Fluvial record of Late Pleistocene and Holocene geomorphic change in Northern Tunisia: global, regional or local climatic causes?

13 Science Highlights PAGES NEWS, VOL.13, N°1, APR. 2005 Fluvial Record of Late Pleistocene and Holocene Geomorphic Change in Northern Tunisia – Global, Regional or Local Climatic Causes? D. FAUST1, C. ZIELHOFER1, F. DIAZ DEL OLMO2 AND R. B. ESCUDERO2 1Dept. of Physical Geography, Dresden University of Technology, 01062 Dresden, Germany; dominik.f[email protected]u-dresden.de 2Dept. of Physical Geography, University of Seville, 41004 Seville, Spain; [email protected] Geomorphical-sedimentological approaches with high-resolution stratigraphic records round out Quaternary paleoclimatic and paleo-process research. Our investigations focus on Late Pleistocene and Holocene fl oodplain evolution. The central Medjerda Valley, Northern Tunisia (Fig. 1) is a tectonic depression zone that has its origin in the orogenesis of the Atlas Mountains. An up to 10 kmwide fl oodplain has been incised into it by the meandering Medjerda River. The fl oodplain is fi lled with Quaternary sediments, the latest of which are of Late Pleistocene and Holocene age. The fl oodplain sedimentation allowed the development of predominantly horizontal, well-stratifi ed layer sequences, interrupted by soil-forming processes (Fig. 1). Methodology From 1999 to 2002, fi eldwork led to the selection and sampling of more than 20 representative key profi les. Field observations and pedological analyses (granulometry, organic matter, CaCO3 content of clay and silt fraction, heavy minerals, thin section analyses) allow us to distinguish fi ve main stratigraphical series. About 60 AMS radiocarbon samples establish a late Quaternary chronology. In several cases, charcoal pieces are remnants of fi resites and cultural layers, so we assume we have dated in situ material. In our investigation, magnetic intensity [mA m-1] and magnetic susceptibility [Ko = 10-6 SI units], as well as inclination and declination [°], were recorded. Because of signifi cant similarity with secular variation curves from Sicily (Tanguy et al., 2003), our declination and inclination values give major chronological information about fl oodplain evolution. On the basis of chronological fi ndings (paleomagnetics, 14C data, artifacts) and the corresponding sediment thickness of the layers, we can reconstruct a curve for average sedimentation rate. In doing so, the sedimentary conditions between two chronologically well-known marks are regarded as homogeneous. Records of sedimentation rates give detailed information about late Pleistocene and Holocene alluvial history. Results Detailed morphostratigraphical fieldwork, paleomagnetics and radiocarbon dates provide a centennial-scale record of Northern Tunisian fl uvial sequences (Fig. 2). Alternating sediment texture, sedimentation rates and soil formation within Medjerda overbank deposits indicate short-term fl uctuations in late Pleistocene and Holocene fl uvial dynamics. Mediterranean river systems show a direct reaction to climatic shifts even of short duration. Stratigraphically, the base of the synthetic profi le is composed of a well-developed, rubefi ed soil. This soil is formed in a fl oodplain sediment in which interbedded mollusces give a radiocarbon age of approx. 42.8 ky (uncal). The red soil is covered by fi ne sedimentation (silty-clayey) at about 13.5 ky cal BP. This sedimentation continues until 12.6 ky cal BP, under climatic conditions similar to modern times (macroremnants of a dated fi resite). According to our datings, the Younger Dryas begins at 12.4 ky cal BP and ends about 11.8 ky cal BP. During the Younger Dryas, coarse sediments and gravels were deposited. The onset of the Holocene (11.8 ky cal BP) is marked by fi ne sedimentation lasting until 6.6 ky cal BP. This is a long period of geomorphic stability, even if sedimentation seems to accelerate at about 8.2 ky cal BP, indicated by the paleomagnetic records of some profi les. At 6.6 ky cal BP, the stable period is interrupted by sedimentation of coarse material. The period that follows is characterized by fi ne sedimentation in most parts of the fl oodplain in which a distinct soil is formed (Mid-Holocene Stability). Around 4.8 ka cal BP, poorly sorted sediments cover the mid- Fig. 1: Area of investigation; excellent exposure conditions allowed detailed sampling. 