Enewetak Research Articles

Strontium-isotope stratigraphy of Enewetak Atoll

/sup 87/Sr//sup 86/Sr ratios determined for samples from a 350 m core of Neogene lagoonal, shallow-water limestones from Enewetak Atoll display a remarkably informative trend. Like the recently published data for Deep Sea Drilling Project (DSDP) carbonates, /sup 87/Sr//sup 86/Sr at Enewetak increases monotonically but not smoothly from the early Miocene to the Pleistocene. The data show intervals of little or no change in /sup 87/Sr//sup 86/Sr, punctuated by sharp transitions to lower values toward greater core depths. The sharp transitions correlate with observed solution disconformities caused by periods of subaerial erosion, whereas the intervals of little or no change in /sup 87/Sr//sup 86/Sr correspond to intervals of rapid accumulation of shallow-water carbonate sediments. When converted to numerical ages using the published DSDP 590B trend, the best-resolved time breaks are at 282 m (12.3 to 18.2 Ma missing) and 121.6 m (3.0 to 5.3 Ma missing) below the lagoon floor. At Enewetak, Sr isotopes offer a stratigraphic resolution for these shallow-marine Neogene carbonates comparable to that of nannofossil zonation in deep-sea carbonates (0.3-3 m.y.). In addition, the correlation of times of Sr-isotope breaks at Enewetak with times of rapid Sr-isotope change in the DSDP 590B samples confirms the importance ofmore » sea-level changes in the evolution of global-marine Sr isotopes and shows that the Sr-isotope response to sea-level falls is rapid.« less

Geologic Reconnaissance of Natural Fore-Reef Slope and a Large Submarine Rockfall Exposure, Enewetak Atoll: ABSTRACT

In 1958 a submarine rockfall exposed a cross section through the reef and fore-reef deposits along the northwestern margin of Enewetak Atoll, Marshall Islands. Removal of more than 10/sup 8/ MT of rock left a cirque-shaped submarine scarp 220 m high, extending back 190 m into the modern reef, and 1000 m along the reef trend. The scarp exposed older, steeply dipping beds below 220 m along which the rockfall detached. They sampled this exposure and the natural fore-reef slope surrounding it in 1984 and 1985 using a manned submersible. The natural slope in this area is characterized by three zone: (1) the reef plate, crest, and near fore reef that extends from sea level to -16 m, with a slope of less than 10/sup 0/, (2) the bypass slope that extends from -16 to -275 m, with slopes of 55/sup 0/ decreasing to 35/sup 0/ near the base, and (3) a debris slope of less than 35/sup 0/ below -275 m. Vertical walls, grooves, and chutes, common on other fore-reef slopes, are sparse on the northwestern slope of Enewetak. The scarp exposes three stratigraphic units that are differentiated by surficial appearance: (1) a near-vertical wall from the reef crestmore » to 76 m that appears rubbly, has occasional debris-covered ledges, and is composed mainly of coral; (2) a vertical to overhanging wall from -76 m to -220 m that is massive and fractured, and has smooth, blocky surfaces; and (3) inclined bedding below -220 m along which the slump block has fractured, exposing a dip slope of hard, dense, white limestone and dolomite that extends below -400 m. Caves occur in all three units. Open cement-lined fractures and voids layered with cements are most common in the middle unit, which now lies within the thermocline. Along the sides of the scarp are exposed fore-reef boulder beds dipping at 30/sup 0/ toward the open sea; the steeper (55/sup 0/) dipping natural surface truncates these beds, which gives evidence of the erosional nature of the bypass slope.« less

Halimeda growth and diversity on the deep fore-reef of Enewetak Atoll

The deep fore-reef at Enewetak has been examined from the submersible Makali'i. Green algae grow to about-150 m at photon flux densities of approximately 1 μEm-2s-1. Halimeda cover is ≧50% at many sites down to-90 m. Halimeda populations are important within the zone of scleractinian corals down to about-65 m, while a Halimeda zone with low coral cover or lacking corals between-65 m and-150 m probably is an important source of reef carbonate. Halimedas of the deep fore-reef, like those of the lagoon, constitute an important structural component in reef building. Other calcareous green algae such as Tydemania are less important on the deep fore-reef, but growth of coralline red algae continues to over-200m. Halimeda diversity is high down to near the base of the euphotic zone.

