Migration of volcanism with time in the Marquesas Islands,French Polynesia

Samples from five islands of the Marquesas Island chain (southeast Pacific Ocean) have been dated by the K-Ar method and exhibit a northwest to southeast volcanic migration rate of 9.9 cm/yr. This movement is in the same direction but of intermediate magnitude to results from the Austral Island chain (lower rate) and the Hawaiian Island chain (considerably higher rate). The rate of migration of volcanism in the Marquesas Islands is consistent with the model of rigid Pacific plate movement over a fixed “hot spot” or mantle “plume” provided that the pole of rotation for the Pacific plate for the last 5 my is located near 55°S, 170°E.     

Seismic detection of underwater volcanism: The example of French Polynesia

We present a review of the principal methods used for the seismic detection and identification of active underwater volcanism, based on our experience in French Polynesia. In particular, we descrobe the 5-year activity in the Tahiti-Mehetia area, during which more than 32000 earthquakes were detected by the Polynesian network. We discuss the use of the following three types of seismic waves: conventional (mostly body waves), seismic tremor, andT waves propagated in the low-velocity acoustic channel of the ocean. For each of these waves, we discuss the principal characteristics of the signals, their spectral content, the type of information they provide on the activity of the volcano, and the various limitations faced by their use in detection or monitoring of underwater volcanic edifices. We present a review of the principal swarms monitored by the Polynesian network, and discuss their characterization as either volcanic or tectonic.     

Identification Criteria for Sources of T Waves Recorded in French Polynesia

—?From a data set of 150 digital records of T phases from 71 sources obtained on seismometers of the Polynesian Seismic Network, we define a discriminant separating earthquake and explosion sources, which uses the maximum amplitude of recorded ground velocity, measured on its envelope, e _Max (in μm/s), and the duration of the phase measured at 1/3 of maximum amplitude, τ_1/3 (in seconds). Earthquake sources and man-made explosions are effectively separated in a log-log space by the straight line ¶¶log₁₀ e _Max = 4.9 log₁₀τ_1/3 - 4.1 .¶¶Other criteria in both the time and frequency domains fail to reliably separate the populations of the various kinds of events. The application of this technique to analog records of large-scale man-made explosions carried out in the 1960s confirms that it provides an adequate discriminant over 3.5 orders of magnitude of ground velocity.     

The 2010 and 2011 Tsunamis in French Polynesia: Operational Aspects and Field Surveys

We present a detailed timeline of the warning procedures as they unfolded at the Laboratoire de Géophysique in Papeete, Tahiti, during the nights of 26–27 February 2010 (Maule, Chile tsunami) and 10–11 March 2011 (Tohoku tsunami). In particular, we discuss how the flow of information available to the warning center (including seismic evaluations obtained both locally and from other warning centers, as well as maregraph and DART buoy data) built up and eventually led to red alerts, which the local authorities used in both cases to impose an evacuation of low-lying areas on 68 islands. While the alerts were successful in Polynesia, a difficulty arose in 2011 when the alert had to be reinstated immediately as the all clear was being declared, since the maximum amplitude was carried by the fourth wave packet. We also present a complete dataset of 119 values of run-up and inundation surveyed in the aftermath of the two tsunamis, principally in the Marquesas Islands where their effects were maximal, and on Tahiti and Moorea for the 2011 event. The highest run-up (4.45 m) was observed in 2011 in the Bay of Taipivai on Nuku Hiva, where seven houses were flooded. We find no clear correlation between run-up values at the same locations in 2010 and 2011, suggesting that local responses are controlled by details specific to each tsunami. In 2010, in the village of Puamau on Hiva Oa (Marquesas), a delayed harbor response, probably due to resonance of the bay upon arrival of short-period components dispersed outside the shallow-water approximation, flung a launch onto a wharf, 7 h after the first arrivals, and 2.5 h after issuance of the all clear.     

