Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
In the Oman ophiolite crustal section, a contact zone between the gabbro unit and the volcanics and diabase sheeted dikes, called the root zone of the sheeted dike complex, has been recently mapped at a fine scale in a selected area. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main gabbro unit and the root zone of the sheeted dike complex. With a few exceptions accounted for, this horizon has a fairly constant thickness, similar to 100 m, and a crude internal pseudo-stratigraphy. At the base of the root zone are isotropic ophitic gabbros interpreted as a thermal boundary layer. This layer is transitional between the magmatic system of the melt lens, convecting at 1200 degrees C, and a high-temperature (< 1100 degrees C) hydrothermal system, convecting within the root zone. Above this level, the isotropic gabbros have been, locally, largely molten due to an influx of seawater, at similar to 1100 degrees C, thus generating varitextured ophitic and pegmatitic gabbros. These latter gabbros constitute the upper part of the root zone and are associated with trondjhemitic intrusions as screens in the lower sheeted dikes. Diorites and trondjhemites were also generated by hydrous melting, at temperatures below 1000 degrees C. The whole root zone is a domain of very sharp average thermal gradient (similar to 7 degrees C/m). At the top of the root zone, a new thermal boundary layer, with diabase dikes hydrated in amphibolite facies conditions, separates the preceding high- temperature convective system from the well-known greenschist facies (< 450 degrees C) hydrothermal system operating throughout the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone are intruded by protodikes with distinctive microgranular margins and an ophitic center. Protodike swarms are exceptional because, intruding a medium at similar to 1100 degrees C, they are largely destroyed by dike-in-dike intrusions and by hydrous melting. However, they demonstrate that this zone was generated by melt conduits issued from the underlying melt lens. Each dike of the sheeted dike complex is thus fed by one protodike. As this zone has been recently drilled by IODP in the eastern Pacific Ocean, a brief comparison is proposed.
ISSN: 1525-2027

hal-00411331
https://hal.archives-ouvertes.fr/hal-00411331
DOI : 10.1029/2007GC001918

Éditeur(s) : HAL CCSD Wiley
Résumé :
How Quaternary climatic and geological disturbances influenced the composition of Neotropical forests is hotly debated. Rain fall and temperature changes during and/or immediately after the Last Glacial Maximum (LGM) are thought to have strongly affected the geographical distribution and local abundance of tree species. The paucity of the fossil records in Neotropical forests prevents a direct reconstruction of such processes. To describe community-level historical trends in forest composition, we turned therefore to inferential methods based on the reconstruction of past demographic changes. In particular, we modelled the history of rainforests in the Eastern Guiana Shield over a time scale of several thousand generations, through the application of Approximate Bayesian Computation (ABC) and Maximum Likelihood (ML) methods to diversity data at nuclear and chloroplast loci in eight species or subspecies of rainforest trees. Depending on the species and on the method applied, we detected population contraction, expansion or stability, with a general trend in favour of stability or expansion, with changes presumably having occurred during or after the LGM. These findings suggest that Guiana Shield rain forests have globally persisted, while expanding, through the Quaternary, but that different species have experienced different demographic events, with a trend towards the increase in frequency of light-demanding, disturbance-associated species.
ISSN: 0962-1083

hal-01512150
https://hal.archives-ouvertes.fr/hal-01512150
https://hal.archives-ouvertes.fr/hal-01512150/document
https://hal.archives-ouvertes.fr/hal-01512150/file/Barthe_et_al-_%7B0B6A8C5E-DDFF-4C4E-822C-4E7BC22A6C9B%7D.pdf
DOI : 10.1111/mec.13949

