Central South Pacific Ocean Research Articles

Short-wavelength gravity lineations and unusual flexure results at the Puka Puka volcanic ridge system

Short-wavelength gravity lineations aligned parallel to the direction of absolute motion of the Pacific plate, and a newly discovered series of linear, elongate volcanic ridges in the south-central pacific Ocean are just two of the many geophysical and geochemical anomalies that have been observed in this area. These Puka Puka volcanic ridges can be traced for 2600 km along the trough of a major gravity lineation and stretch from close to the East Pacific Rise, in the east, to the Tuamotu Islands in the west. The ridges were the focus of a recent cruise to this area that collected high quality gravity and multi-beam swath bathymetry data, in addition to dredge samples that are suitable for radiometric age dating. A complete 2-D lithospheric flexure analysis of these new data reveals that each of the volcanic ridges is associated with an unusually low effective elastic plate thickness. Previous workers showed this region to be characterised by lower than expected elastic thickness values, which were interpreted in terms of both regional and more localised thermal anomalies in the oceanic lithosphere. The new flexure results obtained for the volcanic ridges in this study confirm these low values. Lithospheric stretching and small-scale convection models that have been put forward to explain the origin of the lineations and volcanic ridges have been re-examined in light of these new results but neither is found to satisfactorily explain all of the observations.

A comparison of the vegetation communities from the islands of the Pitcairn Group

The vegetation of the raised coralline Henderson Island and Oeno Atoll (Pitcairn Group, south-central Pacific Ocean) is described from cover-abundance values of the vascular flora sampled in 10 times 10 m quadrats. Exploratory data analysis by detrended correspondence analysis and two-way indicator species analysis was used to summarize the resulting data matrices for each island, and to aid definition of vegetation types. For Henderson, six major vegetation communities are described, which are often closely linked to geomorphology: beachfront, embayment forests, open limestone scrub, cliff and ledge, exposed cliff top, and plateau forests. For Oeno, four main vegetation communities are described: open sandy littoral, Argusia scrub, closed forest, and coconut grove. For each island, the main vegetation types were further subdivided into a number of communities depending on the proportions of various taxa. Brief qualitative descriptions of the vegetation of Ducie and Pitcairn are also given. Anthropogenic communities, some of which are highly invasive, are widespread on Pitcairn. Adequate conservation measures must be given to all islands in the Pitcairn Group; this will protect not only the endemic-rich and possibly unique plateau forests of Henderson, but also the species-poor and scientifically important analogues of vegetation types which occur elsewhere in the Pacific islands.

The flora of the Pitcairn Islands: a review

Abstract The vascular plant flora of the Pitcairn Islands, south-central Pacific Ocean, is described based on extensive new collections made in 1991 and previously published records. Two vascular plants occur on Ducie Atoll; one ( Pemphis acidula ) is a new record. Sixty-three native vascular plants occur on Henderson, of which nine are endemic; Canavalia rosea , Operculina turpethum , Psilotum nudum and Solanum americanum are new records for the island. Oeno Atoll has 16 native vascular plants; the single endemic ( Bidens hendersonensis var. oenoensis ) was not found in 1991 despite careful searches. Triumfetta procumbens was new for Oeno. Sixty-six native vascular plants have now been recorded from Pitcairn Island, there are two endemic ferns and seven endemic angiosperms in this number. A number of non-native taxa were new to Pitcairn. Some of the previously described taxa could not be found on Pitcairn, probably because they are very rare and only a small amount of time was spent collecting on Pitcairn. Many of the Pitcairn taxa are threatened by the spread of introduced species, especially Syzygium jambos

A comparison of the vegetation communities from the islands of the Pitcairn Group

