Variability in intermediate water circulation of the western Tyrrhenian margin (NE Corsica) over the past 56 kyr

The Marion Dufrene core MD01-2472 made of hemipelagic fine-grained sediments (silt and clays) was collected at 501 m depth on the East Corsica continental slope in 2001 and studied in detail in its 12 uppermost meters. The correlation between sedimentological parameters (Sortable Silt), isotopic data and 14C dating allowed to establish the chronology of main climate events (Younger Dryas/YD, Bölling-Alleröd/B-A, Heinrich events/HS) on this record and to evaluate the impact of major climate oscillations on bottom water condition variability. The sea temperature changes are identified thanks to the planktonic foraminifera assemblages. HS are marked by planktonic foraminifers with peaks of the polar species N. pachyderma (left-coiling), whilst interstadials are marked by warm planktonics that become very abundant during the B-A and Holocene. The occurrence of reworked ostracod species (originating from the continental shelf) and the presence of shallow water Elphidium/Ammonia benthic foraminifera are used to estimate the degree of along-slope transport at the core site. This has revealed two intervals of along-slope transport also associated with coarse-grained contourites, deposited during the YD and HS2 episodes. Intervals with Krithe spp. (ostracod), C. wuellerstorfi (benthic foraminifera) indicate bottom water oxygenation during stadials, whereas interstadials are typified by A. acuminata and Paracypris sp (ostracods) indicating low oxygenated environments. The Last Glacial Maximum is dominated by the planktonic foraminifer T. quinqueloba suggesting high surface primary productivity associated with the establishment of mestrophic bottom conditions. During the Holocene, benthic assemblages indicate oligo-mesotrophic conditions and weak hydrodynamic bottom regime. We hypothesize that there is relationship between the Levantine Intermediate Water (LIW) intensification during cold rapid climate events and benthic fauna assemblages due to changes in: 1) bottom water ventilation, corresponding to a significant reinforcement of the LIW velocity, and 2) the export of nutrients (generating changes in trophic conditions) and/or sediment particles by bottom currents (contributing to the formation of contourites)

Distribution of ostracod and benthic foraminiferal assemblages during the last 550kyr in the East-Corsica basin, western Mediterranean Sea: A paleo-environmental reconstruction

The study of Quaternary sediments covering the last 550 kyr from the borehole GDEC-4-2, East-Corsica basin, was performed through the study of benthic meiofaunal assemblages (ostracoda and benthic foraminifera) whose distribution responds to climate changes, and glacial–interglacial variability in particular. The interglacial ostracod group is mainly composed of the Argilloecia acuminata, Cytheropteron alatum, Henryhowella sp., Polycope sp. and Cytherella sp., which indicate an enhanced surface productivity and possibly high bottom water temperature during these warm intervals. Benthic foraminifera, such as Hoeglundina elegans, Sigmoilopsis schlumbergeri, Bigenerina nodosaria and Hyalinea balthica are predominant during interglacial periods and also show an increasing surface productivity associated to warm intervals. Increase in surface productivity would contribute to an enhanced export of organic matter to sea floor at the GDEC-4-2 site during interglacial periods. The bottom temperature drop coupled with the important sedimentary inputs (associated to a significant quantity of refractory organic matter) during glacial periods was highlighted by the predominance of ostracoda, such as Paradoxostoma sp., Cytheropteron testudo, Bathycythere vanstraateni, Macrocypris sp., Echinocythereis echinata and benthic foraminifera as Bolivina spathulata, Bulimina costata, and Bulimina marginata. The distribution of some benthic foraminifera species describing a turnover system allowed the characterization of the last three climate cycles (MIS 8–9, MIS 6–7, MIS 1–5) probably marked by changes of bottom water paleoceanographic conditions in terms of temperature, salinity, bottom current intensity and surface productivity. The cyclic variations of the allochthonous group recorded over the last 550 kyr suggest a major role played by the eustatic factor, with the latter possibly favouring down slope transport. The distribution of this group allowed thus establishing a link between shelf-basin sediment transfer and climate, with enhanced shelf-basin sediment transfer during glacial low stands.L’étude des sédiments du Quaternaire provenant du forage GDEC-4-2 couvrant les derniers 550 ka, dans le bassin Est-Corse, a été faite grâce à la distribution des assemblages de méiofaunes benthiques (les ostracodes et les foraminifères benthiques) dont la distribution répond aux changements climatiques, et particulièrement, à la variabilité glaciaire–interglaciaire. Le groupe interglaciaire des ostracodes est principalement composé de Argilloecia acuminata, Cytheropteron alatum, Henryhowella sp. Polycope sp. et Cytherella sp. qui indiquent une amélioration de la productivité de surface et probablement des températures élevées au fond pendant ces intervalles chauds. Les foraminifères benthiques, tels que Hoeglundina elegans, Sigmoilopsis schlumbergeri, Bigenerina nodosaria et Hyalinea balthica prédominent pendant les périodes interglaciaires et montrent également une augmentation de la productivité de surface associée à ces intervalles chauds. L’augmentation de la productivité de surface a pu contribuer à une amélioration, pendant les interglaciaires, de l’apport de la matière organique au fond au niveau du site du forage GDEC-4-2. La chute de la température du fond combinée à un apport sédimentaire important (associée à une bonne quantité de matière organique réfractaire) pendant les périodes glaciaires a été mise en évidence par la prédominance des ostracodes, tels que Paradoxostoma sp., Cytheropteron testudo, Bathycythere vanstraateni, Macrocypris sp., Echinocythereis echinata et des foraminifères benthiques comme Bolivina spathulata, Bulimina costata, et Bulimina marginata. La distribution de certains foraminifères benthiques, décrivant un système de turnover, a permis la caractérisation des trois derniers cycles climatiques (MIS 8–9, MIS 6–7, MIS 1–5) probablement marqués par des changements des conditions paléocéanographiques des eaux profondes en termes de : température, salinité, intensité du courant au fond et productivité de surface. Les variations cycliques du groupe allochtone, enregistrées pendant les 550 000 dernières années, suggèrent le rôle major joué par les facteurs eustatiques pouvant favoriser le transport de la pente vers le bassin profond. La distribution de ce group a donc permis d’établir un lien entre le transfert sédimentaire plateau continental-bassin profond et le climat avec un renforcement du transfert durant les bas niveaux marins