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The Nile delta sedimentation constitutes a continuous high resolution (1.6 mm/year) record of Ethiopian African monsoon regime intensity. Multiproxy analyses performed on core MS27PT recovered in hemipelagic Nile sediment margin (<90 km outward of the Rosetta mouth of the Nile) allow the quantification of the Saharan aeolian dust and the Blue/White Nile River suspended matter frequency fluctuations during the last 21 cal. ka BP. The radiogenic Sr and Nd isotopes, clay mineralogy, bulk elemental composition and palynological analyses reveal large changes in source components, oscillating between a dominant aeolian Saharan contribution during the LGM and the Late Holocene (~4 to 2 cal. ka BP), a dominant Blue/Atbara Nile River contribution during the early Holocene (15 to 8.4 cal. ka BP) and a probable White Nile River contribution during the Middle Holocene (8.4 to 4 cal. ka BP). The following main features are highlighted: 1. The rapid shift from the LGM arid conditions to the African Humid Period (AHP) started at about 15 cal. ka BP. AHP extends until 8.4 cal. ka BP, and we suggest that the Ethiopian African Monsoon maximum between 12 and 8 cal. ka BP is responsible for a larger Blue/Atbara Nile sediment load and freshwater input into the Eastern Mediterranean Sea. 2. The transition between the AHP and the arid Late Holocene is gradual and occurs in two main phases between 8.4 and 6.5 cal. ka BP and 6.5 to 3.2 cal. ka BP. We suggest that the main rain belt shifted southward from 8.4 to ~4 cal. ka BP and was responsible for progressively reduced sediment load and freshwater input into the eastern Mediterranean Sea. 3. The aridification along the Nile catchments occurred from ~4 to 2 cal. ka BP. A dry period, which culminates at 3.2 cal. ka BP, and seems to coincide with a re-establishment of increased oceanic primary productivity in the western Mediterranean Sea. We postulate that the decrease in thermo-haline water Mediterranean circulation could be part of a response to huge volumes of fresh-water delivered principally by the Nile River from 12 to 8.4 cal. ka BP in the eastern Mediterranean. We propose that the large hydrological change in Ethiopian latitude could be a trigger for the 8.2 ka cooling event recorded in high latitude. Revel R., Colin C., Bernasconi S., Combourieu-Nebout N., Ducassou E., Grousset F.E., Rolland Y., Migeon S., Brunet P., Zhaa Y., Bosch D., Mascle J.,. "21,000 years of Ethiopian African moonsoon variability recorded in sediments of the western Nile deep sea fan", Regional Environmental Change,
ISSN: 1436-3798

hal-00836300
https://hal.archives-ouvertes.fr/hal-00836300

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Located in central Europe at the junction of the Southern Alps and the Dinarides, the ≈ 100 km long Idrija fault, striking N310 and dipping ≈ 80°NE is often considered as the potential source of the historical 1511 earthquake (estimated macroseismic magnitude: Mm 6.8). Using 1/25,000 to 1/5000 topographical maps, satellite images (SPOT5), 12.5 m illuminated DEM and airborne LiDAR data-derived 1-m-DEM, we examined in detail the Idrija fault trace along a 20-km-long swath in the central portion of the fault. Combining topographic and remote sensing data allows characterizing the recent activity along the Idrija fault and estimating its cumulative displacement along strike.Between Tolmin and Godovič, the fault trace is mostly linear but appears divided into three disconnected segments of 11–20-km-long. Morphological evidences such as offset streams and ridges and major drainage abandonments suggest ongoing movement. At three sites, field observations and LiDAR-DEM analysis allow measuring cumulative dextral offsets comprised between 35 and 65 m. We found two larger offsets of ≈ 140 m and ≈ 360 m at the southeasternmost site. The vertical component is significant and revealed by a rake ranging from 0 to 33°. The derived N303–N310 striking slip-vector is in agreement with the motion along this fault as suggested by the geodesy. Assuming that the smallest measured offsets have been recorded after the last glacial maximum (LGM, about 20 ka ago), the minimum slip-rate for the Idrija fault is about 1.5 mm/yr over this period of time.
ISSN: 0040-1951

hal-01115724
https://hal.archives-ouvertes.fr/hal-01115724
DOI : 10.1016/j.tecto.2014.04.043

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Despite the unique nature of endemic species, their origin and population history remain poorly studied. We investigated the population history of 28 coral reef fish species, close related, from the Gambier and Marquesas Islands, from five families, with range size varying from widespread to small-range endemic. We analyzed both mitochondrial and nuclear sequence data using neutrality test and Bayesian analysis (EBSP and ABC). We found evidence for demographic expansions for most species (24 of 28), irrespective of range size, reproduction strategy or archipelago. The timing of the expansions varied greatly among species, from 8,000 to 2,000,000 years ago. The typical hypothesis for reef fish that links population expansions to the Last Glacial Maximum fit for 14 of the 24 demographic expansions. We propose two evolutionary processes that could lead to expansions older than the LGM: (a) we are retrieving the signature of an old colonization process for widespread, large-range endemic and paleoendemic species or (b) speciation; the expansion reflects the birth of the species for neoendemic species. We show for the first time that the demographic histories of endemic and widespread reef fish are not distinctly different and suggest that a number of processes drive endemism.
ISSN: 2045-2322

hal-01469339
https://hal-univ-perp.archives-ouvertes.fr/hal-01469339
https://hal-univ-perp.archives-ouvertes.fr/hal-01469339/document
https://hal-univ-perp.archives-ouvertes.fr/hal-01469339/file/population%20expansions.pdf
DOI : 10.1038/srep40519

