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International audience; Within the framework of EPOCA (European Project on OCean Acidification ) and the French INSU project PHARE, we are adapting the boron isotope technique to ancient corals with the scope to reconstruct “past” ocean pH changes. In this study, we applied the technique to surface seawater pH reconstructions based on tropical 20th century corals from Fiji. Models estimated a pH drop close to 0.07 pH units in the South Western Equatorial Pacific since the onset of the industrial era (Sabine et al., 2004). To reconstruct such a change in pH, the isotopic composition of boron (δ11B) in coral material has to be determined with a precision better than ±0.2‰. This analytical criteria was meet on a Multi-Collector ICPMS Neptune. We selected a Porites coral for the reconstruction of the time dependent evolution of pH. Our results show a progressive decrease of seawater pH between 1900 and 2000 of 0.08 +/- 0.02 pH units. This decrease in pH agrees with projections of surface ocean pH for the Fiji area obtained with the biogeochemical ocean circulation model NEMO-PISCES. Our results further reveal that seawater pH changes in the Fiji area are strongly affected by regional processes such as the South Pacific Convergence Zone (SPCZ) tightly linked the Pacific Decadal Oscillation (PDO). This last observation highlights the potential of the δ11B-pH technique for studying past changes of ocean dynamics. Hönisch, B., Hemming, N. G., Grottoli, A. G., Amat, A., Hanson, G. N. & Bijma, J. (2004). Assessing scleractinian corals as recoders for paleo-pH: Empirical calibration and vital effects. Geochimica et Cosmochimica Acta, 68(18), 3675-3685. Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J. L., Wanninkhof, R., Wong, C. S., Wallace, D. W. R., Tilbrook, B., Millero, F. J., Peng, T. H., Kozyr, A., Ono, T. & Rios, A. F. (2004). The Oceanic Sink for Anthropogenic CO2. Science, 305, 367-371.

The spatial and temporal distribution of volcanic eruptions in the Canarian Archipelago are a consequence of their origin and evolution, in response to hotspot activity. Therefore, the probability that eruptive phenomena will occur is greater in the western islands (Tenerife, La Palma and El Hierro), the closest to the hotspot vertex, where hundreds of eruptions have been localized in the last 20.000 years. By contrast, in La Gomera (presently in the volcanic repose stage) and in the easterly islands of Gran Canaria, Fuerteventura and Lanzarote (the most distant and oldest, in the post-erosive stage of development), few eruptions, if any, have taken place during this period. Rift structures known as «dorsals» have been defined in the more active westerly islands, where the greater part of the recent volcanic activity has been concentrated. Objective data therefore exist that permit the assessment and zonification of eruptive hazards in the archipelago. In this work the assessment of volcanic hazards in the island of Tenerife is presented, whichare concentrated in the volcanic system formed by the central edifice of Mt. Teide and the Northwest Rift. Numerous radiometric datings (14C and K/Ar) and the preparation of detailed digitalized and georeferenced geological mapping (GIS) have permitted an initia1 analysis and assessment of the volcanic hazards in this island, which presents the greatest risk of eruptions because of the number of recent eruptions and their complexity, in addition to demographic density.La distribución en el tiempo y el espacio de las erupciones volcánicas en el Archipiélago Canario es consecuencia de su origen y evolución, como respuesta a la actividad de un punto caliente. Por consiguiente, la probabilidad de ocurrencia de fenómenos eruptivos es mayor en las islas occidentales (Tenerife, La Palma y El Hierro), las más próximas a la vertical del punto caliente, donde se han localizado centenares de erupciones en los últimos 20.000 años. En cambio, en La Gomera (actualmente en la fase de reposo volcánico) y en las islas orientales de Gran Canaria, Fuerteventura y Lanzarote (las más alejadas y antiguas, en la fase post-erosiva de desarrollo), no se han producido erupciones en este período o sólo han sido unas pocas. Dentro de las islas occidentales más activas se han definido unas estructuras – los rifts o dorsales – donde se concentra la mayor parte de la actividad volcánica reciente. Existen, por consiguiente, datos objetivos que permiten la evaluación y zonificación del riesgo eruptivo en el Archipiélago. En este trabajo se presenta la evaluación de la peligrosidad volcánica de la isla de Tenerife, concentrada en el sistema volcánico formado por el edificio central del Teide y la dorsal Noroeste. Abundantes dataciones radiométricas (14C y K/Ar) y la elaboración de una cartografía geológica de detalle digitalizada y georeferenciada (SIG) han permitido la realización de un primer análisis y evaluación de los peligros volcánicos de esta isla, la de mayor riesgo eruptivo por el número de erupciones recientes y su complejidad, y por su densidad demográfica.

