SW Pacific Research Articles

The competing effects of sulfide saturation versus degassing on the behavior of the chalcophile elements during the differentiation of hydrous melts

AbstractThere is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile‐rich magmas from the Eastern Manus Back‐arc Basin (EMBB) only reach sulfide saturation following magnetite‐driven reduction of the melt: the so‐called “magnetite crisis.” If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile‐rich phase, which may contribute to the sporadic occurrence of magmatic‐hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile‐rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga arc‐proximal Valu Fa Ridge (VFR; SW Pacific). Cu‐Se‐Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation‐triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate that the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely that the evolving VFR and EMBB were saturated in a S‐bearing volatile phase prior to magnetite fractionation. Hence, the metal‐depleting magnetite crisis may be restricted to back‐arc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high pressures in the continental crust.

Effects of oceanic circulation and volcanic ash-fall on calcite dissolution in bathyal sediments from the SW Pacific Ocean over the last 550ka

Abstract The effects on calcite dissolution of both oceanic circulation and volcanic ash-fall were evaluated in lower bathyal sediments over the last 550 ka record from core MD 97-2114, recovered on the northern slope (depth of 1936 m, in the Pacific Deep Water, PDW) of the Chatham Rise (east of New Zealand, SW Pacific Ocean). This area has been impacted by changes in glacial/interglacial circulation and ocean chemistry as well as by the explosive volcanic activity of the Taupo Volcanic Zone. Several micro-paleontological dissolution proxies, based on planktonic foraminifera and calcareous nannofossils, were analysed in order to evaluate the calcite dissolution of the deep-sea sediments. These were compared with a couple of proxies of primary productivity (benthic foraminiferal epifaunal/infaunal ratio and δ13Cbenthic foraminifera) and the abundance of volcanic glass. The dissolution proxy data from MD 97-2114 were compared with two nearby ODP sites, ODP 1123 (3290 m deep, bathed by the lower Circumpolar Deep Water, LCDW) and ODP 1125 (1365 m deep, bathed by the Antarctic Intermediate Water, AAIW). The results suggest: (1) the calcite dissolution/preservation cycles at all three core sites show Glacial–Interglacial (G–I) periodicities that match the previously described “Pacific-style” CaCO3 cycles; (2) several short-term dissolution events do not follow this general scheme and occur following tephra deposition. The dissolution related to the tephra deposition seems to have mostly affected calcareous nannofossils, thus we hypothesise that the ash-fall induced a temporary reduction of the surface water pH (below 7.8), which affected the coccolithophores that inhabit the surface waters. (3) Other short-term dissolution events (1000 years) are unrelated to tephra deposition and are possibly driven by the slowing of deep-sea circulation and a reduced Deep Western Boundary Current (DWBC). This lead to the dominance of older, more corrosive Pacific Deep Water (PDW) flowing in to the region, resulting in coeval dissolution episodes at all three core sites (depth range from 1365 to 3290 m).

A Paleozoic Japan-type subduction-accretion system in the Beishan orogenic collage, southern Central Asian Orogenic Belt

