) micrite in deep seawater undersaturated with respect to HMC but supersaturated with respect to LMC (probably near the sediment-water interface). Mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the Thamama Group (Cretaceous, Persian Gulf; see Moshier, 1989). Equant microcrystals of calcite in the Oligocene micrites are similar to those in microporous parts of the Thamama Group. Widespread microporosity in LMC limestones could form by alteration of HMC micrite in mixing zones or in seawater undersaturated with respect to HMC but supersaturated with respect to LMC.

Fat Cyclicity, Predicted Migratory Flight Ranges, and Features of Wintering Behavior in Pacific Golden-Plovers

Annual cyclicity of body components in wintering Pacific Golden-Plovers (Pluvialisfulva), combined with ecological and behavioral features were variously studied from 1978 through 1987. The primary research site was in Hawaii at the Bellows Air Force Station (BAFS), Oahu. After fall arrival, body weights trended downward for several weeks, possibly in response to the energy required for the behaviors associated with establishing (and reestablishing) residency combined with molting. Major premigratory weight gains began in late March about I month prior to migration. Statistical correlations between body weight and fat content are described. Premigratory dehydration as an adaptation to increase flight range and/or energy stores at the migratory destination was likely. Fat-free dry weights (FFDW) at BAFS were least in the fall and greatest in the spring. For juveniles, this difference relates to overall body growth during the first wintering season. With older birds, it may reflect the environmental conditions at the respective end points of migrationpredictably favorable for southbound migrants, unpredictable for northbound birds. Plovers wintering on Enewetak Atoll and Wake Island had significantly higher FFDWs than the birds at BAFS. Possible factors in this relationship are discussed. The BAFS population contained many territorial birds, and showed high rates of survival and site fidelity over successive years. Juveniles arriving on the wintering grounds for the first time probably experience considerable mortality as they compete with established adults for space and resources. Based on wing lengths, many of the plovers involved in this study were from Alaska breeding grounds. Flyways to the tundra may involve staging areas, but specific information is lacking. Using current formulae, we describe the relationship between body weight and flight range, and provide estimates of the fat required to reach various landfalls.

The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater: 2. Copper

Atmospheric deposition contributes copper to the surface ocean. The biogeochemical importance and fate of this copper is poorly understood for open ocean regions. Atmospheric aerosols collected at Enewetak Atoll, in the tropical North Pacific, were exposed to seawater and artificial rainwater in laboratory experiments. Aerosol copper during the high‐dust season at Enewetak Atoll is made up of aluminosilicate, oceanic, and possibly soil organic matter components. During the low‐dust season, aerosol copper appears to be essentially all of oceanic origin. Virtually all nonaluminosilicate copper in marine aerosols collected at Enewetak is soluble in seawater. Dissolved organic matter and possibly cations in seawater increase the dissolution of aerosol copper. The net atmospheric flux of soluble copper to the tropical North Pacific near Enewetak is approximately 0.13 nmol cm−2 yr−1 out of a total net atmospheric copper flux of 0.14 nmol cm−2 yr−1. Atmospheric deposition supplies roughly the same quantity of soluble copper to tropical open North Pacific surface waters as does upwelling to eastern North Pacific surface waters. Atmospheric copper deposition, which appears to be primarily of natural origin, may be the most important input of copper to the surface waters of the central gyre of the North Pacific.

Geohydrology of the Laura fresh-water lens, Majuro atoll: A hydrogeochemical approach

In small limestone islands, the depositional history and subsequent chemical interactions between ground water and the aquifer host rock play critical roles in the occurrence, movement, and chemical quality of ground water. The hydrogeochemistry of the Laura fresh-water lens, Majuro atoll, Marshall Islands, is an example of these relations. Laura is underlain by two principal hydrologic units. The upper unit is a back-reef-marginal-lagoonal deposit which formed during the Holocene interglacial stage. It is composed of moderately permeable carbonate sediments. The lower hydrologic unit consists of highly permeable limestone that was subaerially exposed, most likely during a Pleistocene glacial lowstand. Similar stratification is found at Bikini and Enewetak atolls. The upper hydrologic unit contains a calcium bicarbonate-rich fresh-water lens, in which a potable fresh-water nucleus as much as 14 m thick occurs on the lagoon side of the island. Storage in the fresh-water nucleus ranged from 1.70 x 10 6 to 2.08 x 10 6 m 3 during 1984-1985. Ground-water occurrence and flow are governed by an asymmetric distribution of lithofacies about the longitudinal axis of the island and an abrupt increase in permeability at the contact between the upper and lower hydrologic units. The highly permeable lower hydrologic unit contains sea water and truncates the fresh-water-sea-water mixing zone. The fresh-water lens and associated fresh-water-sea-water mixing zone are the site of continuously occurring diagenetic reactions that significantly affect the porosity and permeability of the aquifer. Non-equilibrium dissolution-precipitation reactions, coupled with variations in CO 2 input, control the chemical evolution of Laura ground water. At the present rate of chemical weathering, 465 m 3 of sediment are being dissolved and transported to the sea by ground water each year. This dissolution results in an annual increase in porosity of 0.01%. The primary factors controlling the occurrence and flow of ground water in the leeward reef islet of Laura are (1) the depositional history of the upper hydrologic unit, which has resulted in a greater accumulation of low-permeability (fine-grained) sediments beneath the lagoon side of the island and a high- to low-permeability (coarse-to fine-grained sediment) gradation between the ocean and lagoon; and (2) the diagenetic history of the lower hydrologic unit, which has resulted in a highly permeable basement.

