• European Marine Science
• 2022-2022close

Ce rapport présente un bilan des résultats obtenus à partir des données récoltées au cours des projets Circareef et Circatax sur les habitats circalittoraux d’intérêt de la zone N2000 « côte basque rocheuse et extension au large ». Ces résultats montrent un intérêt écologique et biogéographique remarquable des sites prospectés avec des habitats abritant une biodiversité originale non connue. L’analyse des prélèvements réalisés et l’identification des taxons présents confortent ces observations. En parallèle, les résultats d’un indice basé sur une approche écosystémique Ecosystem-Based Quality Index (EBQI) sont présentés. à partir de ce bilan, des orientations sont déterminées pour approfondir la connaissance de ces habitats remarquables dans le contexte des directives européennes de conservation. This report presents the results obtained from the data collected during the Circareef and Circatax projects on the circalittoral habitats of interest in the N2000 zone "rocky Basque coast and offshore extension". These results show a remarkable ecological and biogeographical interest of the prospected sites with habitats sheltering an unknown original biodiversity. The analysis of the samples taken and the identification of the taxa collected confirm these observations. In parallel, calculations of an index based on an Ecosystem-Based Quality Index (EBQI) ecosystem approach are presented. From this report, orientations are determined to improve the knowledge of these remarkable habitats in the context of European directives of conservation.

The Ordos Block in the western part of the North China Craton is enigmatic in having contrasting topographic structure in the northern and southern parts, while previous geophysical studies show little difference in crustal and upper mantle structure across the region. Here we present a new model of upper mantle structure in the Ordos Block region in order to test the importance of mantle heterogeneity for topographic differences. Our model is based on P- and S-wave seismic receiver functions calculated for data from 171 stations. It documents the presence of an uppermost mantle low-velocity zone between the Mid Lithospheric Discontinuity (MLD) and the Lehmann discontinuity. Clear converters at the 410 and 660 km discontinuities show constant Mantle Transition Zone (MTZ) thickness within the Ordos Block region, which indicates that no deep mantle thermal anomaly affects its present dynamics. However, the amplitude of the MTZ-converters is higher in the southern than the northern Ordos Block. In contrast, the conversions from MLD and the Lehmann discontinuity are strongest in Northern Ordos, which we interpret as a block with essentially preserved cratonic lithospheric mantle. We speculate that smaller amplitudes of the MLD and Lehmann converters in Southern than Northern Ordos may be related to either rheological weakening of cratonic lithosphere during the Mesozoic convergence between the North and South (Yangtze) China Cratons, or northeast extrusion of Tibetan lower crust and upper mantle in the Cenozoic caused by the India-Asia collision.

Groundwater seepage leads to the formation of theater-headed valleys (THVs) in unconsolidated sediments. In bedrock, the role of groundwater in THV development remains disputed. Here, we integrate field and remote-sensing observations from Gnejna Valley (Maltese Islands) with numerical modeling to demonstrate that groundwater seepage can be the main driver of THV formation in jointed limestone overlying clays. The inferred erosion mechanisms entail (1) widening of joints and fractures by fluid pressure and dissolution and (2) creeping of an underlying clay layer, which lead to slope failure at the valley head and its upslope retreat. The latter is slower than the removal of the talus by creep and sliding on the valley bed. The location and width of THVs are controlled by the location of the master fault and the extent of the damage zone, respectively. The variability of seepage across the fault zone determines the shape of the valley head, with an exponential decrease in seepage away from the fault giving rise to a theater-shaped head that best matches that of Gnejna Valley. Our model may explain the formation of THVs by groundwater in jointed, strong-over-weak chemical sedimentary lithologies, particularly in arid terrestrial settings. peer-reviewed

Microorganisms do not exist as individual population in the environment. Rather, they form complex assemblages that perform essential ecosystem functions and maintain ecosystem stability. Besides the diversity and composition of microbial communities, deciphering their potential interactions in the form of association networks has attracted many microbiologists and ecologists. Much effort has been made toward the methodological development for constructing microbial association networks. However, microbial profiles suffer dramatically from zero values, which hamper accurate association network construction. In this study, we investigated the effects of zero-value issues associated with microbial association network construction. Using the TARA Oceans microbial profile as an example, different zero-value-treatment approaches were comparatively investigated using different correlation methods. The results suggested dramatic variations of correlation coefficient values for differently treated microbial profiles. Most specifically, correlation coefficients among less frequent microbial taxa were more affected, whichever method was used. Negative correlation coefficients were more problematic and sensitive to network construction, as many of them were inferred from low-overlapped microbial taxa. Consequently, microbial association networks were greatly differed. Among various approaches, we recommend sequential calculation of correlation coefficients for microbial taxa pairs by excluding paired zero values. Filling missing values with pseudo-values is not recommended. As microbial association network analyses have become a widely used technique in the field of microbial ecology and environmental science, we urge cautions be made to critically consider the zero-value issues in microbial data.