Teide volcano, the highest volcano on earth (3,718 m a.s.l., > 7 km high) after Mauna Loa and Mauna Kea in the Hawaiian Islands, forms a volcanic complex in the centre of the island of Tenerife. Its most recent eruptive activity (last 20 Ka) is associated with the very active NW branch of the 120" triple rift system of the island. Most of the eruptions of Tenerife during the past 20 Ka have occurred along these volcanic features, frequently in the production of extensive mafic and felsic lava flows, many of which reached the coast, crossing what is now one of the most densely populated areas of Tenerife and of any oceanic island in the world. However, despite numerous previous studies, very important basic geological information is still lacking, in particular dating of these flows to construct a geochronological framework for the evolution of the Teide-NW rift system, and a scientifically based, much needed volcanic hazard assessment. New carbon- 14 ages, obtained via coupled mass spectrometry (other in progress), provide important time constraints on the evoliition of Teide's volcanic system, the frequency and distribution of its eruptions, and associated volcanic hazards. Most of the eruptions are not related to the Teide stratovolcano, which apparently had only one eruption in the last 20 Ka about 1,240 f 60 years BP (between 1,287 CAL years BP and 1,007 CAL years BP, corresponding to a time interval between the VI1 and X centuries, 663 years AD to 943 years AD), but to the Pico Viejo volcano (17,570 f 150 years BP), flank parasitic vents (Mña. Abejera upper vent, 5,170 f 110 years BP; Mña. Abejera lower vent, 4,790 f 70 years BP; Mña. de La Angostura early, 2,420 f 70 years BP; Mña. La Angostura late, 2,010 f 60 years BP and Roques Blancos, 1,790 f 60 years BP) and the NW rift (Mña. Chío, 3,620 f 70 years BP). Although the volcanic activity during the past 20 Ka involved at least 7 voluminous phonolitic flank vents in the northem, more unstable slopes of Teide, it took place without any apparent response of the volcano; on the contrary, these eruptions seemed to progressively buttress and enhance the stability of Teide Volcano. Conversely, the occurrence of these flank eruptions, combined with the Pico Viejo and NW rift eruptions, poses a very high lava-flow risk to the now densely populated areas in north and west Tenerife, which have been almost entirely resurfaced during the past 20,000 years.El Teide, el edificio volcánico más alto del planeta (3.718 m sobre el nivel del mar, > 7 km desde el fondo oceánico) después del Mauna Loa y Mauna Kea en las islas Hawaii, forma un complejo volcánico en el centro de la isla de Tenerife. Su actividad eruptiva reciente (últimos 20 Ka) está asociada con la rama NO del rift triple (120") que ha configurado la etapa reciente de construcción de la isla. La mayoría de las erupciones de Tenerife en este período se han localizado en estas estructuras volcánicas, generando frecuentes y extensas coladas máficas y félsicas, muchas alcanzado la costa e invadiendo lo que es ahora una de las zonas más densamente pobladas de Tenerife y, probablemente, de cualquier isla oceánica del planeta. Sin embargo, y a pesar de los numerosos estudios y proyectos previos, falta aún información geológica básica para este importante sistema volcánico, en particular la datación de las diferentes erupciones que lo componen, con objeto de reconstruir el marco geocronológico indispensable para conocer su evolución y determinar científicamente los riesgos volcánicos, de perentoria necesidad habida cuenta de su naturaleza y entidad, y de la población potencialmente afectada. Nuevas dataciones de Carbono-14 y K/Ar aportan ahora importante información a este respecto. La mayoría de las erupciones de los últimos 20 Ka no están relacionadas con el estratovolcán Teide, que sólo ha tenido una hace 1.240 f 60 años -entre 663 y 943 AD una vez calibrada la edad-, sino con el volcán Pico Viejo (17.570 f 150 a BP), los centros eruptivos adventicios fonolíticos de la base del Teide (Mña. Abejera centro alto, 5.170 f 110 a BP; Mña. Abejera centro bajo, 4.790 f 70 a BP; Mña. de La Angostura inicial, 2.420 f 70 a BP; Mña. de La Angostura, 2.010 f 60 a BP y Roques Blancos, 1.790 f 60 a BP), y en el Rift NO (Mña. Chío, 3.620 f 70 a BP). Aunque la actividad volcánica de los últimos 20 Ka incluye al menos 7 erupciones fonolíticas de gran volumen de centros eruptivos situados en la cara norte del Teide, supuestamente muy inestable, esto no parece haber generado respuesta alguna del sistema volcánico. Más bien ha contribuido a asegurar la estabilidad estructural del conjunto, al aumentar y reforzar la base del estratovolcán. Por el contrario, la ocurrencia de estas erupciones y las del Rift NO imponen un elevado riesgo por flujo de coladas en el norte y oeste de Tenerife, zona ahora densamente poblada y que ha sido casi totalmente recubierta con lavas en los últimos 20.000 años.