Magmatic arcs ascribed to oceanic lithosphere subduction played a dominant role in the construction of the accretionary Central Asian Orogenic Belt (CAOB). The Beishan orogenic collage, situated between the Tianshan Orogen to the west and the Inner Mongolia Orogen to the east, is a key area to understanding the subduction and accretionary processes of the southern CAOB. However, the nature of magmatic arcs in the Beishan and the correlation among different tectonic units along the southern CAOB are highly ambiguous. In order to investigate the subduction-accretion history of the Beishan and put a better spatial and temporal relationship among the tectonic belts along the southern CAOB, we carried out detailed field-based structural geology and LA-ICP-MS zircon U-Pb geochronological as well as geochemical studies along four cross-sections across crucial litho-tectonic units in the central segment of the Beishan, mainly focusing on the metamorphic assemblages and associated plutons and volcanic rocks. The results show that both the plutonic and volcanic rocks have geochemical characteristics similar to those of subduction-related rocks, which favors a volcanic arc setting. Zircons from all the plutonic rocks yield Phanerozoic ages and the plutons have crystallization ages ranging from 464±2Ma to 398±3Ma. Two volcanic-sedimentary rocks yield zircons with a wide age range from Phanerozoic to Precambrian with the youngest age peaks at 441Ma and 446Ma, estimated to be the time of formation of the volcanic rocks. These new results, combined with published data on ophiolitic mélanges from the central segment of the Beishan, favor a Japan-type subduction-accretion system in the Cambrian to Carboniferous in this part of the Paleo-Asian Ocean. The Xichangjing-Niujuanzi ophiolite probably represents a major suture zone separating different tectonic units across the Beishan orogenic collage, while the Xiaohuangshan-Jijitaizi ophiolitic mélange may represent a Carboniferous back-arc basin formed as a result of slab rollback ascribed to northward subduction of the Niujuanzi oceanic lithosphere. Subduction of this back-arc basin probably took place in the early Carboniferous, generating the widespread arc-related granitoids including adakitic plutons, and overlapping earlier arc assemblages. The Beishan orogenic collage is not the eastern extension of the Chinese Central Tianshan, but it was generated by the same north-dipping subduction system separated by the Xingxingxia transform fault, as revealed by available regional data. This contribution implies that in addition to fore-arc accretion, back-arc accretion ascribed to opening and closure of a back-arc basin may also have been a common process in the construction of the CAOB, resembling that of the Mesozoic-Cenozoic subduction-accretion system in the SW pacific.

Using stable isotopes of albacore tuna and predictive models to characterize bioregions and examine ecological change in the SW Pacific Ocean

Broad-scale food web inferences of 534 albacore tuna, Thunnus alalunga, in the south-west Pacific Ocean in 2009 and 2010 were made using bulk nitrogen (δ15N) and carbon (δ13C) stable isotopes. Condition was also examined for the same fish using C:N ratios. A Generalized Additive Modeling (GAM) approach was used to analyze spatio-temporal, biological and environmental drivers that impact the distribution of tuna isotopes and to create oceanographic maps. Based on model formulations, five bioregions with distinct isotopic signatures were identified and were related to known biological, nutrient cycling and oceanographic (temperature, primary productivity and eddy) features associated with the East Australian Current. δ15N values showed a large-scale, uniform latitudinal gradient decreasing from the south to north, in a region encompassing oligotrophic waters in the Coral Sea. In contrast, δ13C values were lower in the nutrient rich Tasman Sea waters and offshore East Australia. C:N ratios suggested that albacore occupying southern and offshore waters were in better condition. Ontogenetic trends in all three biochemical parameters were identified and related to differences in size distribution. Regional-specific temporal variations were detected including similar monthly changes for both isotopes and significantly less enriched δ13C (by 1.9‰) than in previous work undertaken in 2006, potentially signifying a substantial shift in the carbon cycle that supports food webs off central east Australia. Our results showed that isotopic measurements in tuna and the GAM framework provide powerful tools to assess ecosystem functioning and change by linking sources of nutrients and organic matter to local food web assembly. Such knowledge is vital to support an ecosystem based approach to fisheries management including biogeochemical whole-of-ecosystem models and monitoring programs at regional and landscape-scales.

The agglutinated foraminifera from the SW Pacific bathyal sediments of the last 550kyr: Relationship with the deposition of tephra layers