A layered aquifer model of atoll island hydrology: Validation of a computer simulation

The finite-element model FEMWATER was used to simulate groundwater movement in the two-dimensional vertical plane of an atoll island section. The model island had dimensions consistent with Enjebi Island (Enewetak Atoll), a surficial (Holocene) aquifer to a depth of 15m with hydraulic conductivity = 6.9 × 10 −4 ms −1, overlying a (Plio-Pleistocene) aquifer with K = 6.9 × 10 −2 ms −1. A tidally varying water level was applied to the ocean and lagoon boundaries, and the model was run until pseudo steady-state conditions were achieved in the groundwater variations. Hydraulic head contours in the vertical section demonstrate that water motion under the supratidal portion of the island is almost exclusively vertical. Calculated water table tidal lags and efficiencies are in excellent agreement with measured values from shallow wells on Enjebi Island, confirming the validity of the layered aquifer model and the approximate magnitudes of the hydraulic conductivities. Model results near island margins can be interpreted to explain other features of hydrologic field data from atoll islands. The results emphasize the potential importance of tidal effects and vertical water flow on reef island groundwater systems, and cast doubt on the utility of traditional models using the Dupuit assumptions of essentially horizontal flow.

Precipitation scavenging of aerosol particles over remote marine regions

There is increasing evidence that the deposition of atmospheric participate material has a significant influence on the chemistry of trace elements in many oceanic regions1–4. One significant but unknown parameter is the efficiency of in-cloud and below-cloud scavenging processes as a function of aerosol particle size. Studies in continental regions have indicated that the largest particles are scavenged most efficiently by rain5. We report here, for the first time, an evaluation of particle scavenging by tropical showers, based on data obtained at Eniwetok Atoll and American Samoa in the tropical Pacific Ocean. We find no clear evidence of a particle size dependence for scavenging in this remote marine region. This indicates that estimates of global atmospheric deposition over the ocean based on extrapolation of measurements made on land may be invalid.

Long-range transport of continentally-derived particulate carbon in the marine atmosphere: evidence from stable carbon isotope studies

Since 1979, we have investigated marine and non-marine sources of particulate carbon in the marine atmosphere from measurements of carbon concentration and isotopic composition 13 C/ 12 C). Aerosol samples were collected, mostly during the Sea/Air Exchange (SEAREX) Program experiments, in the northern and southern hemispheres (Sargasso Sea, Enewetak Atoll, Peru upwelling, American Samoa, New Zealand, Amsterdam Island). The concentration and the isotopic composition of particulate carbon of marine origin are about the same in both hemispheres (C mean =O.O7 μgC m -3 , δ 13 C mean C = -21‰) This component is primarily associated with large sea-salt particles (diameter > 3 μm). Particulate carbon of continental origin displays a variable isotopic composition (δ 13 C range: - 23 to - 28‰,) and is primarily found attached to the smallest aerosol particles (diameter < 1 μm). This continental component shows little temporal variability, but its concentration is much lower in the southern hemisphere (0.06 μgC m -3 ) than in the northern hemisphere (0.45 μgC m -3 ). Also, the northern hemisphere samples appear strongly depleted in 13 C compared to those from the southern hemisphere. This can be explained on the basis of our isotopic data over various continental areas (Paris region, Ivory Coast, Congo) as due to the predominance in the northern hemisphere of carbonaceous aerosols of anthropogenic origin derived either from industrial combustion processes or from biomass burning. On the other hand, our data set from the southern hemisphere appears to reflect primarily natural land-derived biogenic emissions. DOI: 10.1111/j.1600-0889.1986.tb00184.x

