Seasonal and event-controlled export of organic matter from the shelf towards the Gulf of Lions continental slope

To investigate the role of coastal canyons in the transfer of organic matter from the shelf to the slope and basin, we deployed sediment trap/current meter pairs at the head of five canyons in the Gulf of Lions (GoL) between November 2003 and May 2004. Analysis of organic carbon, biogenic silica, Corg isotopic composition, Corg/total nitrogen, chloropigments, and amino acids clearly shows the seasonal influence and effect of extreme meteorological events on the composition of collected particles. The sampling period was divided into three “scenarios”. The first corresponded to a large easterly storm and flood of the Rhone river during stratified water column conditions; the composition of material collected during this event was influenced by increased transfer of riverine and coastal particulate matter, with a lower Corg content. During the second “fall-winter” scenario, northern and northwestern winds blowing over the shelf caused cooling and homogenization of the shelf water column; particles collected at this time reflected the homogeneous source of particulate matter transported through canyons; particles sitting in the vicinity of canyon heads are most likely swept downslope by the general south-westward circulation. Organic tracers indicate a degraded origin for organic matter transported during this period. A third “spring” scenario corresponded to northern winds alternating with eastward windstorms that triggered and/or enhanced the cascading of dense waters accumulated on the bottom of the shelf due to previous cooling. These conditions occurred in conjunction with increased phytoplankton productivity in shelf surface waters. Organic matter advected mainly by dense shelf water cascading was fresher due to the transport of newly produced particles and a variable terrestrial fraction; this fraction depended on the proportion of resuspended material accumulated during previous high discharge periods that was involved in each transport pulse. The tight link shown between meteorological conditions and organic matter transport is important for continental margin geochemical studies as future changes in climatic conditions may lead to dramatic changes in carbon sequestration capability and in the ecosystems of deep margin environments.     

Impact of winter dense water formation on shelf sediment erosion (evidence from the Gulf of Lions,NW Mediterranean)

A 1-year survey of sediment dynamics on the Têt inner-shelf in the south-western part of the Gulf of Lions was conducted as part of the EUROSTRATAFORM program (European Margin Strata Formation) from October 2004 to November 2005. Several bottom instruments (ADCP, wave gauge and altimeters) were deployed at 28 m water depth on the Têt prodelta to measure forcing responsible for sediment erosion and transport on the inner-shelf.     

The next step in shallow coral reef monitoring: Combining remote sensing and in situ approaches

Most current coral reef management is supported by mapping and monitoring limited in record length and spatial extent. These deficiencies were addressed in a multidisciplinary study of cyclone impacts on Aboré Reef, New-Caledonia. Local knowledge, high thematic-resolution maps, and time-series satellite imagery complemented classical in situ monitoring methods. Field survey stations were selected from examination of pre- and post-cyclone images and their post-cyclone coral communities documented in terms of substrata, coral morphologies, live coral cover, and taxonomy. Time-series maps of hierarchically defined coral communities created at spatial scales documenting the variability among communities (29-45 classes) and suggesting the processes that affected them. The increased spatial coverage and repeatability of this approach significantly improved the recognition and interpretation of coral communities’ spatio-temporal variability. It identified precise locations of impacted areas and those exhibiting coral recovery and resilience. The approach provides a comprehensive suite of information on which to base reef-scale conservation actions.     

Why Is the Climate Forcing of Sulfate Aerosols So Uncertain?

l. IntroductionAlthough the aerosol has been recognized as an important factor which has innuence onthe past, present and future climate for a long time, it still has much uncertainty in assessingits climate forcing. The direct radiative forcing of sulfate aerosols has been estimated rangingfrom --0.3 W/ m2 to --0.9 W/ m2 in recent publications (Charlson et al., l992, Kiehl andBriegleb l993; Taylor and Penner 1994, Boucher and Anderson l995, Kieh1 and Rodhe l995;Chuang et al., l997, Penne…     

Spatial Variation of Sea-Spray Fluxes over a Mediterranean Coastal Zone Using a Sea-State Model

We first deal with sea-spray flux estimates for short fetch conditions in coastal Mediterranean areas. To this end, a sea-state dependent model for the whitecap fraction was included in three different formulations for the sea-spray source function. A comparison with the sea-spray fluxes, calculated on the basis of aerosol size distributions measured at the island of Porquerolles located south off the French Riviera, evaluates the predictions of different whitecap dependant flux formulations. Then we deal with the spatial distribution of the whitecap fraction and the sea-spray fluxes in the study area. To achieve this, a whitecap dependant flux formulation was forced by a wave numerical model that was implemented in the study area. Experimental results on wave conditions have been used to adjust the model in the Mediterranean coastal area. Numerical simulations of wave and whitecap coverage have been carried out during typical regional wind events, and they show a nonhomogeneous distribution of the sea-surface production over the northern Mediterranean as a consequence of the spatial variation of the sea state. In particular, we note the occurrence of a narrow band of high sea-surface production following the northern coast and along the east part of the Gulf of Lions.     