Éditeur(s) : Elsevier
Résumé : High-resolution sedimentological and micropaleontological studies of several deep-sea cores retrieved from the levees of the Celtic and Armorican turbidite systems (Bay of Biscay - North Atlantic Ocean) allow the detection of the major oscillations of the British-Irish Ice Sheet (BIIS) and 'Fleuve Manche' palaeoriver discharges over the last 30,000 years, which were mainly triggered by climate changes. Between 30 and 20 cal ka, the turbiditic activity on the Celtic-Annorican margin was weak, contrasting with previous stratigraphic models which predicted a substantial increase of sediment supply during low sea-level stands. This low turbidite deposit frequency was most likely the result of a weak activity of the 'Fleuve Manche' palaeoriver and/or of a reduced seaward transfer of sediments from the shelf to the margin. However, two episodes of turbiditic activity increase were detected in the Celtic-Armorican margin, during Heinrich events (HE) 3 and 2. This strengthening of the turbiditic activity was triggered by the meltwater releases from European ice sheets and glaciers favouring the seaward transfer of subglacial material, at least via 'Fleuve Manche' palaeoriver. At around 20 cal ka, a significant increase of turbidite deposit frequency occurred as a response to the onset of the last deglaciation. The retreat of the European ice sheets and glaciers induced a substantial increase of the 'Fleuve Manche' palaeoriver discharges and seaward transfer of continentally-derived material into the Armorican turbidite system. The intensification of the turbiditic activity on the Celtic system was directly sustained by the widespread transport of subglacial sediments from the British-Irish Ice Sheet (BIIS) to the Celtic Sea via the Irish Sea Basin. A sudden reduction of turbiditic activity in the Armorican system, between ca. 19 and 18.3 cal ka, could have been triggered by the first well known abrupt sea-level rise ('meltwater pulse', at around 19 cal ka) favouring the trapping of sediment in the 'Fleuve Manche' palaeoriver valleys and the decrease of the seaward transfer of continentally-derived material. The maximum of turbiditic activity strengthening in the Celtic-Armorican margin, between ca. 18.3 and 17 cal ka, was induced by the decay of European ice sheets and glaciers producing the most extreme episode of the 'Fleuve Manche' palaeoriver runoff and a great seaward transfer of subglacial material into the Bay of Biscay, Between ca. 17.5 and 16 cal ka, the turbiditic activity significantly decreased in both Celtic and Armorican turbidite systems in response to a global re-advance of glaciers and ice sheets in Europe. The last episode of ice sheet retreat, between ca. 16 and 14 cal ka, is well expressed in the Celtic system by a new increase of the turbiditic activity. The major episode of sea-level rise at around 14 cal ka ('Meltwater Pulse 1A'), precluding the seaward transfer of sediments, induced the end of turbiditic activity in both the Celtic and the Armorican system. Although two main phases of global sea-level rise seem to have had an effect on the Celtic-Armorican margin, this work proposes the BUS retreat and associated riverine discharges as the main trigger mechanisms of the turbiditic activity in this region during the last 30,000 years.
Marine Geology (0025-3227) (Elsevier), 2008 , Vol. 247 , N. 1-2 , P. 84-103

Droits : 2008 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2008/publication-3729.pdf
DOI:10.1016/j.margeo.2007.08.006
http://archimer.ifremer.fr/doc/00000/3729/

Éditeur(s) : Elsevier Sci Ltd
Résumé : A detailed morphological analysis of the outer shelf and continental slope of the Western Gulf of Lion is presented, based on swath bathymetry data together with sub-bottom profiles and high resolution seismic reflection profiles. These data reveal two main erosive features, of very different dimensions: the axial incision and the canyon's major valley. The height of axial incisions' flanks with respect to the canyon deepest point (the thalweg) ranges from 40 to 150 m. It creates a small axial erosive path within the canyon's major valley, which is typically bounded by flanks of more than 700 m in height. We interpret the axial incision observed in the sea floor as the imprint of turbidity currents that eroded the floor of canyons during phases of connection to rivers (hyperpycnal turbidity current). Such currents are most likely to have formed during the Last Glacial Maximum (LGM) as both proximity of the shoreline (due to the lowstand of sea level) and high detrital sediment supply (due to glacial abrasion upstream) increased the flow of sediments delivered to the canyon heads. Fossil axial incisions, observed in seismic lines, are related to equivalent conditions. The axial incision, however, has a key influence on canyon evolution as it triggers mass wasting of different sizes that affect the canyon's major valley (head and flanks). We interpret the geometry of the canyon's major valley as the result of recurrent activity of axial incisions. These periods of activity occurred during low sea levels at glacial maxima and show a cyclicity of 100,000 years for the last 400,000 years.
Marine And Petroleum Geology (0264-8172) (Elsevier Sci Ltd), 2005-06 , Vol. 22 , N. 6-7 , P. 805-826

Droits : 2005 Elsevier Ltd All rights reserved
http://archimer.ifremer.fr/doc/2005/publication-353.pdf
DOI:10.1016/j.marpetgeo.2005.03.011
http://archimer.ifremer.fr/doc/00000/353/