International audience; Holocene paleohydrology reconstruction is derived combining planktonic and benthic stable oxygen and carbon isotopes, sea surface temperatures (SSTs) and oxygen isotope composition of seawater (δ18Ow) from a high sedimentation core collected in the South Adriatic Sea (SAS). Core chronology is based on 10 AMS 14C measures on planktonic foraminifera and tephra layers. Results reveal two contrasted paleohydrological periods that reflect (i) a marked lowering of δ18Ow/salinity during the early to mid-Holocene (11.5 ka to 6.3 ka), including the two-step sapropel S1 deposition, followed during the mid- to upper Holocene by (ii) a prevailing period of increased salinity and enhanced arid conditions in the South Adriatic Basin. Superimposed on these trends, short-term centennial-scale hydrological events punctuated the Holocene period in the SAS. During the early to mid-Holocene, two main SST coolings together with prominent δ18Ow/salinity lowering delineate the sapropel S1 interruption and the post-sapropel phase between 7.3 to 6.3 ka. After 6 ka, centennial-scale δ18Ow and G. bulloides δ13C lowering, mostly centered between 3 to 0.6 ka, reflect short-term hydrological changes related to more intensive runoff of the Po and/or Apennine rivers. These short-term events, even of lesser amplitude compared to the early to mid-Holocene period, may have induced a lowering of sea surface density and consequently reduced and/or inhibited the formation of deep bottom waters in the SAS. Comparison of the emerging centennial- to millennial-scale hydrological record with previous climatic records from the central Mediterranean area and north of the Alps reveal possible synchronicities (within the radiocarbon-dating uncertainty) between phases of lower salinity in the SAS and periods of wetter climatic conditions around the north-central Adriatic Sea. Finally, wavelet analyses provide new clues about the potential origin of climate variability in the SAS, confirming the evidence for a mid-Holocene transition in the central Mediterranean climate and the dominance of a ~1670-yr periodicity after 6 ka, reflecting a plausible connection with the North Atlantic climate system.

International audience; A detailed tephrostratigraphic study supported by stable isotope (δ18O) analyses and AMS 14C dating was carried out on a high sedimentation rate deep-sea core recovered in the northern Ionian Sea. Eight tephra layers were recognised, all originated from explosive eruptions of southern Italian volcanoes. These tephra layers are correlated with terrestrial proximal counterparts and with both marine and lacustrine tephra already known in the central Mediterranean area. The oldest tephra (dated at ca. 19.4 ka cal BP) is tentatively correlated to the Monte Guardia eruption from Lipari Island. Two other rhyolitic tephra layers were correlated with the explosive volcanic activity of Lipari Island: Gabellotto-Fiumebianco/E-1 (8.3 ka cal BP) located close to the interruption of Sapropel S1 deposit, and Monte Pilato (ca. AD 1335) in the uppermost part of the core. The Na-phonolitic composition of the other five recognised tephra layers indicates the Somma-Vesuvius as the source. The composition is quite homogeneous among the five tephra layers, and fits that of the Mercato proximal deposits. Beyond the striking chemical similarity with the Mercato eruption, these tephra layers span over ca. 2000 years, preventing correlation with the single well known Plinian eruption of the Somma-Vesuvius. Therefore, at least two of these tephra layers were assigned to an interplinian activity of the Somma-Vesuvius between the eruptions of Mercato and Avellino, even though these eruptions remains poorly constrained in the proximal area. By contrast, the most prominent tephra layer (2 mm white tephra visible at naked eyes) was found within the S1a Sapropel interval. Despite the possible complication for the presence of similar eruption with different ages we argue that Mercato is probably a very good marker for the onset of sapropelic condition in the Ionian Sea and can be used for land-sea correlations for this important climatic event. More in general, these data allow a significant update of the knowledge of the volcanic ash dispersal from Lipari and Somma-Vesuvius volcanoes.