Physical and chemical control of the phytoplankton of Ahe lagoon, French Polynesia

The environmental characteristics of Ahe deep lagoon (Tuamotu Archipelago, French Polynesia) were studied over 3 years with the aim of explaining the spatial and temporal variability of the natural food available for pearl oysters with a special focus on phytoplankton biomass and global photosynthesis/respiration ratio of the lagoon. Chlorophyll averaged 0.34±0.01 μg L(-1) and our findings did not confirm increased phytoplankton biomass in deep lagoonal waters. Phytoplankton production appears to be limited firstly by nitrogen and respiratory processes overpass photosynthetic processes at least in the north-eastern edge of the atoll. Grazing by pearl oysters in culture seems to decrease the POC concentration but not the phytoplankton biomass. Oysters graze mainly on non chlorophyllian particles.     

Hydrothermal alteration associated with rift zones at Fangataufa atoll (French Polynesia)

 The Mururoa and Fangataufa atoll basement consists of superimposed submarine and subaerial lava flows which have been intruded by late volcanics. The intrusions have developed large hydrothermal alteration haloes throughout the basaltic wall rock. The cuttings of the Natice-1 and Mitre-1 holes, drilled into the submarine volcanic pile at Fangataufa atoll, show a vertical zonation of clay minerals ranging from 270 to 850 m depth. The newly formed clay minerals occurring from top to bottom of the altered pile are: dioctahedral aluminous smectites, saponite, an intimate assemblage of saponite with two random chlorite/saponite mixed layers and an intimate assemblage of one random chlorite/saponite mixed-layer with one ordered chlorite/saponite mixed layer and one chlorite below 816 m depth. These clay mineral assemblages indicate a general increase in the chloritic component with depth. They are associated throughout the pile with secondary carbonates and quartz. The ∂¹⁸O and ∂¹³C of calcite and ∂¹⁸O of clay minerals, on the one hand, and the intimate mixtures of trioctahedral species, on the other, suggest a general cooling with the evolution of a paleogeothermal gradient from approximately 300  °C/km during the crystallization of chlorite to 150  °C/km for the late calcite precipitation. Received: 2 October 1995 / Accepted: 14 January 1997     

Uncoupled viral and bacterial distributions in coral reef waters of Tuamotu Archipelago (French Polynesia)

This study examined the distribution of virioplankton and bacterioplankton in two coral reef systems (Ahe and Takaroa atolls) in the Tuamotu Archipelago, in comparison with the surrounding oligotrophic ocean. Mean concentrations of 4.8×10(5) and 6.2×10(5)cellsml(-1) for bacteria and 8.1×10(6) and 4.3×10(6)VLP(virus-like particle) ml(-1) were recorded in Ahe and Takaroa lagoons, respectively. Chlorophyll-a concentrations and dissolved organic matter were higher in Ahe whereas (3)H thymidine incorporation rates were higher in Takaroa. First data on lytic and lysogenic strategies of phages in coral reef environments were discussed in this paper. The fraction of visibly infected cells by viruses was negligible regardless of the lagoon station (mean=0.15%). However, the fraction of lysogenic cells ranged between 2.5% and 88.9%. Our results suggest that the distribution patterns of virioplankton are apparently not coupled to the spatial dynamics of the bacterioplankton communities.     

The influence of pearl oyster farming on reef fish abundance and diversity in Ahe,French Polynesia

Many cultured pearl farms are located in areas of the Pacific that have thriving, highly diverse fish communities but the impacts of farming on these communities are poorly understood. We studied the effects of pearl oyster farming on shore fish abundance and diversity in the lagoon of Ahe, French Polynesia by adapting roving diver census methods to the coral reef bommies of the lagoon and compared 16 sites with high pearl farming impact to others with no direct impact. Pearl farming has a slightly positive effect on reef fish abundance (N) and no significant impact on fish diversity (H) or community composition. This is important when considering the ecological sustainability of pearl farming in French Polynesia and suggests that a potential synergy between pearl farms and marine conservation should be further explored.     