Study of the total lightning activity in a hailstorm

A thunderstorm that developed over northeastern Spain on 16 June 2006 is analyzed. This severe thunderstorm produced hailstones as large as 40 mm and had a lifetime of 3 h and 30 min. Radar cross-sections show strong vertical development with cloud echo tops reaching an altitude of 13 km. The specific characteristics of the lightning activity of this storm were: (i) a large amount (81%) of negative cloud-to-ground (−CG) flashes with very low peak currents (< 10 kA in absolute value), (ii) a very large proportion of intra-cloud (IC) flashes with an IC/CG ratio reaching about 400, (iii) a large number of “short” IC flashes (with only 1-VHF source according to SAFIR detection), (iv) a large increase of the −CG flash rate and of the CG proportion near the end of the storm. The rate of −CG flashes with a low peak current were observed to evolve similarly to the rates of IC flashes. Most of them have been assumed to be IC flashes misclassified by the Spanish Lightning Detection Network (SLDN). They have been filtered as it is usually done for misclassified +CG flashes. After this filtering, CG flash rates remained very low (< 1 min^− 1) with +CG flashes sometimes dominant. All the particular lightning activity characteristics similar to those observed in the Severe Thunderstorm Electrification and Precipitation Study (STEPS) campaigns support the hypothesis that this thunderstorm could have had an inverted-polarity or complex charge structure. The maximum IC flash rate (67 min^− 1) peaked 24 min before the presence of reflectivity higher than 60 dBZ. The IC activity abruptly decreased during the period when reflectivity was dramatically increasing. The time of maximum reflectivity observed by radar was consistent with the times of reported hail at the ground.     

Adaptation of the optimal fingerprint method for climate change detection using a well-conditioned covariance matrix estimate

The “optimal fingerprint” method, usually used for detection and attribution studies, requires to know, or, in practice, to estimate the covariance matrix of the internal climate variability. In this work, a new adaptation of the “optimal fingerprints” method is presented. The main goal is to allow the use of a covariance matrix estimate based on an observation dataset in which the number of years used for covariance estimation is close to the number of observed time series. Our adaptation is based on the use of a regularized estimate of the covariance matrix, that is well-conditioned, and asymptotically more precise, in the sense of the mean square error. This method is shown to be more powerful than the basic “guess pattern fingerprint”, and than the classical use of a pseudo-inverted truncation of the empirical covariance matrix. The construction of the detection test is achieved by using a bootstrap technique particularly well-suited to estimate the internal climate variability in real world observations. In order to validate the efficiency of the detection algorithm with climate data, the methodology presented here is first applied with pseudo-observations derived from transient regional climate change scenarios covering the 1960–2099 period. It is then used to perform a formal detection study of climate change over France, analyzing homogenized observed temperature series from 1900 to 2006. In this case, the estimation of the covariance matrix is only based on a part of the observation dataset. This new approach allows the confirmation and extension of previous results regarding the detection of an anthropogenic climate change signal over the country.     

La couverture océanique des ultrabasites de Lanzo (Alpes occidentales) : arguments lithostratigraphiques et pétrologiques

Résumé

De nombreuses analogies entre les évolutions pétro-logiques et métamorphiques des ultrabasites de Lanzo et des ophiohtes liguro-piémontaises ont été soulignées récemment. Ces analogies concernent: la nature des ultrabasites (Iherzolites à plagioclase peu appauvries), la présence de gabbros et de filons basaltiques superficiels intrusifs dans les péridotites, l’existence de reliques de paragenèses métamorphiques de haute température attribuables à un épisode océanique, l’empreinte du métamorphisme alpin de haute pression. Ces faits ont conduit certains auteurs à proposer que les péridotites de Lanzo représentent un fragment du plancher océanique téthysien.

Nous avons tenté de vérifier cette hypothèse en recherchant la couverture océanique (volcanique et/ou sédimentaire) des péridotites de Lanzo, dans la partie occidentale du massif où des pincées de métabasites et de schistes lustrés s.l. sont visibles associées aux serpentinites.

Aux environs du village de Richiaglio, à l’Est de la cicatrice de Viù, les successions lithostratigraphiques reconstituées comprennent schémati-quement : les ultrabasites serpentinisées du corps de Lanzo, des métabasites, des quartzites manganésifères (comparables à celles du Mont Viso par exemple), et des calcschistes. Ces successions s’apparentent à celles décrites dans les couvertures d’autres ophiolites piémontaises, ce qui conduit à proposer que ces ensembles métavolcaniques et Dimetaires dimentaires représentent des résidus de la couverture océanique originelle des péridotites. L’analyse des paragenèses éclogitiques des métasédiments et des métabasites permet de montrer que l’évolution P,T alpine de ces lambeaux est compatible avec celle du corps de Lanzo. Cette interprétation et ses implications sont discutées.     

Rhythmic pelagic successions and terrestrial orbital variations. The example of the Lower Cretaceous from Angles (SE France)

An harmonic analysis of limestone bed and marl interbed thicknesses of pelagic Berriasian-Barremian interval from the Angles area (SE France) shows that marly parts of the succession contain frequencies significantly different from calcareous ones. Some of the resulting periods are in the Milankovitch frequency band but many others are different from those predicted by the orbital theory.