Comment and Reply on "Strontium-isotope stratigraphy of Enewetak Atoll"

Comment and Reply on "Strontium-isotope stratigraphy of Enewetak Atoll"

Deposition and diagenesis of the windward reef of Enewetak Atoll

Six cores from the northeast reef of Enewetak Atoll reveal that 25 to 50 ft (7.5 to 15 m) of Holocene deposits unconformably overlie at least 200 ft (60 m) of Pleistocene limestone. The Holocene-Pleistocene boundary and four other unconformities within the Pleistocene section are distinguished by paleosols. The unconformities record repeated periods of subaerial emergence and death of the Pleistocene reef associated with low sea-level stands. During emergent periods, meteoric-water diagenesis altered the Pleistocene section. The resulting mineralogy and diagenetic textures are characteristic of the meteoric-vadose diagenetic environment. Aragonite generally composes from 30 to 70% by weight of the Pleistocene limestone and is absent only locally; magnesium calcite is not present. Cementation by calcite is variable but generally increases with depth. Much of the section is poorly cemented and retains high primary and secondary porosity. Micrite and mixed micrite-microspar are the most abundant cements throughout the section. They occur as meniscus, uneven, and irregularly distributed (“patchy”) pore-fill cement. In the upper parts of depositional units, the following paleosol-associated cements are common: caliche crusts, rhizocretions, needle-fiber cement, and lath-crystal cement. Phreatic-type cements are present, but generally are unimportant volumetrically. A reconstruction of the Pleistocene paleohydrology of the atoll indicates that the section has occupied the meteoric-vadose environment during emergent periods. The similarity of Pleistocene limestones beneath Enewetak and other Pacific atolls studied by drilling suggests that for many atolls, most alteration of shallow subsurface Pleistocene limestones has occurred in the meteoric-vadose diagenetic environment.

Rhinebothrium devaneyi n. sp. (Eucestoda: Tetraphyllidea) and Echinocephalus overstreeti Deardorff and Ko, 1983 (Nematoda: Gnathostomatidae) in a Thorny Back Ray, Urogymnus asperrimus, from Enewetak Atoll, with Phylogenetic Analysis of Both Species Groups

The new species is a member of an apparently monophyletic group within the genus that includes R. flexile, R. walga, R. himanturi, R. burgeri, R. euzeti, R. hawaiiensis, R. urobatidium, R. paratrygoni, R. ditesticulum, R. tetralobatum, R. margaritense, R. biorchidum, and R. spinicephalum. All of these species have bothridia with medial longitudinal septa, a constriction at mid-bothridium, and, primitively, at least 42 loculi per bothridium and 17-22 testes per proglottid. Of the above, the new species is apparently most closely related to R. burgeri, with which it shares an increased number of testes (30-43) per proglottid, a V-shaped ovary, and a muscular genital pore. The new species is distinct by virtue of possessing 94-152 loculi per bothridium--no other known species has more than 78. This is the second report of Echinocephalus overstreeti from a stingray. It represents a new host, U. asperrimus, and a new location, Enewetak Atoll. Phylogenetic and biogeographic analysis of each species group suggests an ancient Tethys Sea-circum-Pacific origin and evolution. This supports the hypothesis of ancient Pacific origins for potamotrygonid stingrays.