Abstract The agglutinated foraminiferal content from the last 550 kyr record of the IMAGES core MD 97-2114 (Chatham Rise, New Zealand) was analysed in order to detect the possible linkage existing between the composition of the grains forming the agglutinated tests and the deposition of tephras. The core was collected east of New Zealand, about 680 km from the active Taupo Volcanic Zone (TVZ) located on the North Island, thus it contains numerous macro- and microscopic tephra layers. Scanning Electron Microscopy and Energy Dispersive Spectroscopy analyses were carried out on entire agglutinated foraminifera as well as on sectioned specimens, sampled around and within the tephra layers. The analyses show that the studied foraminifera built structurally complex tests picking and selecting mineral and biogenic particles on the basis of their availability and abundance in the substratum, as well as their composition, size and shape. In most of the studied species, belonging to the order Textulariida, the composition of the agglutinated grains does not change when the deposition of the tephra layer strongly enriched the substratum in volcanic glass shards. Only the species Karreriella novangliae changed significantly its grain composition, mostly selecting volcanic glass fragments to cover the test surface. Nevertheless, the tephra deposition seems to influence the wall microstructure of the agglutinated tests. Textulariid specimens coming from the volcanoclastic layers have a thinner wall which is also characterised by a less abundant calcareous matrix with respect to the specimens sampled above or below the tephra layer. We hypothesise that the volcanic ash deposition probably interfered with the normal agglutinating process by causing the development of more aggressive waters at the sea floor which, in turn, could have induced carbonate dissolution. Our observations therefore suggest that the sediment type of the substratum is not the only controlling factor in the construction of the agglutinated foraminiferal test and grain selection, which appears to be species-dependent.

A radiolarian-based palaeoclimate history of Core Y9 (Northeast of Campbell Plateau, New Zealand) for the last 160 kyr

Abstract Sea-surface temperatures (SSTs) based on radiolarian assemblage changes are estimated for the last 160 kyr, from a sediment core (Y9) recovered from Pukaki Saddle, northeast of Campbell Plateau. Site Y9 lies beneath Subantarctic Surface Water (SAW) immediately to the north of the Subantarctic Front (SAF), which in this region is bathymetrically constrained by the edges of Campbell Plateau and defines the northern boundary of the Antarctic Circumpolar Current (ACC). Radiolarian assemblages are characterised by an exceptionally high abundance of the Antarctic to subantarctic species Antarctissa spp. (up to 68%), especially during glacial intervals. SST estimates are derived using Factor Analysis and the Modern Analog Technique. Both methods capture the glacial-interglacial (G-I) pattern. The SST reconstructions show the changing relative influence of distinct water masses during the past G-I cycle, with major temperature variations of the order of 7–9 °C at glacial Terminations. Glacials (marine isotope stages (MIS) 6 and 2) are associated with particularly cool SSTs that are indicative of a more vigorous SAF/ACC and an enhancement of the inflow through Pukaki Saddle and/or frequent development of cold-core eddies at the SAF. By contrast, the influence of warmer waters and relaxation of the ACC during interglacials can be inferred from temperatures slightly warmer (e.g., mid-Holocene) and/or comparable to present day (e.g., MIS 5e). During these intervals, relatively warmer temperatures most likely indicate a higher warm-core eddy activity due to a strengthened Subtropical Front and/or a weakened inflow of cool water through Pukaki Saddle and/or an increased stratification in the Campbell Plateau region. Furthermore, the SST record is characterised by an abrupt warming at ca. 10 kyr (i.e., Termination I), the occurrence of a reversal at Termination I, and a warming event at the end of MIS 4, coinciding with the A4 event in the Byrd ice core. These characteristics, together with the pronounced G-I cycle shown by the SST estimates, suggest that Site Y9 is influenced by major oceanographic changes in the SW Pacific and responds to thermal changes at high southern latitudes.

Ramsi Ten Years On

The archipelagic nation of Solomon Islands in the sw Pacific experienced a debilitating internal conflict between 1998 and 2003. What began as an ethnic conflict evolved into a wider breakdown of law and order that led to the progressive collapse of government, closure of commercial enterprises and threat of national bankruptcy. In mid-2003 the Regional Assistance Mission to Solomon Islands (ramsi) was mobilised and deployed under the auspices of the Pacific Islands Forum. Led and largely funded by the Australian government, ramsi sought to restore security and stability to the troubled nation through a combination of policing and law enforcement, institutional strengthening with central government agencies and measures aimed at reviving and growing the national economy. Ten years later and the mission is undergoing drawdown and the transition of its development programs into regular bilateral and multilateral aid programs. While ramsi has made a substantial contribution to the restoration of security and stability in the aftermath of conflict, many outstanding challenges remain. These include issues of political economy and how these are impacting on the quality of governance, service delivery and nation-building, as well as longstanding structural issues with the formal economy, set against prevailing patterns of population growth and internal migration. These challenges are examined in the context of Solomon Islands socio-economic characteristics and recent history with a view to assessing the country’s prospects for enduring stability in the post-ramsi era.