Radiaxial Calcite in Lower Miocene Strata, Subsurface Enewetak Atoll

ABSTRACT Radiaxial calcite is abundant in Lower Miocene strata, 377-819 m deep on Enewetak Atoll. These radiaxial crystals have equant, bladed, and fibrous morphologies and occur as isopachous crusts 0.05-2.0 mm thick. Radiaxial terminations consist of fibrous subcrystals bundled together into larger acute, obtuse, or rounded shapes. Enewetak radiaxial calcite ranges from clear to very inclusion-rich. Most inclusions are microcavities caused by organic infestations (Videtich 1985). Alternating bands of inclusion-rich and inclusion-poor calcite mimic existing termination patterns. Radiaxial calcites fill intergranular pores, sponge borings, fractures, and aragonite molds. Aragonite has been pervasively dissolved from the Lower Miocene strata containing radiaxial calcite. Banded radiaxial calcit can be traced from intergranular pores into aragonite molds. Radiaxial calcites have a mean magnesium concentration of 3.2 mole % MgCO3 with a range from 1.6 to 11.1 mole % MgCO3. Inclusion-rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations. Enewetak radiaxial calcite has 13C values of +1.3 to +2.5 (PDB) and 18O values of -1.8 to +0.4 (PDB). Petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater. Precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition, probably in a burial environment. Dissolution of aragonite by deep marine water is suggested by: 1) abundant marine cement associated with aragonite dissolution; 2) a lack of meteoric cements associated with aragonite dissolution; and 3) no evidence of subaerial exposure in Lower Miocene strata between 400 and 819 m. Modern Pacific Ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed (375-850 m). Tidal fluctuations and temperature profiles indicate that Lower Miocene strata containing radiaxial calcite are in open communication With modern Pacific Ocean water. Therefore, aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater (undersaturated with respect to aragonite) circulating through the atoll. Variations in magnesium concentration in Enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater. The first alternative is preferred. Precipitation of radiaxial calcite at different rates (at different degrees of saturation) could cause variable magnesium concentrations. More deeply buried radiaxial calcite may have precipitated more slowly in deeper, less supersaturated water, resulting in lower magnesium concentrations. Likewise, slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion-rich bands. Enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric: 1) precipitation at fluctuati g rates, and 2) formation in waters undersaturated with respect to aragonite.

Mass spawning of Caesio teres (Pisces: Caesionidae) at Enewetak Atoll, Marshall Islands

Mass spawning of Caesio teres occurred between March and August, 1983 on a reef emerging from deep water just inside the East Channel of Enewetak Atoll, Marshall Islands. Aggregations and spawning were observed from one day before until three days after the full moon. Spawning aggregations were not observed during the new or quarter moons. Spawning commenced after high tide, when the current began to flow out the pass from lagoon to ocean. The spawning aggregation of close to 1000 individuals migrated to the spawning site. Spawning occurred when the aggregate ascended to near the water's surface. Subgroups dashed horizontally within the aggregate, releasing a highly visible gamete cloud. Predation on spawning adults was not observed. Predation on spawned eggs was noted. The eggs of C. teres are described.

Oligocene sea-level falls recorded in mid-Pacific atoll and archipelagic apron settings

Drilling results from mid-Pacific atoll and archipelagic apron sites in the Line Islands and Marshall Islands provinces lead to the conclusion that Oligocene sea-level falls detected in Atlantic passive margin sequences are also recorded in a mid-plate-tectonic setting in the Pacific Basin. The mid-Pacific sea-level falls are recorded by (a) the presence of distinct, coarse-grained, graded beds of turbidite origin, rich in reef-derived skeletal debris of Oligocene, Eocene, and Cretaceous age, that were redeposited in deep-water archipelagic apron carbonate sequences of middle and late Oligocene age now flanking the atolls and (b) a marked stratigraphic hiatus and solution unconformity in the subsurface of Enewetak atoll which dates an Oligocene period of atoll emergence correlative with both the deposition of the turbidites and the coastal offlap events discerned in Atlantic passive margins. Correlation of the subsidence path of Enewetak atoll with the development of the Oligocene solution unconformity shows that ca. 30 Ma sea level was as much as 100 m lower than at present.