Multi-scale distribution and dynamics of bivalve larvae in a deep atoll lagoon (Ahe,French Polynesia)

Bivalve larvae and hydrographic parameters were sampled over a range of spatio-temporal scales in a deep atoll lagoon. Bivalve larvae abundances were very high throughout the year: 18,550 m⁻³ in average. Larvae were (i) concentrated at mid-depth with nocturnal ascent and diurnal descent, (ii) heterogeneously dispersed at the lagoon scale, (iii) subject to day-to-day variation in abundance and (iv) transferred between different parts of the lagoon providing evidence of intra-lagoonal connectivity. The primacy of physical factors was seen on large spatial scale with the diluting effect of water renewal and transfers by hydrodynamics. On smaller spatial scale, the primacy of biological processes was recognised, with larval swimming activity leading to dial vertical migration correlated with food concentration. Variations in larval abundance were driven by bivalve reproductive activity correlated with meteorological conditions (i.e. windy periods). Finally, relationship between bivalve larvae patterns and pearl oyster (Pinctada margaritifera) settlement structuring is discussed.     

Multi-scale distribution and dynamics of bivalve larvae in a deep atoll lagoon (Ahe, French Polynesia)

Bivalve larvae and hydrographic parameters were sampled over a range of spatio-temporal scales in a deep atoll lagoon. Bivalve larvae abundances were very high throughout the year: 18,550 m(-3) in average. Larvae were (i) concentrated at mid-depth with nocturnal ascent and diurnal descent, (ii) heterogeneously dispersed at the lagoon scale, (iii) subject to day-to-day variation in abundance and (iv) transferred between different parts of the lagoon providing evidence of intra-lagoonal connectivity. The primacy of physical factors was seen on large spatial scale with the diluting effect of water renewal and transfers by hydrodynamics. On smaller spatial scale, the primacy of biological processes was recognised, with larval swimming activity leading to dial vertical migration correlated with food concentration. Variations in larval abundance were driven by bivalve reproductive activity correlated with meteorological conditions (i.e. windy periods). Finally, relationship between bivalve larvae patterns and pearl oyster (Pinctada margaritifera) settlement structuring is discussed.     

Pearl oysters Pinctada margaritifera grazing on natural plankton in Ahe atoll lagoon (Tuamotu archipelago, French Polynesia)

In atoll lagoons of French Polynesia, growth and reproduction of pearl oysters are mainly driven by plankton concentration. However, the actual diet of black-lip pearl oysters Pinctada margaritifera in these lagoons is poorly known. To fill this gap, we used the flow through chamber method to measure clearance rates of P. margaritifera in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia). We found: (i) that pearl oysters cleared plankton at a rate that was positively related to plankton biovolume, (ii) that nanoflagellates were the main source of carbon for the pearl oysters, and (iii) that the quantity and origin of carbon filtrated by pearl oysters was highly dependent on the concentration and composition of plankton. These results provide essential elements for the comprehension of growth and reproduction variability of pearl oysters in atoll lagoons of French Polynesia.     

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.     

Metazooplankton communities in the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia): Spatiotemporal variations and trophic relationships

Metazooplankton abundance, biomass (<80μm, 200-500μm and >500μm) and community structure in the Ahe atoll were studied together with their relationships with environmental factors (temperature, salinity, wind) and trophic factors (phytoplankton, bacteria, heterotrophic nanoflagellates (HNF) and ciliates) during three periods in 2008-2009. Meroplankton, mainly bivalve and gastropod larvae, was dominant. Holoplankton was dominated by copepods, the main species being Oithona spp., Paracalanus parvus, Clausocalanus spp., Corycaeus spp., Acartia fossae and Undinula vulgaris. The results suggest a clear wind influence on the structure and horizontal distribution of the zooplankton communities. The metazooplankton appeared to be controlled mainly by food resources, suggesting a bottom-up control. The low nanophytoplankton biomass in contrast to the high abundance of picophytoplankton, HNF and nano-particle grazers (mainly Oithona spp., Paracalanus and bivalve larvae) highlighted the importance of the microbial loop in the food web.     