Description of a new species of Microstoma (Pisces, Microstomatidae) from the southwestern Pacific Ocean.

A new species of the microstomatid genus Microstoma is described from specimens collected in the SW Pacific Ocean off New Zealand and Australia. Microstoma australis n. sp. differs from M. microsotma of the Mediterranean and Atlantic Ocean in having a higher number of gill rakers and vertebrae. Both species are compared with available data for NE Pacific specimens.

Structural analysis of extended Australian continental crust: Capel and Faust basins, Lord Howe Rise

Abstract The Capel and Faust basins (northern Lord Howe Rise) are located in the SW Pacific between Australia, New Zealand and New Caledonia. New seismic, gravity, magnetic and bathymetry data and rock samples have enabled the construction of a three-dimensional geological model providing insights into the crustal architecture and basin stratigraphy. Multiple large depocentres up to 150 km long and 40 km wide, containing over 6 km of sediment, have been identified. These basins probably evolved through two major Early Cretaceous rifting episodes leading to the final break-up of the eastern Gondwanan margin. Pre-break-up plate restorations and potential field data suggest that pre-rift basement is a collage of several discrete terranes, including a Palaeozoic orogen, pre-rift sedimentary basins and rift-precursor igneous rocks. It is likely that a pre-existing NW-trending basement fabric, inherited from the New England Orogen (onshore eastern Australia), had a strong influence on the evolution of basin architecture. This basement fabric was subjected to oblique rifting along an east–west vector in the ?Early Cretaceous to Cenomanian and NE–SW-oriented orthogonal rifting in the ?Cenomanian to Campanian. This has resulted in three structural provinces in the study area: Eastern Flank, Central Belt and Western Flank.

Extreme cyclone wave climate in the Southwest Pacific Ocean: Influence of the El Niño Southern Oscillation and projected climate change

This paper describes the first use of a stochastic cyclone model (SCM) to quantify the extreme significant wave height from tropical cyclones across the Southwest Pacific Ocean. The median extreme significant wave heights across the entire SW Pacific Ocean were 7.5, 10 and 11m for annual exceedance probabilities (AEPs) of 0.1, 0.02 and 0.01 respectively. Maximum significant wave heights in the region were approximately 1.5 times these values for the same AEP. Tables of extreme significant wave heights are provided for selected inhabited locations. The SCM was used to quantify the effects of the El Niño-Southern Oscillation (ENSO) on extreme significant wave heights, and also the effects of projected climate change on cyclone intensity and frequency of occurrence. West of the International Dateline in the region of the Vanuatu archipelago, the extreme cyclone wave climate was relatively consistent during all phases of the ENSO cycle, but highest during El Niño. Cyclone formation and propagation eastward of the Dateline are more likely to occur during El Niño conditions, however these cyclones tended to be more intense, particularly during extreme El Niño events, leading to a higher long-term extreme wave climate in the eastern SW Pacific, despite the relatively low cyclone observation rate there. Simulations of climate change cyclone intensity increases of 10–20% of the most intense cyclones (categories 4 and 5) along with 10–20% reduction in number of cyclones indicated little change in extreme significant wave heights for low-occurrence AEPs of 1/20 or less. These changes were much less than induced by present-day ENSO variability, suggesting that future changes in extreme wave climate will be sensitive to climate change influences on the frequency and intensity of ENSO events. These results are significant in the light of indications that the frequency of extreme El Nino events might double in response to greenhouse warming.