14 Science Highlights PAGES NEWS, VOL.13, N°1, APR. 2005 Holocene soil. This indicates an enhancement of morphodynamic processes. Sediments then fineup until Punic times. After 3.0 ka cal BP, increased morphodynamic processes take place for a short time (dating of two firesites in one exposure). This aggradation of sandy deposits ends about 2.7 ka cal BP. About 2.0 ka cal BP, morphodynamic stability and soil formation may be observed, although the timing of the beginning of soil formation is still unknown. At 1.7 ka cal BP, an accentuated restart of fl uvial dynamics takes place. At this time, the morphodynamically stable Punic-Roman period ends and most of the soils are buried under coarse sediments (Post Roman Crisis). Certain sediment strata with ages between 1.2 and 0.8 ka cal BP show humic enrichment due to soil formation. Subsequent minor fl ooding yields only fi ne sediments. The top of this sediment-sequence also shows a slight humifi cation. At 0.4 ka cal BP (dating of a fi resite), devastating fl oods occur in the entire Medjerda Basin, leaving thick laminated sediments, referred to as the Youngest Layer. Conclusions The Medjerda river system within the semi-arid to sub-humid subtropics of Mediterranean North Tunisia was sensitive to paleoclimatic shifts. Dominant climatic infl uences were not due to changes in temperature but to shifts in humidity. According to Rohdenburg (1983), geomorphic activity in the subtropics responds to aridifi cation of the climate. The centennial-scale synthetic profi le indicates that: a) Changes in fluvial dynamics during the late Pleistocene to early Holocene can be assigned to global climatic shifts. Climate changes due to shifts in the North Atlantic deep water formation also seem to be refl ected in the central Mediterranean region. However, the paleogeomorphological response of the Younger Dryas appears to be signifi cantly shorter. b) In the course of the early to mid-Holocene, major shifts in the Medjerda fl uvial dynamics correlate with changes in Saharan climate. Brief Saharan aridifi cation around 5 ka corresponds with the 4.7 ka collapse within the Medjerda River system. In contrast, the worldwide documented 8.2 ka EMTH event cannot clearly be deduced from the Medjerda overbank deposits. c) Medjerda overbank sediments document increased fl uvial activity around 6.6 to 6.0 ka. The 6.6 ka event is not well described in other paleoclimatic records. This indicates the possibility of major changes in aridifi cation, even at a regional to local scale. d) The comparison of mid- to late Holocene sedimentation rates of the Medjerda with the Haematite curve of Bond et al. (2001) reveals a parallel progression of late Holocene North Atlantic coolings and increased alluviation in Mediterranean Tunisia. Short-term periods of geomorphic activity in Northern Tunisia match well with North Atlantic Bond events. The fi ndings indicate a climatic link from the 4.7 ka collapse until today. During the last 2,000 years, climatic optima caused landscape stability regardless of the land-use intensity (Faust et al. 2004). Our fi ndings provide evidence that late Holocene fl uvial dynamics in Northern Tunisia were chiefl y driven by climate. Anthropogenic impact at most intensifi ed or attenuated the geomorphic processes. ACKNOWLEDGEMENTS We would like to thank the Deutsche Forschungsgemeinschaft (DFG) for supporting this project (FA 239/2-1 and FA 239/2-2). REFERENCES BOND, G.C., KROMER, B., BEER, J., MUSCHELER, R., EVANS, M.N., SHOWERS, W., HOFFMANN, S., LOTTI-BOND, R., HAIDAS I. AND BONANI, G., 2001: Persistent solar infl uence on north Atlantic climate during the Holocene. Science, 294: 2130-2136. FAUST, D., ZIELHOFER, C., BAENA, R. AND DIAZ DEL OLMO, F., 2004: High resolution fl uvial record of late Holocene geomorphic change in northern Tunisia: climatic or human impact? Quaternary Science Reviews, 23(16-17): 1757-1775. ROHDENBURG, H., 1983: Beiträge zur allgemeinen Geomorphologie der Tropen und Subtropen. Geomorphodynamik und Vegetation – Klimazyklische Sedimentation – Panplan/Pediplain-Pediment-Ter- rassentreppen. Catena, 10: 393-438. TANGUY, J.-C., LE GOFF, M., PRINCIPE, C., ARRIGHI, S., CHILLEMI, V., PAIOTTI, A., LA DELFA, S. AND PATANÈ, G., 2003: Archeomagnetic dating of the Mediterranean volcanics of the last 2100 years: validity and limits. Earth and Planetary Science Letters, 211: 111-124. Fig. 2: Synthetic profi le of the Medjerda Valley, Northern Tunisia. Chronostratigraphy is based on the analyses of 20 key profi les.