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.     

Contrasting time trends of organic contaminants in Antarctic pelagic and benthic food webs

We demonstrate that pelagic Antarctic seabirds show significant decreases in concentrations of some persistent organic pollutants. Trends in Adélie penguins and Southern fulmars fit in a general pattern revealed by a broad literature review. Downward trends are also visible in pelagic fish, contrasting sharply with steady or increasing concentrations in Antarctic benthic organisms. Transfer of contaminants between Antarctic pelagic and benthic food webs is associated with seasonal sea-ice dynamics which may influence the balance between the final receptors of contaminants under different climatic conditions. This complicates the predictability of future trends of emerging compounds in the Antarctic ecosystem, such as of the brominated compounds that we detected in Antarctic petrels. The discrepancy in trends between pelagic and benthic organisms shows that Antarctic biota are still final receptors of globally released organic contaminants and it remains questionable whether the total environmental burden of contaminants in the Antarctic ecosystem is declining.     

Long-term Field Observations on Seasonality in Chlorophyll-<Emphasis Type="Italic">a</Emphasis> Concentrations in a Shallow Coastal Marine Ecosystem,the Wadden Sea

Analyses of long-term field observations (1974–2007) on chlorophyll-a concentrations in the western Wadden Sea showed no long-term trends in the timing of the wax and wane of phytoplankton spring blooms. There is weak evidence, however, that the height of the autumn bloom has decreased since the early 1990s. This fading of the autumn bloom may have had consequences for the carbon transfer to higher trophic levels, currently hampering primary consumer species that mostly rely on food supply during late summer. Current and other findings suggest a shortening of the growing season due to the fading of the autumn bloom in the Wadden Sea and a lengthening of the growing season due to an advancement of the spring bloom in the North Sea. These regionally different changes in seasonality may have contributed to the coinciding decrease in bivalve filtering capacity in the western Wadden Sea and the large-scale offshore shift of juvenile plaice from the Wadden Sea to the adjacent North Sea.     

Springtime changes in snow chemistry lead to new insights into mercury methylation in the Arctic

Seasonal snow is an active media and an important climate factor that governs nutrient transfer in Arctic ecosystems. Since the snow stores and transforms nutrients and contaminants, it is of crucial importance to gain a better understanding of the dynamics of contaminant cycling within the snowpack and its subsequent release to catchments via meltwater. Over the course of a two-month field study in the spring of 2008, we collected snow and meltwater samples from a seasonal snowpack in Ny-Ålesund, Norway (78°56′N, 11°52′E), which were analyzed for major inorganic ions and some organic acids, as well as total, dissolved, bioavailable mercury (THg, DHg, BioHg, respectively) and monomethylmercury (MMHg) species. We observe a seasonal gradient for ion concentrations, with surface samples becoming less concentrated as the season progressed. A significant negative correlation between BioHg and MMHg was observed in the snowpack. MMHg was positively and significantly correlated to methanesulfonate concentrations. Based on these results, we propose a new model for aerobic methylation of mercury involving species in the dimethylsulfoniopropionate cycle.     

Premières mesures françaises de gravimétrie absolue dans les régions polaires Antarctique (terre Adélie) et Arctique (Spitzberg)First French measurements of absolute gravity in the Antarctic (Terre Adélie) and Arctic (Spitsbergen) polar regions

This study is devoted to the first French determinations of absolute gravity in the Antarctic (Dumont-d'Urville, Terre Adélie) and Arctic (Ny-Alesund, Spitsbergen) polar regions. The measurements in Dumont-d'Urville were performed in 2000 with the help of the FG5#206 absolute gravimeter belonging to the French geodetic community since beginning 1997; they show a strong noise that causes an uncertainty close to 11 μGal in the determination of the mean gravity value, which will be compared, to a new determination planned for 2005. The Ny-Alesund measurements show on the contrary a noise that, although very variable in time, leads to a gravity determination of better than 5 μGal. The comparison of our value with a previous one two years before suggests a gravity decrease of about 4 μGal which is related to the vertical uplift measured by Very Large Baseline Interferometry (VLBI) at the same site. However, the uncertainty does not allow discriminating between height-to-gravity conversion factors originating from different models. To cite this article: J. Hinderer et al., C. R. Geoscience 334 (2002) 819–826.     

The Haymiliyah Sulphide Ores (Haylayn Massif,Oman Ophiolite): In-situ Segregation of PGE-poor Magmatic Sulphides in a Fossil Oceanic Magma Chamber