Dissolved copper isotope biogeochemistry in the Tasman Sea, SW Pacific Ocean

Described as a ‘Goldilocks element’ in seawater, the free copper ion (Cu2+) concentration in open ocean waters is generally low enough to avoid causing toxicity to marine algae and bacteria, but high enough to be utilised in a range of bio-essential metallo-proteins and enzymes. The measurement of stable copper (Cu) isotopic compositions in seawater is proposed as a means to better understand the biogeochemical cycling of this important micronutrient in marine systems. Processes including changes in oxidation state, biological uptake and scavenging have all been shown to fractionate Cu isotopes and it is hoped that their measurement in seawater will shed some light on the processes that govern the regulation of the bioavailable pool of Cu in the ocean. However, measurement of Cu isotopic compositions in seawater is analytically challenging. This work utilised solvent-extraction coupled with anion-exchange and multi-collector inductively-coupled plasma mass spectrometry to spatially characterise their distribution in the Tasman Sea, south-west Pacific Ocean. Three profiles were measured to 3500m depth along a broad transect covering the oligotrophic north Tasman Sea, the Tasman Front and the productive waters of the Southern Ocean in the south. The Cu isotope composition of all 3 profiles was relatively homogenous and corresponding measurements of organic Cu complexation (Thompson et al., 2014. Dissolved copper speciation in the Tasman Sea, SW Pacific Ocean) showed similarly homogenous conditional stability constants for the complexation of Cu by natural organic ligands. Greater than 99% of dissolved Cu is complexed to natural organic ligands; thus, the strong complexation by organic Cu-binding ligands likely stabilises the heavy isotopic composition of dissolved Cu in the Tasman Sea at around 0.70±0.23‰ (n=47, 2SD). It is hypothesised that the breakdown of dissolved organic Cu complexes may provide a source of isotopically heavy Cu that could be removed from the oceanic system as a sink in certain environments such as anoxic basins.

Eocene and Oligocene basins and ridges of the Coral Sea-New Caledonia region: Tectonic link between Melanesia, Fiji, and Zealandia

This paper presents 34 geochemical analyses, 24 Ar-Ar ages, and two U-Pb ages of igneous rocks from the back-arc basins and submarine ridges in the Coral Sea-New Caledonia region. The D'Entrecasteaux Ridge is a composite structural feature. Primitive arc tholeiites of Eocene age (34–56 Ma) are present along a 200 km length of the ridge and arguably were part of the initial line of subduction inception between Fiji and the Marianas; substantial Eocene arc edifices are only evident at the eastern end where Bougainville Guyot andesite breccias are dated at 40 ± 2 Ma. The South Rennell Trough is confidently identified as a 28–29 Ma (early Oligocene) fossil spreading ridge, and hence, the flanking Santa Cruz and D'Entrecasteaux basins belong in the group of SW Pacific Eocene-Early Miocene back-arc basins that include the Solomon Sea, North Loyalty, and South Fiji basins. The rate and duration of spreading in the North Loyalty Basin is revised to 43 mm/yr between 28 and 44 Ma, longer and faster than previously recognized. The direction of its opening was to the southeast, that is, parallel to the continent-ocean boundary and perpendicular to the direction of coeval New Caledonia ophiolite emplacement. Medium- and high-K alkaline lavas of 23–25 Ma (late Oligocene) age on the northern Norfolk Ridge are an additional magmatic response to Pacific trench rollback.

