R. Abu-Zied

A dynamic concept for eastern Mediterranean circulation and oxygenation during sapropel formation

We propose that intermittent bottom water ventilation occurred throughout periods of sapropel deposition, and that the recently reported sapropel ‘interruptions’ represent centennial-scale episodes of enhanced frequency/intensity of that process. In essence, the modern high-frequency variability in deep water formation (affected by climatic variability over the northern basins on seasonal to longer time scales) prevailed also at times of sapropel deposition, although the overall ventilation state was much reduced. This concept is supported by: detailed multiple-species isotope records for three Aegean cores; the presence of abundant Globorotalia truncatulinoides within especially sapropels S7 and S8 in the western Levantine basin; observations of three rapid benthic repopulations within sapropel S6 in the deep western Levantine basin; a report of continuous benthic presence through sapropel S1 at intermediate-deep locations offshore Libya; and further supporting information from the literature. In the Aegean records, concomitant abundance of low-oxygen tolerant benthic foraminifera and presence of the more oxyphilic benthic foraminifer Uvigerina mediterranea, with surface-similar δ13C values, indicate repeated deep water re-oxygenation events throughout the deposition of S1. The observations of a continuous benthic presence through S1 (offshore from Libya) imply that no persistent anoxia developed at mid-depth levels in that region, which is far removed from direct deep ventilation influences. The abundance of deep mesopelagic G. truncatulinoides through several sapropels from the western Levantine basin also suggests the presence of bio-available oxygen at many hundreds of meters of depth. Moreover, the rapid/intermittent benthic repopulations within sapropels from the deep eastern Mediterranean imply that bottom water anoxia was spatially restricted and/or of a highly intermittent nature. The short time scales of these repopulation events are incompatible with titration of an extensively anoxic water column and subsequent re-establishment of water-column anoxia. We suggest that where anoxic/azoic conditions were present, they most likely were restricted to a veneer at the sediment/water interface. The extent of such an anoxic ‘blanket’ depends on the balance between advective oxygen supply into the deep sea, and biological and chemical oxygen demand. The demand functions imply a decoupling of oxygenation from water mass advection, allowing export production and Corg posting rates to the sea floor to delimit the extent of the anoxic blanket in both space and time. Low-productivity regions would develop no anoxic blanket, allowing for the observed persistence of deep dwelling planktonic and bottom dwelling benthic faunas.

BENTHIC FORAMINIFERAL RECOVERY AFTER RECENT TURBIDITE DEPOSITION IN CAP BRETON CANYON, BAY OF BISCAY

The vertical distribution of living (rose Bengal stained) benthic foraminifera from three multiple cores from Cap Breton Canyon, Bay of Biscay, France, has been investigated with the objective of monitoring the recolonization and subsequent evolution of the foraminiferal fauna following the deposition of a turbidite layer. The first samples, taken in May 2000 in the axis of Cap Breton Canyon, contained a young turbiditic sequence, most likely deposited during the heavy storm of December 1999. Four months after this sedimentological event, the composition of the living benthic foraminiferal fauna was almost monospecific in the >150 μm fraction, which contained mainly Technitella melo, a species which is otherwise very rare or absent in the Cap Breton Canyon and open slope assemblages. This species was accompanied in the 63–150 μm fraction by adult specimens of Cassidulina carinata and Fursenkoina bradyi, and exclusively juvenile specimens of Bolivina subaenariensis and Bulimina marginata. This fauna represents the first stage of foraminiferal colonization after the turbidite deposition. The samples taken one year later, in June and September 2001, at approximately the same location, contained a more variable foraminiferal assemblage strongly dominated by Bolivina subaenariensis. Foraminiferal assemblages in samples taken just below the successive turbidite sequences contained nearly the same faunal elements as the surface assemblages sampled in 2001. The benthic foraminiferal assemblages from the canyon axis sampled in 2001 show the same composition as other canyon axis faunas dominated by B. subaenariensis. We suggest that the recovery of the foraminiferal faunas in this extremely unstable environment takes about 6–9 months, and that the community structure more or less permanently stays in an early stage of ecosystem recolonization.

Recent turbidite deposition in the eastern Atlantic: Early diagenesis and biotic recovery

An interface core taken in Capbreton canyon shows a succession of sedimentary facies interpreted as classical Bouma turbiditic sequences. Activities of 234Th and 210Pb suggest that the deposition of the most recent turbidite was triggered by the violent storm that affected the Atlantic coast of southern France on the 27th of December 1999, about four months before the sampling of the core. This turbidite allows us to study the ongoing diagenesis of the new sediment layer and of the previous sediment-water interface, which has been buried and only slightly eroded. A study of benthic foraminiferal populations informs us about the rate of benthic ecosystem recovery after such a major ecosystem disturbance event. The composition of the benthic foraminiferal fauna suggests that the benthic ecosystem in Capbreton canyon remains in an early stage of colonization. The rare agglutinant taxon Technitella melo appears to be the first colonizing species. It is suggested that Technitella melo is advantaged by the food-impoverished conditions in the days following turbidite deposition. Almost all of the turbidite layer and the previous oxic sediment-water interface contain reduced dissolved metal species and were anoxic. The buried interface contains Fe- and Mn-oxides inherited from its recent oxic past. The reduction of manganese oxides was in progress at the time of core collection. The reduced Mn remained trapped in the sediment as Mn-containing carbonates. Iron-oxides did not undergo significant reductive dissolution. The top of the newly deposited turbidite formed an oxic layer, which was rapidly enriched in metal-oxides. The enrichment of manganese oxides was mostly due to the oxidation of dissolved Mn2+, which diffused from below. The enrichment of iron oxides is explained both by the oxidation of the upward flux of dissolved Fe2+, and by the input of detrital iron oxide after, or as a result of the turbidite deposition.