Modeling atmospheric transport to the Marshall Islands

Isentropic trajectory analyses are presented which support the hypothesis that atmospheric continental material found at Enewetak Atoll (11.3°N, 162.3°E) during the Sea‐Air Exchange (SEAREX) experiments in 1979 had its origin primarily in Asia in the springtime (dry season experiment) and in North and Central America in the summer (wet season experiment). Fields of wind, Montgomery potential, and pressure on isentropic surfaces are obtained from global isobaric analyses by vertical interpolation. Trajectories backward in time from the area at and upwind of the experiment site were calculated using these fields. In April and May 1979 the atmospheric chemistry at Enewetak was influenced strongly by long‐range transport from Asia; this transport was primarily in the potential temperature range 305–315 K with travel times of 8–13 days. Westerly winds over Asia at 350–600 mbar carry continental materials over the ocean, and as the air moves southward, subsidence occurs until the air is entrained in the trade wind flow. During July and August 1979 the transport paths were from open ocean areas and from near North and Central America at 305–310 K with travel times of 17–21 days. The trajectories remained at low levels within the boundary layer during this period. Also discussed is a meteorological analysis of dust storms in China, which shows that a mechanism exists for lifting eolian material to the upper troposphere. There is substantial uncertainty in the individual trajectories, and the factors limiting their accuracy are discussed. Given the consistency of the trajectory analyses with the chemical results, we now have a coherent picture of some of the processes responsible for long‐range transport to the subtropical open ocean.

Nonrandom mating with respect to mate size in the crabTrapezia(Brachyura, Xanthidae).

Crabs of the genus Trapezia, obligate symbionts of reef corals, typically occur as heterosexual pairs, with a single pair of a given species per coral colony. Pair formation, and the effects of size on pairing, were studied in T. intermedia and T. digitalis in Hawaii, in T. tigrina, T. rufopunctata, and T. dentata at Enewetak Atoll, Marshall Islands, and in T. corallina at Panama. When a male and female of the same species were placed on separate corals, either the male or the female usually moved to the colony inhabited by the other crab, resulting in the formation of a pair. Both males and females paired with the larger of two prospective mates significantly more often than expected on the basis of random pairing; the “preference” for large mates did not vary with the relative sizes of the crabs. The distribution of injuries which occurred in the experiments indicated that there was aggressive competition for mates between members of the same sex. While the experimental results can be explained by intra...

Correction to “Atmospheric trace elements at Enewetak Atoll: 2. Transport to the ocean by wet and dry deposition” by R. Arimoto et al.

Correction to “Atmospheric trace elements at Enewetak Atoll: 2. Transport to the ocean by wet and dry deposition” by R. Arimoto et al.

Predation in Insular Plant Dynamics: An Experimental Assessment of Postdispersal Fruit and Seed Survival, Enewetak Atoll, Marshall Islands

Predation in Insular Plant Dynamics: An Experimental Assessment of Postdispersal Fruit and Seed Survival, Enewetak Atoll, Marshall Islands

Atmospheric trace elements at Enewetak Atoll: 2. Transport to the ocean by wet and dry deposition

The concentrations of trace elements in precipitation and dry deposition are presented for samples collected at Enewetak Atoll (11°N, 162° E) during SEAREX experiments in 1979. The concentrations of Al, Sc, Mn, Fe, Co, and Th in rain are dominated by crustal material, and for these elements, wet deposition evidently exceeds dry deposition. For most of these elements the present rates of atmospheric deposition at Enewetak are similar to their mean rate of accumulation in sediments over the past 5–10,000 years, suggesting that the air‐to‐sea exchange of particles is closely tied to the sedimentary cycle of the mid‐Pacific. Noncrustal sources govern the concentrations of Pb, Zn, Cu, Se, and Cd in wet and dry deposition samples. Analyses of dry deposition collected from a flat plastic plate indicate that the amount of material recycled from the sea surface varies markedly between samples, and even though these estimates do not necessarily reflect the dry deposition to the ocean surface, the results suggest that recycled sea spray often amounts to more than 50% of the total dry deposition of the enriched elements. Recycled sea spray also makes up a significant fraction of the total wet deposition of the enriched elements. The net deposition rates of elements such as Cu and Zn are greater than or equal to their inputs from vertical mixing, but the net deposition of Pb clearly exceeds the input from upwelling. The current net deposition rates of the enriched elements are also similar to their rates of removal to sediments. These results indicate that air‐sea exchange processes may significantly affect the chemistry of trace metals in the open ocean.