Dissolved copper speciation in the Tasman Sea, SW Pacific Ocean

The speciation of copper (Cu) in seawater is dominated by organic complexation but the sources, distribution and nature of these natural organic ligands remain unclear. The spatial distribution and character of organic copper binding ligands at two sites in the Tasman Sea were investigated using competitive ligand exchange adsorptive cathodic stripping voltammetry, utilising salicylaldoxime as the competing ligand. The first station was located in the oligotrophic north Tasman Sea and was contrasted with a second, mesotrophic, station in the south of the region where sampling coincided with a coccolithophore bloom. The northern P1 site (0–3000m) was characterised by detection of a single, strong, class of ligand with a uniformly distributed conditional stability constant (log K′CuL=12.97±0.26). At the two shallowest depths of the southern P3 site (13 and 28m), two ligands were detected. A single, stronger, ligand was detected at all other depths for this site. Below 120m, conditional stability constants at the southern P3 site (0–3500m) were uniformly distributed (log K′CuL=13.32±0.21). At both stations, ligand concentrations usually exceeded the total dissolved Cu concentration and mirrored the bio-intermediate Cu profile at depth. Dissolved Cu was more than 99% complexed by organic ligands at all depths except one, when dissolved Cu concentration exceeds the dissolve ligand concentration. The variability in ligand concentrations was greatest in the top 0–50m at the northern P1 site and 0–80m at the southern site. Based on a combination of phototrophic cell counts and biomarker pigment concentrations, the source of organic Cu-binding ligands in the north Tasman Sea is thought to be from cyanobacterial production, namely Synechococcus spp which dominates in the region. The detection of two organic Cu binding ligand classes at the southern P3 station indicates a more complex Cu-binding ligand pool in the surface waters. At this site, the ligands are likely sourced from cyanobacteria and eukaryotes, namely coccolithophores. At the northern P1 station only, a relationship between the proportion of unbound ligand available and the dissolved oxygen concentration was obtained. Given the simpler ligand pool dynamics of the northern station relative to the south, this relationship may reveal that complexation of organic Cu-binding ligands is important to prevent or slow ligand degradation in the deep sea.

Geochemical constraints on the contribution of Louisville seamount materials to magmagenesis in the Lau back-arc basin, SW Pacific

Seamounts are an integral part of element recycling in global subduction zones. The published trace element and Pb-Sr-Nd isotope data for basaltic lavas from three key segments (Central Lau Spreading Ridge (CLSR), Eastern Lau Spreading Ridge (ELSR), and Valu Fa Ridge (VFR)) of the Lau back-arc basin were compiled to evaluate the contribution of Louisville seamount materials to their magma genesis. Two geochemical transitions, separating three provinces with distinct geochemical characteristics independent of ridge segmentation, were identified based on abrupt geochemical shifts. The origin of the geochemical transitions was determined to be the result of drastic compositional changes of subduction components added into the mantle source, rather than the transition from Indian to Pacific mid-ocean ridge basalt (MORB) mantle, or due to variable mantle fertilities. The most likely explanation for the drastic shifts in subduction input is the superimposition of Louisville materials on ‘normal’ subduction components consisting predominantly of aqueous fluids liberated from the down-going altered oceanic crust and minor pelagic sediment melts. Quantitative estimation reveals that Louisville materials contributed 0–74% and 21–83% of the Th budget, respectively, to CLSR and VFR lavas, but had no definite contribution to the lavas from the ELSR, which lies farthest away from the subducted Louisville seamount chain (LSC). The spatial association of the subducted LSC with the Louisville-affected segments suggests that the Louisville signature is regionally but not locally available in the Tonga subduction zone. Besides, the preferential melting of subducted old Cretaceous LSC crust instead of the old normal Pacific oceanic crust at similar depths implies that elevated temperature across the subduction interface or seamount erosion and rupture were required to trigger melting. A wider implication of this study, thus, is that seamount subduction may promote efficiency of element recycling in subduction zones.

Spatial patterns of Aquificales in deep-sea vents along the Eastern Lau Spreading Center (SW Pacific)

