High frequency climate variability and deep-water ventilation in the Eastern Mediterranean at times of sapropel formation: A dynamic concept.
Abstract
We propose that the recently reported sapropel ‘interruptions’ represent centennial-scale episodes of enhanced frequency/ intensity of intermittent bottom-water ventilation and that this ventilation process continued to occur throughout periods of sapropel deposition. In essence, the modern high-frequency variability in deep-water formation (annual to decadal in frequency), 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 is derived from: detailed multiple-species isotope records for 3 Aegean cores; the presence of abundant Globorotalia truncatulinoides within especially S7 and S8 in the Western Levantine basin; observations of three rapid benthic repopulations within S6 in the deep Western Levantine basin; a report of continuous benthic presence through S1 at intermediate-deep locations offshore Israel; 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 d13C values, indicate repeated bottom water re-oxygenation events throughout the deposition of S1. This appears to occur at a frequency greater than our sample resolution or at least once every 50-75 years. In addition, the observations of benthic continuation through S1 (offshore from Israel) 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: their short time scales are incompatible with titration of an extensively anoxic water column and subsequent reestablishment 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.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....4169C