Air‐to‐sea fluxes of lipids at Enewetak Atoll

We report data for the Enewetak site of the SEAREX program from the rainy season in 1979. The concentrations of n‐alkanes, n‐alkanols, sterols, n‐alkanoic acids and their salts, and total organic compounds in rain are reported, as well as the apparent gaseous hydrocarbon concentrations. These data and information on the particulate forms are analyzed in conjunction with ancillary chemical and meteorological data to draw inferences about sources, fluxes, and chemical speciations. While the higher molecular weight lipid biomarker components are exclusively terrestrial, the organic carbon in rain may be derived from atmospheric transformations of terrestrial carbon. Distinctively marine components are nearly absent. Comparison of the scavenging ratios of the organic components in rain vs. those for clays reveals that the alkanoic acids and the higher molecular weight alkanols behave as essentially particulate materials, whereas lower alkanols and most hydrocarbons show much higher scavenging ratios, probably due to the involvement of a gaseous phase or sampling artifact. Vaporization in the atmosphere and scavenging of a gas phase would lead to higher scavenging ratios; vaporization during sampling would give low aerosol concentrations and high gas‐phase concentrations, leading to high scavenging ratios. The major fluxes at Enewetak result from rain rather than dry deposition, and extrapolating the measured values to meaningful annual averages requires adjustment for seasonally varying source intensity and rain dynamics. Aerosol data for other seasons and other substances are used to correct for source‐strength intensity variations, and a 210Pb/organic compound correlation is established and extrapolated to adjust for rainfall volume effects. These corrections, assumed independent and applied together, yield inferred fluxes 2.5–9 times larger than the fluxes calculated for mean concentrations. The inferred fluxes to the ocean, while small compared to primary production, are large enough to have potential impacts in the cycle of dissolved organic carbon and the sedimentary geochemistry of refractory lipid components.

The Effect of Shading by the Table Coral Acropora Hyacinthus on Understory Corals

Field surveys at Enewetak Atoll, Marshall Islands, show that coral density and diversity is much lower beneath Acropora table corals than in adjacent unshaded areas. Additionally, the understory community is predominantly composed of massive and encrusting species, while branching Acropora and Pocillopora predominate in unshaded areas. Results of experiments in which coral fragments were transferred to the shade of table Acropora and to adjacent unshaded areas show that shading slows the growth and leads to higher mortality of branching species, while massive and encrusting species are unaffected. Light measurements made beneath table Acropora show that illumination and irradiance values fall to levels at which most hermatypic corals do not occur. The fast—growing but fragile table Acropora are abundant in a wide variety of atoll habitats and grow rapidly to form a canopy °50 cm above the substrate. However, table Acropora also have high mortality rates, so that there is continuous production of unshaded areas. The growth and death of tables thus create local disturbances, and the resulting patchwork of recently shaded and unshaded areas may enhance coral diversity in areas of high coral cover.

Redistribution of fallout radionuclides in Enewetak Atoll lagoon sediments by callianassid bioturbation.

The lagoon sediments of Enewetak Atoll in the Marshall Islands contain a large selection of fallout radionuclides as a result of 43 nuclear weapon tests conducted there between 1948 and 1958. Studies of the burial of fallout radionuclides have been conducted on the islands and in several of the large craters, but studies of their vertical distribution have been limited to about the upper 20 cm of the lagoon sediments. We have found elevated fallout radionuclide concentrations buried more deeply in the lagoon sediments and evidence of burrowing into the sediment by several species of callianassid ghost shrimp (Crustacea: Thalassinidea) which has displaced highly radioactive sediment. The burrowing activities of callianassids, which are ubiquitous on the lagoon floor, facilitate radionuclide redistribution and complicate the fallout radionuclide inventory of the lagoon.