The microbial diversity associated with actively venting deep-sea hydrothermal deposits is tightly connected to the geochemistry of the hydrothermal fluids. Although the dominant members of these deposits drive the structure of the microbial communities, it is less well understood whether the lower abundance groups are as closely connected to the geochemical milieu, or driven perhaps by biotic factors such as microbial community interactions. We used the natural geochemical gradients that exist in the back-arc basin, Eastern Lau Spreading Center and Valu-Fa Ridge (ELSC/VFR) in the Southwestern Pacific, to explore whether the chemolithotrophic Aquificales are influenced by geographical location, host-rock of the vent field or deposit type. Using a combination of cloning, DNA fingerprinting (DGGE) and enrichment culturing approaches, all genera of this order previously described at marine vents were detected, i.e., Desulfurobacterium, Thermovibrio, Aquifex, Hydrogenivirga, Persephonella and Hydrogenothermus. The comparison between clone libraries and DGGE showed similar patterns of distribution of different Aquificales whereas results differed for the enrichment cultures that were retrieved. However, the use of cultivation-based and -independent methods did provide complementary phylogenetic diversity overview of the Aquificales in these systems. Together, this survey revealed that the ELSC/VFR contains some of the largest diversity of Aquificales ever reported at a deep-sea vent area, that the diversity patterns are tied to the geography and geochemistry of the system, and that this geochemical diverse back-arc basin may harbor new members of the Aquificales.

Cretaceous fore-arc basalts from the Tonga arc: Geochemistry and implications for the tectonic history of the SW Pacific

The Tonga fore-arc preserves a complex history of subduction initiation, back-arc basin formation and arc volcanism which has extended from the Cretaceous to the present. In this paper, we discuss the geochemistry of a Cretaceous basalt/dolerite/gabbro suite recovered in two dredges from the Tonga fore-arc at ~19°S. The geochemistry of the Tonga fore-arc suite is unlike that of the uniformly depleted MORB basalts of the subducting Pacific Plate and therefore is unlikely to be accreted from Pacific Cretaceous crust. The ~102Ma age obtained for one Tongan fore-arc dolerite is contemporaneous with a major phase of Cretaceous subduction-related volcanism, recorded both in detrital zircon age populations and associated volcanics from New Caledonia and New Zealand. We believe that the Tonga fore-arc basalts are a remnant of a hypothesized, once extensive Cretaceous back-arc basin, called the East New Caledonia Basin, which we propose to have existed from ~102 to 50Ma. The allochthonous Poya Terrane of New Caledonia is geochemically very similar to the Tonga fore-arc basalts and represents a younger (~84–55Ma) remnant of the same basin. Subduction-related Cretaceous volcanics from the SW Pacific, representing both arc and back-arc settings, all appear to have similar Zr/Nb values, suggesting a common mantle component in their petrogenesis. The Tonga fore-arc basalts are also similar to fore-arc basalts recovered from the Izu-Bonin-Mariana fore-arc, but unlike these basalts they are not associated with subduction initiation.

Ovoviviparous reproduction in Australian specimens of the intertidal isopod Cirolana harfordi

Cirolana harfordi is an intertidal isopod found in the SW Pacific (Eastern Australia: New South Wales and Victoria), NW Pacific (Japan and Russia) and the Pacific coast of North America. C. harfordi from North America has been reported to reproduce by depositing eggs into a marsupial pouch formed by oostegite plates that grow from the base of the 3rd to 6th pereopods (walking legs), so as to incubate embryos and mancas (young that resemble the adult except the seventh pair of pereopods are absent) external to the ventral cuticle of the mother. In this study, however, C. harfordi individuals from New South Wales, Australia were found to incubate embryos and mancas inside the pereon (thorax) cavity. C. harfordi in Australia is, therefore, displaying ovoviviparous reproduction and not incubation in a marsupial pouch external to the pereon as previously thought. Embryos have a direct development pathway to the internal manca stage that is held within the mother’s body cavity until the time of live birth. Additionally, internal mancas show some morphological differences to adults, such as a lack of cusps on the mandibles.