Abstract The vegetation of the raised coralline Henderson Island and Oeno Atoll (Pitcairn Group, south-central Pacific Ocean) is described from cover-abundance values of the vascular flora sampled in 10 10 m quadrats. Exploratory data analysis by detrended correspondence analysis and two-way indicator species analysis was used to summarize the resulting data matrices for each island, and to aid definition of vegetation types. For Henderson, six major vegetation communities are described, which are often closely linked to geomorphology: beachfront, embayment forests, open limestone scrub, cliff and ledge, exposed cliff top, and plateau forests. For Oeno, four main vegetation communities are described: open sandy littoral,Argusiascrub, closed forest, and coconut grove. For each island, the main vegetation types were further subdivided into a number of communities depending on the proportions of various taxa. Brief qualitative descriptions of the vegetation of Ducie and Pitcairn are also given. Anthropogenic communities, some of which are highly invasive, are widespread on Pitcairn. Adequate conservation measures must be given to all islands in the Pitcairn Group; this will protect not only the endemic-rich and possibly unique plateau forests of Henderson, but also the species-poor and scientifically important analogues of vegetation types which occur elsewhere in the Pacific islands.

Rare and endemic vascular plants of the Pitcairn Islands, south-central Pacific Ocean: A conservation appraisal

The endemic and threatened plants of the Pitcairn Islands (Pitcairn, Henderson, Oeno, Ducie) are listed and their conservation status assessed. The volcanic Pitcairn and the raised atoll Henderson are the most floristically rich of the islands, and each has a number of endemic taxa. Over half of the native Pitcairn flora is either threatened or insufficiently known (but likely to be threatened), the main threats being removal of native vegetation cover and the invasion of exotic taxa, particularly Syzygium jambos. A detailed survey of the Pitcairn flora is urgently needed to determine fully the status, likely threats, and measures required to conserve the native flora. Ex situ conservation of the endangered Pitcairn taxa and protection of suitable remaining habitats are the most urgent conservation requirements in the whole of the island group. Less than 20% of the Henderson taxa are threatened, and only two of the endemic taxa ( Myrsine hosakae and Santalum insulare var. hendersonensis) are considered threatened. Threatened taxa on Henderson are mostly species which occur very locally on the island or in small populations; they include several very widespread species. Oeno and Ducie are atolls with depauperate floras; Oeno contains three threatened taxa, one of which may be a recent colonist, and another may be extinct. We recommend an immediate floristic survey of Pitcairn, the establishment of a Conservation Officer and the implementation of a sustainable development and conservation policy for the island.

An altimetric and bathymetric study of elastic thickness in the central Pacific Ocean

Using a 1-D spectral approach we recover the underlying topography across ten seamounts in the south central Pacific Ocean from high-resolution along-track satellite altimetry data, this in turn yields an estimate for the elastic thickness value. Theoretical modelling showed that after correcting for the effects of one-dimensionality, this approach can clearly indicate areas of low elastic thickness values. Our results show that whilst a number of the seamounts in this region display anomalously low elastic thickness values, some of the seamounts studied have values close to those expected. Coupled with the results from geochemical and heat flow studies, this suggests that the region of low elastic thickness values is not as extensive as previously thought.

Goussia auxidis (Dogiel, 1948) (Apicomplexa: Calyptosporidae) from tuna (Pisces: Scombridae) in the South Pacific

Abstract. A coccidian identified as Goussia auxidis (Dogiel, 1948) is recorded for the first time from the liver and spleen of albacore, Thunnus alalunga; the liver of slender tuna, Allothunnus fallai, skipjack tuna, Katsuwonus pelamis and yellowfin tuna, T. albacares: and in the spleen of Scomber australasicus. All host fish were caught in the western and central South Pacific Ocean. Coccidian spores were not found in the liver of southern bluefin tuna, T. maccoyii, or butterfly tuna, Gasterochisma melampus.

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.

Infrasonic seismic and acoustic measurements in the deep ocean

The authors compare the signal-to-noise ratios obtained on bottomed seismometers, bottomed hydrophones, and buried seismometers from near-surface explosions in the Ngendei Expedition. The data were recorded in 5.5-km-deep water in the south central Pacific Ocean with a triaxial borehole seismograph and four triaxial ocean-bottom seismographs having externally mounted hydrophones. At ranges less than 35 km, the data indicate that the ocean bottom seismometer is a superior signal detector than the ocean-bottom hydrophone, and that the subbottom seismometer is superior in performance to the ocean-bottom seismometer. Above 4 Hz, the seismometer appears to have a 10-dB signal-to-noise advantage over the hydrophone for surface explosions at ranges less than 30 km.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Resolution of low‐frequency attenuation anomalies in the Southern Hemisphere

During Project KIWI ONE [D. G. Browning et al., Nature 282, 820–822 (1979)] anomalously high values of low‐frequency attenuation were observed in the central South Pacific Ocean. Similar results were obtained along a track from New Zealand into the Southern Ocean during Project TASMAN TWO [R. W. Bannister et al., J. Acoust. Soc. Am. 62, 847–859 (1977)]. A recent analysis of oceanographic data by Mellen shows that both regions are included in a relatively high pH contour at the sound channel axis. The corresponding predicted values of attenuation are in reasonable agreement with the measured data.