Marine Diagenesis on Enewetak Atoll: ABSTRACT

Marine waters currently circulate through Eocene and Miocene limestones deposited in slope, fore-reef, and reef-related environments on Enewetak Atoll. Subsurface core samples (375-1,400 m deep) demonstrate that marine waters have extensively altered the original carbonate sediments. Evidence supporting alteration by marine water includes: (1) calcite cements with isotopic compositions characteristic of a marine origin (^dgr13C = 1.3 to 2.5^pmil; ^dgr18O = -1.8 to 0.4^pmil PDB), (2) dolomites with average isotopic compositions that are in equilibrium with cold normal-marine waters (^dgr13C = 2.3^pmil; ^dgr18O = 2.5^pmil PDB), (3) consistent magnesium concentrations in fossil coralline algae, and (4) a lack of diagnostic evidence for meteoric diagenesis. Products of marine diagenesis on Enewetak Atoll are dependent on the CaCO3 saturation state of the diagenetic waters. Above the aragonite saturation depth (350 m), marine diagenesis is dominated by aragonite and high-magnesian-calcite cementation. Between aragonite and calcite saturation depths, marine waters apparently have dissolved aragonite and precipitated low-magnesian, radiaxial calcite cements. Below the calcite saturation depth (1,000 m), deep marine waters have apparently dissolved some calcite and have precipitated dolomite. Effective marine diagenesis requires limestones with good permeability and a hydraulic drive, such as tidal pumping or thermal convection, to move large volumes of marine water through the rock. End_of_Article - Last_Page 304------------

Population genetics of eight species of Trapezia (Brachyura: Xanthidae), symbionts of corals

Starch-gel electrophoresis was used to study gene-enzyme variation in thirteen populations of eight species of the genus Trapezia from Hawaii, Panama, and Enewetak Atoll (Marshall Islands). Between 20 and 30 (mean = 27.8) gene-enzyme systems were resolved in each population, with 20 systems in common among all populations. The distribution of electrophenotypes was in agreement with Hardy-Weinberg-Castle expectations, except for T. digitalis, which consistently showed heterozygote deficiencies. Diagnostic loci among color forms support the hypothesis that color forms are distinct species. Low values of genetic distance among species suggest a recent radiation, perhaps during the Pleistocene. Genetic distance between the Hawaiian and Panamanian populations of T. ferruginea did not significantly differ from zero, indicating that the Eastern Pacific population of T. ferruginea has recently immigrated from the central Pacific, and/or that there is gene flow between the two areas. There were diagnostic loci between T. ferruginea and T. formosa from Enewetak and the populations of these species from Hawaii (T. ferruginea only) and Panama (both species). Therefore, these geographic populations may represent separate species. The level and pattern of genetic variability in Trapezia spp. are in agreement with those observed in most other organisms.

Electron Microprobe Study of Mg Distribution in Recent Mg Calcites and Recrystallized Equivalents from the Pleistocene and Tertiary

ABSTRACT The effect of early freshwater diagenesis on Mg distributions in former Mg calcite cements was determined by electron microprobe analysis of two major groups of samples: 1) Mg calcites in Recent limestones not yet exposed to freshwater, and 2) Tertiary-Recent Mg calcites and recrystallized equivalents which have undergone freshwater exposure. Pristine Mg calcites are characterized by a uniform Mg concentration. In contrast, former Mg calcites from the Miocene of Enewetak Atoll, most of which are now radiaxial, prismatic calcites, have a much greater variation in Mg content, and traverses across the calcites reveal definite trends. Mg content of the calcite is inversely correlated with abundance of inclusions. Some of the cloudy growth layers are due to infestation by endolithic organi ms (fungi?). The small variation in Mg content in original Mg calcite cements is a valuable starting point to be aware of when interpreting Mg trends in recrystallized equivalents. Mg content can be related to inclusion abundance in recrystallized, former Mg calcites, but not in pristine Mg calcites. Therefore, Mg trends in altered materials most likely reflect dissolution/reprecipitation processes, not original Mg trends. A likely cause for the low Mg content associated with inclusion-rich growth layers is relatively high rates of dissolution and reprecipitation in these relatively permeable layers. In some cases, the increased permeability is the result of infestation by endoliths. Such organisms may cause the cloudiness observed in some other radiaxial calcites.