The Semail ophiolite in Oman is one of the few ophiolitic complexes that may display locally magmatic sulphide ores in layer 3 of the crustal section. The ores found in the wadi Haymiliyah plutonic sequence, are composed of low Ni-pyrrhotite, chalcopyrite, pyrite and pentlandite. They are located at the bottom of a thick two-pyroxene and noritic gabbro unit, the Main Laminated Noritic Gabbro Unit (MLNGU) that crystallized from evolved tholeiites according to a calc-alkaline liquid line of descent. The MLNGU rests on coarse-grained layered gabbros of the Main Layered Gabbro Unit (MLGU) crystallized from more primitive MORB-like magmas. A detailed mineralogical study coupled with analyses of S, Se, chalcophile transition metals (Cu, Ni, Platinum-group elements, PGE) and Au allows two stages to be distinguished in the precipitation of the sulphides. Sulphide modal abundances start to increase at the top of the MLGU (up to 4%) where orthopyroxene becomes a major phase. Sulphides in the MLGU are only intercumulus and Cu-rich, indicating a S-undersaturated regime. The fine-grained two-pyroxene gabbros at the bottom of the MLNGU contain up to 16% sulphides (monoclinic pyrrhotite, pyrite, chalcopyrite). However, their precious metal contents are very low and far from economic grade ( PGE+ Au <15 ppb except one concentration at 230 ppb in the richest layer). Sulphide droplets in cumulus silicates indicate that sulphides precipitated in the fine-grained gabbros from a S-saturated magma. Sulphide liquid immiscibility was likely triggered by a set of unusual circumstances, such as a progressive S enrichment in high f_O2 environments resulting from the closure of the Haymiliyah magma subchamber, the lack of a strong Fe-enrichment trend, and possibly a sudden temperature drop. The abundance of disseminated ores suggests that gravity segregation was inefficient; sulphide droplets probably nucleated at the crystallization front of the silicates. This hypothesis (of in-situ crystallization of sulphides) would also account for the very low precious metal contents. All the S present in the magma chamber was mobilized into the ores since the underlying MLNGU and isotropic gabbros are almost devoid of sulphides.     

Discovery of two ophiolite complexes of different ages in the Khoy area (NW Iran)

New field and laboratory studies on the ophiolite of Khoy (northwestern corner of Iran) lead to the discovery that there are not one, but two ophiolitic complexes in the Khoy area: (1) an old, poly-metamorphic ophiolite, whose oldest metamorphic amphiboles yielded a Lower Jurassic apparent ⁴⁰K–⁴⁰Ar age, and whose primary magmatic age should logically be pre-Jurassic (Upper-Triassic?); (2) a younger non metamorphic ophiolite of well dated Upper Cretaceous age. To cite this article: M. Khalatbari-Jafari et al., C. R. Geoscience 335 (2003).     

A Compilation of Silicon and Thirty One Trace Elements Measured in the Natural River Water Reference Material SLRS-4 (NRC-CNRC)

The natural river water certified reference material SLRS-4 (NRC-CNRC, National Research Council-Conseil National de Recherches Canada) has been routinely analysed for major and trace elements by six French laboratories. Most measurements were made using inductively coupled plasma-mass spectrometry. For silicon and thirty one trace elements (rare earth elements, Ag, B, Br, Cs, Ga, Ge, Li, P, Pd, Rb, Se, Th, Ti, Tl, W, Y and Zr), no certified values are assigned by NRC-CNRC. We propose some compilation values and related uncertainties according to the results obtained by the different laboratories.     

Climatic variability during the last 90 ka of the southern and northern Levantine Basin as evident from marine records and speleothems

The influence of the northern Atlantic and tropical monsoonal systems, as recorded by the River Nile, on the climate variability of the southeastern Mediterranean was studied in two cores taken by the R/V Marion Dufresne: one core taken SE of Cyprus representing the northern Levantine Basin (core 9501, 980 m water depth) and the other located ˜380 km further south, represents the southeastern Levantine Basin in an area influenced by the River Nile plume (core 9509, 884 m water depth). The study was performed at relatively high resolution using several proxies: δ¹⁸O of Globigerinoides ruber, sediment characteristics and index colour parameters in core sections representing the last 86 ka. A low-resolution alkenone sea surface temperature record was also measured. The time frame in both cores was mostly constrained by ‘wiggle’ matching with the nearby well-dated δ¹⁸O and δ¹³C record of the Soreq Cave, which is mainly influenced by the eastern Mediterranean water vapor. The sedimentary record of the southern core is strongly influenced by the River Nile contribution throughout the last 86 ka, as evidenced by the higher sedimentation rates compared with the northern core (20 cm/ka vs. 5 cm/ka), continuously darker sediment colour, and higher TOC values (0.6–0.9 vs. 0.25 wt% not including sapropels). During sapropels S1 and S3, present in both cores, the influence of the River Nile became more widespread, reaching as far as Cyprus. Yet, the influence of the River Nile remained stronger in the south, as evident by the higher TOC values in the southern core throughout the entire 90 ka period and the longer duration of S1 in the southern core. An anomalous low δ¹⁸O interval that is not recorded in western Mediterranean occurred between 58 and 49 ka in the Levantine Basin and is more developed in the northern core. This period correlates with D-O interstadial 14 and maximum northern hemisphere insolation during the lastglacial cycle, suggesting that the warming mainly impacted the northern Levant.The Eastern Mediterranean Sea and land area was considerably warmer than the western Mediterranean throughout the LGM – Holocene transition, and the δ¹⁸O_{G. ruber} drop of 4.5‰ is significantly greater than the 3‰ shift found for the western Mediterranean δ¹⁸O_{G. bulloides}, both differences reflecting an increased continental effect from the western to eastern Mediterranean. Comparison between the marine and the land δ¹⁸O records suggests that the origin of rain over the land is composed of mixed signal from the southern and northern Levantine Basin. The study of Δδ¹⁸O_sea–land variations demonstrates that various factors have influenced the sea–land relationship during the last 90 ka. The ‘amount effect’ has an important influence on rainfall δ¹⁸O during interglacial periods (particularly sapropel periods), whereas during glacial periods, increased land distances and elevation differences arising from decrease in sea level may have brought about decrease in δ¹⁸O of rainfall due to Rayleigh distillation processes. These influences were superimposed on those of sea surface water δ¹⁸O changes brought about by continental ice melting, and the strong effects felt in the southern Levantine Basin of the high River Nile input during periods of enhanced monsoonal activity.