Nimbus 7 earth radiation budget wide field of view climate data set improvement: 1. The Earth albedo from deconvolution of shortwave measurements

A deconvolution technique to recover the daily average earth albedo from a set of wide field of view (WFOV) satellite measurements is presented. For each shortwave observation in space a corresponding synthetic WFOV integral is formulated to simulate the satellite value. This is done by including the known earth‐sun‐satellite geometry and a compromise diurnal model, depicting reflection from the earth‐atmosphere system as isotropic and occurring at an effective top of the atmosphere (TOA), and by representing the unknown albedo field as a truncated series of spherical harmonic functions. The series expression for the albedo reduces the measurement integral to a linear equation in the now unknown expansion amplitudes. A typical sampling period leads to many such Satellite measurements and their associated linear representations. This system of linear equations will generally be overdetermined and is solved in a least squares sense for the expansion amplitudes and resulting albedo field. Simulation studies show that a maximum of about 289 pieces of information can be extracted from a set of Nimbus 7 WFOV satellite measurements. These correspond to the number of linearly independent modes of variation definable in the TOA albedo expansion. Zonally, variations through eight waves may be resolved and, meridionally, through 16 nodes. When compared to the current WFOV archived albedo product, the deconvolved albedo field achieves about a 20% reduction in the global rms regional reflected flux density errors. This error reduction is attained within a data collection period containing as few as 6 sampling days and remains constant for longer periods. Global mean measurements are, of course, not changed. An application of these results is used to derive the mean, daily average TOA albedo field for January 1983. An unusually strong and extensive albedo maximum is noted over the south central Pacific Ocean. It has a central maximum of 0.42 and corresponds to the El Niño/Southern Oscillation event of 1982–1983.

Origin of basalts from the Marquesas Archipelago (south central Pacific Ocean): isotope and trace element constraints

Basalts from the Marquesas Archipelago display significant variations according to magmatic type in 143Nd/ 144Nd (0.512710–0.512925) and 87Sr/ 86Sr (0.70288–0.70561) suggesting heterogeneities at various scales in the mantle source, with respectively the highest and lowest values in tholeiites compared to alkali basalts. This relationship is the reverse from that observed in the Hawaiian islands. Systematic indications of magma mixing are recognized from the relationships between trace element and isotopic ratios. Tholeiites from Ua Pou Island which have unradiogenic Sr (about 0.7028) plot close to basalts from Tubuai and St. Helena, i.e. distinctly below the main mantle trend in the Nd vs. Sr isotopic diagram. It is suggested that the source of these tholeiites is ancient subducted lithosphere which has suffered previous extraction of liquid with island arc tholeiite composition. The trace element and isotopic data of the basalts from the other Marquesas Islands imply the contamination of an equivalent source by an enriched component. This latter has trace element characteristics of the upper crust.

Transitional geomagnetic field behavior: Volcanic records from French Polynesia

Here we report the behavior of the earth's magnetic field for transitions recorded on volcanic islands from Polynesia in the South Central Pacific Ocean. Detailed transitional fields are observed on Huahine Island; the data suggest the record of an excursion N‐N following the Kaena event. The Jaramillo termination and the Brunhes onset are recorded in a volcanic sequence from Tahiti Island. The transitional field is mainly characterized by low intensities. While the beginning of the transition appears to be consistent with the zonal model of reversal, large deviations from a north‐south planar distribution of the intermediate directions show that non dipolar fields without preferential axisymmetry control the middle of the transition.

Geochemistry and origin of basaltic lavas from Marquesas Archipelago, French Polynesia

The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii.