International audience

International audience; [1] We measured the oxygen isotopic composition of planktonic and benthic foraminifera in three cores collected at key positions to reconstruct the paleoceanography of the Barents Sea: core ASV 880 on the path of the northern branch of Atlantic water inflowing from the Arctic Ocean, core ASV 1200 in the central basin near the polar front, and core ASV 1157 in the main area of brine formation. Modern seawater d 18 O measurements show that far from the coast, d 18 O variations are linearly linked to the salinity changes associated with sea ice melting. The foraminifer d 18 O records are dated by 14 C measurements performed on mollusk shells, and they provide a detailed reconstruction of the paleoceanographic evolution of the Barents Sea during the Holocene. Four main steps were recognized: the terminal phase of the deglaciation with melting of the main glaciers, which were located on the surrounding continent and islands, the short thermal optimum from 7.8 ka B.P. to 6.8 ka B.P., a cold mid-Holocene phase with a large reduction of the inflow of Atlantic water, and the inception of the modern hydrological pattern by 4.7 ka B.P. Brine water formation was active during the whole Holocene. The paleoclimatic evolution of the Barents Sea was driven by both high-latitude summer insolation and the intensity of the Atlantic water inflow.

Paired 230Th/U and 14C dating were performed on deep-sea corals (Lophelia pertusa and Madrepora oculata) from the northeastern North Atlantic at 730 m bsl to investigate past changes of the thermohaline circulation. These were estimated using the ?14C value of the upper intermediate waters, based on the 14C ages of the top and base of each coral, where possible, and the 230Th/U dating. The reliability of these estimates was checked by dating two very young corals of the species L. pertusa. One of these corals, collected alive in 1999 AD, gave a 230Th/U age of 1995±4 AD after correction for non-radiogenic 230Th. Another coral, the top of which dated to 1969±6 AD, recorded the atmospheric 14C/12C increase due to the nuclear tests in the early 1960s. The calculated ?14C values from these two corals agree with those measured at GEOSECS Station 23 in 1972–1973 [Östlund et al., Earth Planet. Sci. Lett. 23 (1974) 69–86] and 1991–1992 [Nydal and Gisfelos, Radiocarbon 38 (1996) 389–406]. This, together with the 100% aragonite content and the d234U and 230Th/232Th values of all the dated corals, indicates that none of the corals behaved as open systems with respect to their U-series nuclides and that they closely represent the water mass properties in which they lived. The pre-anthropogenic ?14C value of the North Atlantic intermediate waters was estimated at -69±4‰. The reservoir age varies from 400 years to 600 years, and this variation is due to atmospheric 14C/12C changes. A reservoir age of 610±80 years, close to the pre-anthropogenic value, was determined from one coral dated at 10 430±120 cal yr BP, when the global sea level was approximately at -35 m [Bard et al., Nature 382 (1996) 241–244]. This suggests a modern-like pattern of the oceanic circulation prevailed in the Northeast Atlantic Ocean at this time although the deglaciation was not completely achieved.

We present combined 230Th/U and 14C dating and stable isotope analyses on benthic corals from the northeastern North Atlantic in order to investigate past changes of the thermohaline circulation. The reef forming cold-water corals Lophelia pertusa and Madrepora oculata were raised from intermediate depth (~750m bsl) from carbonate mounds along Rockall and Porcupine Bank and Porcupine Seabight. The 230Th/U ages range from today to 247400yr. The d234U, 230Th/232Th and X-ray images indicate negligible alteration of the investigated corals, i.e. open system behavior. Very young deep-sea corals were accurately dated by means of 230Th/U dating. One in situ living Lophelia coral yielded a mean age of 1995AD, matching the date of collection in 1999AD. From this coral. the measured and calculated seawater d14C values are indistinguishable and the reservoir age Rinterm of the upper intermediate waters is 710±80 years. Several modern corals, being dated between 1950AD and 1986AD, recorded the atmospheric 14C/12C increase due to the nuclear tests in the early 60s. The modern pre-bomb d14C value of the North Atlantic intermediate waters was determined at an average of -65±7‰, and the mean reservoir age at 448±62 years. Finally, several investigated benthic coral grew during the second step of the deglaciation and during the Holocene climate optimum (from 10900 to about 8000 CAL yr BP). The reservoir age of average 500±150 years is equivalent to that of today indicating that, during the studied coral growth episodes, a modern type oceanic circulation, as well as the air-sea and surface to deeper adjacent water 14CO2 exchanges prevaled in the Northeast Atlantic Ocean.