Anthropogenic (PBDE) and naturally-produced (MeO-PBDE) brominated compounds in cetaceans — A review

This paper reviews the available data on brominated flame retardants, the polybrominated diphenyl ethers (PBDEs), as well as on the naturally-produced methoxylated polybrominated diphenyl ethers (MeO-PBDEs) in cetacean tissues around the world. Levels and possible sources of both compound classes are discussed. Odontocete cetaceans accumulate higher PBDE concentrations than mysticete species. PBDE contamination was higher in cetaceans from the Northern hemisphere, whereas MeO-PBDE levels were higher in animals from the Southern hemisphere. Southern resident killer whales from NE Pacific presented the highest levels reported in biota, followed by bottlenose dolphins from North Atlantic (U.K. and U.S. coast). Many species presented PBDE concentrations above threshold levels for health effects in odontocetes. Time trend studies indicate that PBDE concentrations in odontocetes from Japan, China, U.S. and Canada coastal zones have increased significantly over the past 30years. Studies from U.K. waters and NE Atlantic showed a decrease and/or stability of PBDE levels in cetacean tissues in recent decades. The highest MeO-PBDE concentrations were found in dolphins from Tanzania (Indian Ocean), bottlenose dolphins from Queensland, Australia (SW Pacific), and odontocetes from coastal and continental shelf waters off southeastern Brazil (SW Atlantic). The upwelling phenomenon and the presence of coral reef complexes in these tropical oceans may explain the large amounts of the naturally-produced organobromines. Considering that these bioaccumulative chemicals have properties that could cause many deleterious effects in those animals, future studies are required to evaluate the potential ecotoxicological risks.

Microtexture and Distribution of Minerals in Hydrothermal Barite‐Silica Chimney from the Franklin Seamount, SW Pacific: Constraints on Mode of Formation

AbstractAn extinct hydrothermal barite‐silica chimney from the Franklin Seamount of the Woodlark Basin, in the southwestern Pacific Ocean, was investigated for mineral distribution and geochemical composition. Six layers on either side of the orifice of a chimney show significant disparity in color, mineral assemblage and major element composition. Electron microscope (SEM) images reveal that the peripheral wall of the chimney is composed of colloform silica, suggesting that incipient precipitation of silica‐saturated hydrothermal fluid initiated the development of the chimney wall. Intermediate layers, between the exterior wall and the inner fluid‐orifice, dominate with barite and sulfides. Low Sr‐to‐Ba ratios (SrO/BaO = 0.015–0.017) indicate restricted fluid‐seawater mixing, which causes relatively high‐temperature formation of the intermediate layers. Whereas the innermost layer bordering the chimney orifice is characterized by more silica and a higher Sr‐to‐Ba ratio (SrO/BaO = 0.023), could have formed due to a paragenetic shift from a high‐temperature active phase to a cooler waning stage of formation. A paragenetic shift is also probably responsible for the change in mineral formation mechanism that resulted in the textural variation of barite and colloform silica developed during different growth phases of this barite‐silica chimney.

Early Paleogene evolution of terrestrial climate in the SW Pacific, Southern New Zealand

We present a long‐term record of terrestrial climate change for the Early Paleogene of the Southern Hemisphere that complements previously reported marine temperature records. Using the MBT′‐CBT proxy, based on the distribution of soil bacterial glycerol dialkyl glycerol tetraether lipids, we reconstructed mean annual air temperature (MAT) from the Middle Paleocene to Middle Eocene (62–42 Ma) for southern New Zealand. This record is consistent with temperature estimates derived from leaf fossils and palynology, as well as previously published MBT′‐CBT records, which provides confidence in absolute temperature estimates. Our record indicates that through this interval, temperatures were typically 5°C warmer than those of today at such latitudes, with more pronounced warming during the Early Eocene Climate Optimum (EECO; ∼50 Ma) when MAT was ∼20°C. Moreover, the EECO MATs are similar to those determined for Antarctica, with a weak high‐latitude terrestrial temperature gradient (∼5°C) developing by the Middle Eocene. We also document a short‐lived cooling episode in the early Late Paleocene when MAT was comparable to present. This record corroborates the trends documented by sea surface temperature (SST) proxies, although absolute SSTs are up to 6°C warmer than MATs. Although the high‐calibration error of the MBT′‐CBT proxy dictates caution, the good match between our MAT results and modeled temperatures supports the suggestion that SST records suffer from a warm (summer?) bias, particularly during times of peak warming.