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

A dynamic concept for eastern Mediterranean circulation and oxygenation during sapropel formation
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.
MEDITERRANEAN, SAPROPEL, OCEAN CIRCULATION, BOTTOM WATER, VENTILATION, GEOLOGY
0031-0182
103-119
Casford, J.S.L.
ab6215ef-d18d-4b90-bb9c-31468744826e
Rohling, E.J.
a2a27ef2-fcce-4c71-907b-e692b5ecc685
Abu-Zied, R.H.
8883b0d9-b7ca-4635-90d2-cc3da2f36a7f
Fontanier, C.
8cd62729-bb6f-4b48-a244-6d9ec43a46dd
Jorissen, F.J.
5e9da85b-8b77-465e-8816-0ed4acf2da63
Leng, M.J.
d3b48578-3020-44f7-a088-827aef7502e7
Schmiedl, G.
e0ad1f9e-6f3e-46df-81cf-15abc837c2ac
Thomson, J.
3395054f-e507-4841-9758-a06ed37f7d6b
Casford, J.S.L.
ab6215ef-d18d-4b90-bb9c-31468744826e
Rohling, E.J.
a2a27ef2-fcce-4c71-907b-e692b5ecc685
Abu-Zied, R.H.
8883b0d9-b7ca-4635-90d2-cc3da2f36a7f
Fontanier, C.
8cd62729-bb6f-4b48-a244-6d9ec43a46dd
Jorissen, F.J.
5e9da85b-8b77-465e-8816-0ed4acf2da63
Leng, M.J.
d3b48578-3020-44f7-a088-827aef7502e7
Schmiedl, G.
e0ad1f9e-6f3e-46df-81cf-15abc837c2ac
Thomson, J.
3395054f-e507-4841-9758-a06ed37f7d6b

Casford, J.S.L., Rohling, E.J., Abu-Zied, R.H., Fontanier, C., Jorissen, F.J., Leng, M.J., Schmiedl, G. and Thomson, J. (2003) A dynamic concept for eastern Mediterranean circulation and oxygenation during sapropel formation. Palaeogeography, Palaeoclimatology, Palaeoecology, 190, 103-119. (doi:10.1016/S0031-0182(02)00601-6).

Record type: Article

Abstract

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.

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Published date: 2003
Keywords: MEDITERRANEAN, SAPROPEL, OCEAN CIRCULATION, BOTTOM WATER, VENTILATION, GEOLOGY

Identifiers

Local EPrints ID: 2021
URI: http://eprints.soton.ac.uk/id/eprint/2021
ISSN: 0031-0182
PURE UUID: 536c28d4-0af3-4d6b-bef5-77cd8254e535
ORCID for E.J. Rohling: ORCID iD orcid.org/0000-0001-5349-2158

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Date deposited: 07 May 2004
Last modified: 16 Mar 2024 02:46

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Contributors

Author: J.S.L. Casford
Author: E.J. Rohling ORCID iD
Author: R.H. Abu-Zied
Author: C. Fontanier
Author: F.J. Jorissen
Author: M.J. Leng
Author: G. Schmiedl
Author: J. Thomson

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