Not only the occurrence of distinct organic-rich intervals (sapropels), but also the relative contents of key major and minor elements and isotopes in the sediments of the eastern Mediterranean, appear to be cyclic and to be astronomically associated. Interpretations of the environmental conditions leading to sapropel formation are based on results from sedimentological, micropalaeontological and geochemical studies of the dark-coloured layers and the cream/brownish sediments that occur above and below them. Part of the signal may be removed by early diagenetic processes. The extent and direction of these processes are ultimately controlled by the amount and reactivity of organic matter. The interval of dark colour associated with a sapropel is often somewhat thicker than that defined by the >2% C(org) definition and usually has a sharp colour change at upper and lower boundaries. A grey so-called 'proto-sapropel' layer of variable thickness underlies most sapropel layers. A few centimeters above the most recent sapropel S1, is usually found a clear dark-brown layer 2-3 cm thickness is usually found, which has a diffuse, often mottled, upper boundary and a relatively abrupt colour transition at its lower boundary. The colour is characteristic of Mn oxyhydroxide enrichments. In the interval from the darkbrown layer to the visible upper S1 boundary, there is usually an increasingly red-brownish colour. The distinct upper manganese Marker-Bed has been related to the Santorini (Minoan) eruption in 3356±18 BP (Bruins, H. J., and Van Der Plicht, J. (1996). The Exodus enigma. Nature, London, 382, 213-214), but is more likely to be associated with a Basin-wide re-ventilation event induced by changing climatic (humidity) conditions. Using barite-Ba as a paleo-productivity indicator, enhanced fluxes, and hence increases in accumulation rates of organic carbon to the seafloor, must have occurred from approximately 9 to 5 ky BP. The perfect correlations between observed C(org) and calculated C(org) from the C(org)/Ba relation in the visible S1 interval, and the total lack of such correlation in the interval between the darkbrown layer and the visible S1 layer are remarkable. It seems, therefore, that S1 deposition lasted from 9 to 5 ky BP but the organic carbon for the upper part has been removed. The double barium peak visible in a number of sediments studied in high-resolution, may be related to the 'Flooding' of the Black Sea that occurred around 7150 BP (Ryan, W. B. F., Pittman, W. C., Major, C. O., Shimkus, K., Moskalenko, V., Jones, G. A., Dimitrov, P., Gorur, N., Sakinc, M., and Yuce, H. (1997). An abrupt drowning of the Black Sea shelf. Marine Geology, 138, 119-126). Using various 'proxies', the (initial) occurrence of eastern Mediterranean sapropels appears cyclic and to be strongly related to the Monsoonal/Insolation Index. However, the relative value of an insolation maximum and initial C(org) content of the corresponding sapropel are not always related in a simple way. This again points to additional (possibly hydrographic) factors determining (the intensity of) sapropel formation. Understanding the mechanisms of sapropel formation and their subsequent preservation is not only necessary if our reconstructions of palaeoenvironmental conditions in the Eastern Mediterranean are to be improved, but may also assist in understanding the present-day situation and in forecasting possible future developments. The observed paleoenvironmental variations are discussed in a geochemical and environmental context, and are illustrated using typical examples from the eastern Mediteranean.

International audience; The 147-m-long sediment sequence recovered from Ocean Drilling Program Leg 152 Site 919 in the Irminger Basin contains several well-preserved volcanic ash layers and ash zones that provide a record of long-range transport of tephra from Iceland toward Greenland during the Pliocene−Pleistocene. A total of eight tephra layers and ash pods as well as three ash zones recovered in two separate holes (919A and 919B) were analyzed for major and trace element chemistry and grain size. Relative ages of the tephra layers were estimated based on oxygen isotope stratigraphy and correlation with other chronostratigraphic markers present in North Atlantic sediments and ice cores in Greenland. Based on their sorting coefficient and grain size, it is inferred that discrete ash layers between 1 and 5 cm thick are the result of ash fallout from large explosive eruptions. The tephra are bimodal (colorless/rhyolitic and sideromelane-tachylite/basaltic glass) or basaltic in composition, with crystal content between less than 15% for the discrete layers to more than 50% for the ash zones. The major element composition of glasses indicates two compositional groups: basaltic and rhyolitic. All of the tephra layers have an affinity with either a tholeiitic or an alkalic source in Iceland. Two separate mixed tephra layers, occurring between 10 and 11 m below seafloor at Site 919, were found to correlate with the ice-rafted ash Zone 2, based on their rhyolitic glass chemistry. Ash Zone 2 is a chronostratigraphic marker dated at about 55−57 ka in marine sediment and at about 52 ka in a Greenland ice core. The rhyolitic mixed tephra are interpreted to have been erupted during two large explosive eruptions of Tindfjallajökull volcano in southern Iceland, at a few hundred years interval. From the current pattern of seasonal variation in the atmospheric circulation over Iceland, it is suggested that the tephra were likely transported by easterly winds occurring at about 30 km elevation in midsummer, followed by fallout in southern Greenland and in the Irminger Basin.