Landscape analysis and stratigraphical and geochemical investigations of playa and alluvial fan sediments in Tunisia...

Full title: "Landscape analysis and stratigraphical and geochemical investigations of playa and alluvial fan sediments in Tunisia and raised bog deposits in Sweden - a possible correlation between extreme climate events and cosmic activity during the late Holocene"

Abstract of poster presentation by Lars G. Franzén(1) and Thomas B. Larsson(2)

  1. Physical Geography, Department of Earth Sciences, University of Göteborg, SE-413 81 Göteborg, Sweden, E-mail: lars[at]gvc.gu.se
  2. Department of Archeology, University of Umea, SE-901 87 Umea, Sweden
Presented at the SIS Conference: Natural Catastrophes during Bronze Age Civilisations (11th-13th July 1997)

Geomorphological signatures, and plays and alluvial fan deposits, all point at a severe flooding catastrophe in central Tunisia c. 1000 B.C. Precipitation must have exceeded 10,000 mm in a very short period. Human activity during this period concentrated around wells, with an increased need of heating as indicated by the very frequent occurrence of unusually large fireplaces with heating stones imbedded in charcoal (Larsson and Franzen, in press). The sediments deposited indicate severe ground frost in an initial phase, i.e. large balls of loose dune sand, imbedded in the lower part of the stratigraphies, objects which could only have been transported in a frozen form. In spite of being found in a 5 m thick sediment sequence the fire places have the same apparent age according to 14 C-datings, indicating an extreme rate of sedimentation. The playa investigated, Chott Nejla, is normally dried out However, after heavy floods, it is a fresh water lake, with a maximum depth of 15 m, if filled up to the thresholds. At the natural playa outlet during pluvial periods, from an archaeological point of view, a very rich site was found with signs of a perhaps continuous settlement from 9000 B.C. to Roman times, uncommon for the Capisien culture (Balout, 1955; Larsson and Franzen, in press). The sediment sequence at the playa centre shows that the most abrupt transition from aridity to very humid conditions at 1000 B.C., was followed by a very long lake stage. The lower lake stage sediments of this event contain glassy spherules (5-100 gm dia) with a very varying composition, from pure Fe-types to types with the same basic composition as those found in the K/T-transition (e.g. Koeberl and Sigurdsson, 1992). Other outstanding wet periods found in the Tunisian material are 900-1000 A.D. and 1600-1700 A.D. In southern Sweden, several raised bog deposits have been investigated for their peat ash composition. Raised bogs, which receive all nutrients from the atmosphere, is an ideal archive of atmospheric fallout, the rate of peat accumulation being rather constant and the peat being easily dated with 14 C. The investigation indicate strongly increased atmospheric circulation in rhythmically appearing periods with high peaks centred around e.g. c.2200 B.C., 1000 B.C., 400 B.C., 200 A.D., 1000 A.D. and 1600 A.D. Harmonic analysis points at a periodicity of 570 years or 1140 years. Like in the playa deposits glassy spherules were found, having the same basic composition as the Tunisian material. In conclusion, both playa and alluvial fan deposits in Tunisia, and peat in Sweden points at a very turbulent Holocene with several sudden and dramatic climate event;, with unknown origin and extent The cultural traces indicate periods of extremely cold and humid conditions. The spherules found could be of volcanic origin but they are not associated with any known major volcanism, and their heterogeneous composition points at other formation mechanisms e.g. cometary impacts in ocean shelf sediments.

References: Balout, L. 1955. La Prehistoire du l'Afrique du Nord. Paris. Koeberl, C. and Sigurdsson, H. 1992. Geochemistry of impact glasses from the K/T boundary in Haiti: relation to smectites and a new type of glass. Geochim. Cosmochim Acta 56(5):2113-2129. Larsson, T.B. and Franzén, L.G. (In press). The Capsien Culture at Ez Zaf Zaf, Chott Nejla, Central Tunisia. Preliminary Report, Univ. of Umeå, Univ. of Göteborg, Sweden. Larsson, T.B. and Franzén, L.G. (In press). The anomalous dwelling sites of Oed Sequia, Central Tunisia. A comparison between fire cracked stones in Tunisia and Northem Europe. Preliminary Report, Univ. of Umeå, Univ. of Göteborg, Sweden.


LARS FRANZEN (Ph.D., Gothenburg University, 1985) is Associate Professor of Physical Geography of the department of Earth Sciences at Göteborg University, Sweden. His research interests include the Holocene climate development as detected in stratigraphies of raised bogs in Sweden and playa deposits of Northern Africa. His studies has lead to the discovery of several abrupt climate shifts in Post-glacial times, the most pregnant of these being centered around 1000 B.C. Cultural and other evidence point at prolonged periods of severe cold and vast floodings associated with some these events.