Geochemistry and origin of basaltic lavas from Society Islands, French Polynesia (south-central Pacific Ocean)

Geochemistry and origin of basaltic lavas from Society Islands, French Polynesia (south-central Pacific Ocean)

Two Severe Freezes in Brazil: Precursors and Synoptic Evolution

Abstract Two freezes with repercussions on world coffee markets struck Brazil in recent years, one during FGGE in 1979, the other in 1981. Data from multiple sources including satellites and drifting buoys were analyzed for early warning signs and synoptic evolution of the freeze events. Two important precursors were found: 1) A slowly moving long-wave pattern in the central South Pacific Ocean amplified greatly one to two days before frontogenesis in South America and four to five days before the freezes in Brazil; 2) the long-wave ridge, when at maximum amplitude, was located near the southern Andes, and the next long-wave trough downstream was located in the South Atlantic at the longitude of Brazil. This configuration channeled subantarctic air northward entirely over land into subtropical latitudes. Frontogenesis in low latitudes in the mid-Pacific was the first sign of wave amplification in the 1979 case. Subsequent frontogenesis in northern Argentina was accelerated by the opportune approach of a s...

Possible evidence of the Southern Oscillation in total ozone at Arosa

Statistical tests of a possible association between total ozone at Arosa and indices of the Southern Oscillation give indefinite but not unpromising answers: linear correlation coefficients between 12-month running means are statistically significant for both halves of the period 1935–1980, whereas the squared coherences are statistically significant in the first but not in the second half of the period. The phases in both periods are consistent, however. Our investigation shows that a relationship between the total ozone amounts at Arosa and the Southern Oscillation, must be in the sense that when the sea level mean pressure is low over the central tropical South Pacific Ocean while it is high in the Indonesian region, the total ozone amounts at Arosa tend to be large, and conversely.

Geochemistry and origin of basaltic lavas from Society Islands, French Polynesia (south central Pacific Ocean)

The Society Islands of French Polynesia (south-central Pacific Ocean) are formed predominatly by basaltic lavas of alkaline affinities. These intraplate rocks are either Ne- or Hy-normative. Ne-normative basaltic lavas are lower in Si and heavy rare earth elements and higher in Ti, V and Sr than Hy-normative. The differences are attributed to the contrasting behaviour of amphibole during melting of the upper mantle, which, in turn, may be related to the variation in the depth of melting. Compared to pyrolite, the upper mantle source of the lavas was enriched in incompatible elements.

Teleseismic location techniques and their application to earthquake clusters in the South-Central Pacific

abstract Improved methods for single-and multiple-event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the South-Central Pacific Ocean. Bayesian statistical methods are used to incorporate a priori information about arrival-time variance into the derivation of hypocenter confidence ellipsoids, permitting a more realistic calculation of critical parameters in the case where the number of stations is small. The diagonal elements of certain projection operators, called “data importances” by Minster et al. (1974), are used to evaluate network balance. The hypocentroid of an event cluster is defined to be the average location of events within the cluster, and the deviations of individual hypocenters from the hypocentroid are called cluster vectors . The problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result, here termed the hypocentroidal decomposition theorem. The algorithm based on this analysis appears to have many advantages over other published methods for multiple-event location, both in its efficient use of available information and its computational speed. The application of this method to three clusters of shallow intraplate seismicity in the South-Central Pacific, designated Regions A, B, and C, demonstrates that the seismicity within each cluster is very localized; the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9, 6, and 12 km, respectively. Estimates of the epicentroids are

Project Kiwi One: an acoustic cross-section of the South Pacific Ocean

The deep ocean sound channel is used to obtain very long range (typically >2,000 km) acoustic transmission1 via totally refracted propagation paths (SOFAR propagation2). Such experiments can therefore determine the acoustic transmission properties of large areas of ocean3. Those acoustic propagation experiments can be used to locate major ocanographic changes4 and to identify specific water masses5. A 10,000-km underwater sound transmission experiment conducted between New Zealand and Peru to obtain an acoustic cross-section of the South Pacific Ocean is described here. Three distinct regions of transmission loss were found. The highest attenuation, which is attributed to Antarctic intermediate water, occurred in the central South Pacific Ocean.