Amount and type of derelict gear from the declining black pearl oyster aquaculture in Ahe atoll lagoon,French Polynesia

Pearl oyster aquaculture is a major activity in French Polynesia atoll lagoons. After the economic decline that characterized the last decade, concerns recently rose about discarded installations and materials that supported aquaculture practices and by facilities abandoned after they had to close their activities. In May 2013, a first inventory of the type and amount of pearl farms derelict gear (PFDG) was achieved on 47 sites in Ahe lagoon. Surveys were conducted within and outside the boundaries of aquaculture concessions. Twenty types of PFDG littered the lagoon floor and the water column. The most impacted areas were near abandoned grafting houses with up to nine types of PFDG. Forty-five percent of the sites were impacted, including outside concessions. While management authorities are fully aware of the problem, this first assessment is a wake-up call to stimulate the cleaning of lagoons, enhance awareness among farmers, and identify potential ecological consequences on lagoon ecosystems.     

Ignimbrite volcanism in North Wales

Many episodes of ignimbrite volcanism have occurred in North Wales. Ignimbrites can be recognised amongst the Pre-Cambrian rocks and from almost every stage of the Ordovician succession. The best known and most instructively exposed ignimbrite volcanies are those of Caradocian age in Snowdonia. In this deeply incised mountainous area it is possible to demonstrate the major characteristics of ignimbrite volcanism and to examine the relationship between numerous intrusive rhyolite masses and the extrusive rocks. Both acid and basic magmas were available in North Wales during Ordovician times, and at several of the volcanic centres the rocks show a differentiation sequence from pyroxeneandesite to alkali-rhyolite. The emergence and growth of volcanic islands upon which the ignimbrites were deposited is revealed in the stratigraphical record. An intimate relationship exists between magmatism and crustal unrest, and it is possible to discuss certain problems regarding the petrogenesis of the rocks, the location and character of the volcanic vents and the palaeogeography of North Wales during the Ordovician. Definition of the terms employed and criteria used in the identification of Welsh ignimbrites is given, and the field relationships and petrology of a number of areas are described in detail.     

The origin of intermediate and evolved lavas in the Marquesas archipelago: an example from Nuku Hiva island (French Polynesia)

The Nuku Hiva Pliocene island (Marquesas, French Polynesia) is composed of a large half-collapsed tholeiitic shield volcano (the Tekao edifice), the caldera of which is filled up by the younger Taiohae volcano. The latter edifice is characterised by a complex magmatic association including minor mafic lavas (olivine tholeiites, alkali basalts and basanites), abundant intermediate lavas (hawaiites with subsidiary mugearites, both covering 47% of the surface of the volcano) and lesser amount of evolved lavas (K-rich and Na-rich trachytes and minor benmoreites, covering 25% of the edifice). Most intermediate and evolved Taiohae lavas are amphibole-rich and crystallised under high oxygen fugacities. The mafic Taiohae lavas originated from lower degree of melting of mantle sources more enriched than that of the shield volcano tholeiites. We show that closed-system fractional crystallisation of the Taiohae basaltic magmas can account for the origin of Taiohae hawaiites and mugearites, provided that separation of substantial amount of amphibole and/or apatite occurred during this process. Similarly, fractionation of benmoreitic magmas involving large amounts of amphibole and mica may account for the genesis of K-rich and Na-rich trachytes, respectively. However, fractional crystallisation cannot account for the derivation of benmoreitic magmas from mugearitic ones: since, this process fails to explain the abrupt increase in K₂O from the latter to the former. In addition, the isotopic signature of trachytes and benmoreites is clearly distinct (more EM II-rich) from that of Taiohae basalts, hawaiites and mugearites. Three hypotheses could account for the genesis of benmoreitic magmas: assimilation of oceanic material with a strong EM II signature, differentiation of non-sampled mafic magmas derived from a mantle source having a EM II-rich signature and partial melting at depth of mafic material with a strong EM II signature. The oxidised character of Nuku Hiva lavas, uncommon in oceanic island settings, suggests interaction with water and/or the contribution of an oxidised (altered?) source material to their genesis.