Predation on Foraminifera by Coral Reef Fish: Taphonomic and Evolutionary Implications

Fish predation or cropping offoraminifera is widespread on coral reefs. At Eniwetok Atoll, live foraminifera are taken by herbivorous grazers that crop close to the substrate, occasionally by browsers nipping higher parts of algae, scrapers like parrot fish, and omnivores feeding on algal turfs. Carnivorous fishes do not take foraminifera, except for fossorial feeders that ingest mostly detrital tests. Planktivores take foraminifera, both oceanic planktonic species and the floating stages of certain benthic species. These feeding activities have taphonomic and evolutionary consequences onforaminifera. Fishes may damage foraminiferal tests, by chipping, gouging or scratching the tests, or they may destroy the tests altogether by dissolution or crushing. The damage could be observable in the fossil record. It may also result in the elimination of some foraminifera from the fossil record of reefs, in part by later diagenesis of weakened tests. Fishes also transport foraminifera from their life habitats to other unsuitable habitats where they may die. Foraminifera have life histories and ecologic and morphologic strategies that decrease chances of individual and population mortality. These include rapid growth and reproduction in commonly cropped habitats, regeneration of damaged tests, and various protective or defensive test morphologies. Foraminifera also occupy habitats avoided by fishes, cryptic habitats, tidepools that exclude fish, and complex habitats where fish predation is ineffectual. The impacts of foraminiferal predatorslcroppers may be evident in the evolutionary record of fossil foraminifera. Furthermore, the taphonomic processes initiated by the feeding activities may well bias the fossil record such that paleoenvironmental interpretation and correlation are difficult or, worse, erroneous. INTRODUCTION

Evolution of Reef and Atoll Margin Carbonates, Upper Eocene Through Lower Miocene, Enewetak, Marshall Islands: ABSTRACT

Two wells drilled along the margin of Enewetak Atoll penetrated approximately 1000 m of upper Eocene, Oligocene, and lower Miocene carbonates. Strontium isotope stratigraphy indicates relatively continuous deposition of carbonate from 40 Ma to 20 Ma. Depositional environments show a gradual basinward progradation of facies with slope carbonates passing upward into fore-reef, reef, back-reef, and lagoonal carbonates. Slope strata contain wackestones and packstones with submarine-cemented lithoclasts, coral, coralline algae fragments, benthic rotaline forams, planktonic forams, and echinoderm fragments. Fore-reef strata are dominantly packstones and boundstones containing large pieces of coral, abundant benthic forams, coralline algae fragments, stromatoporoids( ), and minor planktonic forams. Reef and near-reef sediments include coralgal boundstones and grainstones with abundant benthic forams. Halimeda and miliolid forams are common in lagoonward parts of the back reef. Sponge borings, geopetal structures, and fractures are common in reef and fore-reef strata. Lagoonal strata are wackestones and packstones with common mollusks, coral, coralline algae, and benthic forams (rotaline and miliolid). Diagenesis has extensively altered strata near the atoll margin. Aragonite dissolution and calcite cements (radiaxial and cloudy prismatic) are abundant in fore-reef, reef, and some back-reef strata. Petrographic and geochemical data indicate aragonite dissolution and calcite cementation in seawater at burialmore » depths of 100 to 300 m. Dolomite occurs in slope and deeply buried reefal carbonates. Most dolomitization occurred at burial depths of more than 1000 m in cool marine waters circulating through the atoll. lagoonal strata are not significantly altered by marine diagenesis and still contain abundant primary aragonite and magnesium calcite.« less

Spawning, eggs and larvae of the longnose filefish Oxymonacanthus longirostris, a monogamous coralivore

The longnose filefish (Monacanthidae, Oxymonacanthus longirostris) was studied at a patch reef on Enewetak Atoll. It swims with undulations of its median fins and feeds almost continuously on coral polyps, using its beak-like mouth. The spot in the tail resembles an eye so that the tail looks like a head when it protrudes from the coral during feeding. There is slight dichromatism in that males have brighter pelvic flaps. Most of the fish were color tagged for individual recognition. They tended to remain within a given area, although some fish roamed throughout the patch. Most of the larger adults formed monogamous pairs; the smaller fish were variously alone or in groups of 1–5 individuals who appeared to know one another. Only pairs having the largest males, on the periphery of the patch, were clearly territorial. Aggression was a common event featuring a few distinctive displays, most of which were used in courtship as well. Spawning may be a daily event, occurring around 1600 h. The female probes and selects a tuft of blue-green algae; the male nuzzles the female and they spawn in the algae. The green eggs are about 0.7 mm in diameter, demersal, adhesive, and hidden in the algae; they hatched in 53.5 h, just after sunset. The planktonic larvae fit the monacanthid type B of Leis & Rennis (1983) and were initially 2.5 mm TL; by day 5 their yolk was gone.