Subglacial meltwater drainage can enhance localized melting along grounding zones and beneath the ice shelves of marine-terminating glaciers. Efforts to constrain the evolution of subglacial hydrology and the resulting influence on ice stability in space and on decadal to millennial timescales are lacking. Here, we apply sedimentological, geochemical, and statistical methods to analyze sediment cores recovered offshore Thwaites Glacier, West Antarctica to reconstruct meltwater drainage activity through the pre-satellite era. We find evidence for a long-lived subglacial hydrologic system beneath Thwaites Glacier and indications that meltwater plumes are the primary mechanism of sedimentation seaward of the glacier today. Detailed core stratigraphy revealed through computed tomography scanning captures variability in drainage styles and suggests greater magnitudes of sediment-laden meltwater have been delivered to the ocean in recent centuries compared to the past several thousand years. Fundamental similarities between meltwater plume deposits offshore Thwaites Glacier and those described in association with other Antarctic glacial systems imply widespread and similar subglacial hydrologic processes that occur independently of subglacial geology. In the context of Holocene changes to the Thwaites Glacier margin, it is likely that subglacial drainage enhanced submarine melt along the grounding zone and amplified ice-shelf melt driven by oceanic processes, consistent with observations of other West Antarctic glaciers today. This study highlights the necessity of accounting for the influence of subglacial hydrology on grounding-zone and ice-shelf melt in projections of future behavior of the Thwaites Glacier ice margin and marine-based glaciers around the Antarctic continent. Frontiers in Earth Science, 10 ISSN:2296-6463

Mission Microbiomes Leg 8 took place between Salvador de Bahia and Rio de Janeiro, Brazil. It corresponded to the first AtlantECO “Biodiscovery” leg, the first case Study on "Molecular Bioprospecting" of the mission. The two principal objectives of this leg were 1) to augment plankton biodiversity discovery through replicated omics samplings, and 2) to enable the potential discovery of new bioproducts & chemicals for industrial applications with high socio-economic value, such as new ways to produce pharmaceutical products or natural enzymes able to digest complex molecules such as pollutants or plastics. Given the bioprospecting focus of this leg, sampling stations were all located within international waters, along the Vitória-Trindade Chain (VTC) consisting of 11 heterogeneous seamounts. Due to this location, an additional objective of this leg was 3) to produce samples potentially also valuable for the H2020 EU-funded All-Atlantic sister project iAtlantic (Integrated Assessment of Atlantic Marine Ecosystems in Space and Time), which overall goal is to measure the impact of climate change on the Atlantic, and notably on deep-sea corals of the VTC seamounts. In line with these objectives, the main questions we wish to address are: a) Are we under-sampling the functional and taxonomic biodiversity of marine microbiomes? What are the “rare” species? (Is everything everywhere?); b) What is the nutrient/light dynamics of a DCM? Is the DCM a vertically homogeneous feature or is it made of vertically separated niches? and c) What are the pelagic microbiomes associated with deep coral reefs? Can we characterize the vertical connectivity between the pelagic microbiomes and deep-sea corals (e.g., downward export of organic matter, using a genomic signature)? What is the survival of coral larvae in the pelagic ecosystem?

Climate warming and related drivers of soil thermal change in the Arctic are expected to modify the distribution and dynamics of carbon contained in perennially frozen grounds. Thawing of permafrost in the Mackenzie Delta region of northwestern Canada, coupled with increases in river discharge and coastal erosion, trigger the release of terrestrial organic matter (OMt) from the largest Arctic drainage basin in North America into the Arctic Ocean. While this process is ongoing, well-established, and its rate is accelerating, the fate of the newly-mobilized organic matter, as it transits from the watershed through the delta and into the marine system, remains poorly understood. In the framework of the European Horizon 2020 Nunataryuk programme, and as part of the Work Package 4 (WP4) Coastal Waters theme, four field expeditions were conducted in the Mackenzie Delta region and southern Beaufort Sea from April to September 2019. The temporal sampling design allowed the survey of ambient conditions in the coastal waters under full ice cover prior to the spring freshet, during ice break-up in summer, as well as anterior to the freeze-up period in fall. To capture the fluvial-marine transition zone, and with distinct challenges related to shallow waters and changing seasonal and meteorological conditions, the field sampling was conducted in close partnership with members of the communities of Aklavik, Inuvik and Tuktoyaktuk, using several platforms: helicopters, snowmobiles and small boats. Water column profiles of physical and optical variables were measured in situ, while surface water, groundwater and sediment samples were collected and preserved for the determination of the composition and sources of OMt, including particulate and dissolved organic carbon (POC, DOC), and chromophoric dissolved organic matter (CDOM), as well as a suite of physical, chemical and biological variables. Here we present an overview of the standardized datasets, including hydrographic profiles, remote sensing reflectance, temperature and salinity, particle absorption, nutrients, dissolved organic carbon, particulate organic carbon, particulate organic nitrogen, colored dissolved organic matter absorption, fluorescent dissolved organic matter intensity, suspended particulate matter, total particulate carbon, total particulate nitrogen, stable water isotopes, radon in water, bacterial abundance, and a string of phytoplankton pigments including total chlorophyll. Datasets and related metadata can be found in Juhls et al. 2021. https://doi.pangaea.de/10.1594/PANGAEA.937587.