The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater, 1. Aluminum

Significant quantities of aerosol aluminum are transported from continental regions through the atmosphere to the oceans. Enrichments in the concentration of dissolved aluminum in open ocean surface seawater suggest that dissolution of aerosol aluminum is an important source of dissolved aluminum to these waters. Atmospheric aerosols collected at Enewetak Atoll were exposed to seawater and artificial rain water to determine directly the importance of atmospheric deposition as a source of marine dissolved aluminum. The results of these experiments indicate that ∼ 8–10% of the aluminum in atmospheric aerosols of crustal origin over the North Pacific is soluble in seawater. Approximately 5–6% dissolves very rapidly ( < 0.6 hr). An additional 3–4% dissolves within 60 hr. This bimodal dissolution of aerosol aluminum of crustal origin suggests that this aluminum is present in two forms. The rapidly dissolving fraction is likely aluminum already weathered from primary minerals, while the more slowly dissolving fraction is probably aluminum from the aluminosilicate matrix. Nearly the same amount of aerosol aluminum dissolved in artificial rain water ( pH= 5.5) in 6 hr as dissolved in seawater ( pH= 8) in 60 hr. The lower pH appears to not only increase the dissolution rate but may also increase the quantity of aerosol aluminum that dissolves. Dissolved organic matter in seawater appears to have relatively little effect on aerosol aluminum dissolution. Considering measured total aerosol aluminum fluxes, aluminum dissolution of 5–10% would constitute the major source for dissolved aluminum in surface waters of the open North Pacific. The calculated residence time of dissolved aluminum in the upper 100 m of the tropical North Pacific ranges from 2 to 6 years.

An empirical study of the nuclear explosion‐induced lightning seen on IVY‐MIKE

We report the results of a unique study of the lightninglike phenomena that were seen to accompany the MIKE shot of operation IVY on October 31 1952. MIKE was a thermonuclear surface burst yielding 10.4 MT, which took place on Enewetak Atoll. During the period of approximately 10 ms after detonation, five discrete luminous channels were seen to start from the ground or sea surface at a distance of approximately 1 km from the burst point and to grow up into the clouds. We have reexamined the original photographic records of IVY‐MIKE, obtaining effective brightnesses (optical powers per unit length) for the luminous channels at different altitudes as functions of time. The absolute calibration for the MIKE records was deduced by comparison with the photographic records of other events of that era, laboratory measurements of film sensitivity, and use of atmospheric transmission data taken just prior to the MIKE event. Errors in this analysis lead to an uncertainty of a factor of ∼2 in the brightnesses of the luminous channels. In the laboratory we have used laser‐guided electric discharges to create long (100 cm), arclike plasma channels to simulate the observed luminous channels and to allow determination of an empirical relation between the brightness of the channel and the electric current flowing in the channel. These laboratory discharges had peak currents up to 100 kA and periods of ∼2 ms. Spectroscopic analysis showed that the luminous channels consisted primarily of normal air plasma with typical ground‐level contaminants. Photographic studies showed that these long‐duration discharges are unstable to the m = 1 magnetohydrodynamic (MHD) instability and become severely distorted in less than 1 ms. By direct comparison of the luminous channels seen at MIKE and the laboratory discharges, we deduce: (1) the peak current in the prominent (brightest) channel at MIKE was between 200 and 600 kA. Here the most likely value of the peak current was 250±50 kA, but potential systematic errors in the film calibration and the comparison of MIKE and laboratory data make higher currents possible. (2) The rapid decline in the brightness of the luminous channels seen at MIKE is caused by a combination of the effects of the MHD instability, which eventually leads to a broadening of the current‐carrying channel, and channel cooling by turbulent convective mixing.

Energetics, competency, and long-distance dispersal of planula larvae of the coral Pocillopora damicornis

Pocillopora damicornis (Linnaeus) were collected from Enewetak Atoll, Marshall Islands, in 1980–1981) and Kaneohe Bay, Hawaii, in 1982. Their planula larvae contained 17% protein, 70% lipid, and 13% carbohydrate by dry weight. Calculations based on stored energy reserves and daily metabolic expenditure indicate that planulae could survive approximately 100 d and still settle successfully. Competency experiments demonstrated that larvae settled and metamorphosed after 103 d. This period of time is sufficient to allow immigration of larvae from the Central Pacific to the eastern Pacific, and supports the hypothesis of long-distance dispersal of larvae for the origin of present eastern Pacific populations of P. damicornis.