The calcareous tufa in the Tadrart Acacus Mt. (SW Fezzan, Libya) An early Holocene palaeoclimate archive in the central Sahara Mauro Cremaschi a , Andrea Zerboni a, ⁎, Christoph Spötl b , Fabrizio Felletti a a Dipartimento di Scienze delle Terra “A. Desio”, Università degli Studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy b Institut für Geologie und Paläontologie, Leopold-Franzens-Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria abstract article info Article history: Received 4 October 2008 Received in revised form 24 July 2009 Accepted 12 January 2010 Available online 21 January 2010 Keywords: Calcareous tufa U/Th dating Climate change African monsoon Early Holocene Central Sahara The palaeoclimatic significance of calcareous tufa deposits found in the presently hyperarid Tadrart Acacus massif (central Sahara, SW Libya) is discussed on the basis of U/Th datings, stable isotope analyses, and the regional hydrogeological context. These deposits formed mostly between c. 9600 and 8100 yr BP. The isotopic signature (δ 18 O and δ 13 C) of this tufa indicates that the water forming these carbonates originated as isotopically depleted rainwater as a result of the northward expansion of the SW African Monsoon system during the early Holocene. Carbonate sedimentation in the Tadrart Acacus decreased shortly before c. 8000 yr BP, attributed to a drop in precipitation in the course of the well known 8.2 kyr event. A correlation between tufa development and climate change in N and E Africa and in the Mediterranean basin during the early Holocene is discussed. The effect of early Holocene climate change on both the landscape and the dwellers living in the Tadrart Acacus region is also evaluated. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Thin and discontinuous calcareous tufa deposits occur in shallow caves and rockshelters of the central Tadrart Acacus mountains (SW Fezzan, central Sahara — Fig. 1), connected to fossil springs (Carrara et al., 1998; Zerboni, 2006). At present the tufa deposits are inactive and the area in which they are found is hyperarid: mean annual temperature is 30 °C and mean annual rainfall is between 0 and 20 mm (Fantoli, 1937; Walther and Lieth, 1960). The occurrence of fossil tufa clearly indicates a higher water availability in the recent geological past. Furthermore, freshwater carbonates are unexpected as the bedrock in this area is characterized by carbonate-free sandstones (Kalefa El-Gahali, 2005). The calcareous tufa deposits of the Tadrart Acacus were initially dated by U-series methods (alpha-spectrometry) from the late Pleistocene to the beginning of the Holocene (Carrara et al., 1998), but this chronological attribution has raised some criticism (Kröpelin, 1999; Wendorf and Schild, 2003; Wendorf et al., 2007). The discovery of new outcrops provides the opportunity to re- examine the significance of these tufa deposits using state-of-the-art U/Th dating techniques and stable isotope analyses (Zerboni, 2006). The goal of this paper is to present these new data and to reassess the palaeoclimatic implications in the framework of the early Holocene hydrology of the Tadrart Acacus. The results underscore the out- standing value of carbonate tufa as a palaeoenvironmental archive (Henning et al., 1983; Turi, 1986; Spötl et al., 2002; Garnett et al., 2004; Pentecost, 2005; Andrews, 2006), particularly in continental arid and semi-arid regions (Dramis et al., 2003; Bar-Matthews and Ayalon, 2004; Smith et al., 2004; Moeyersons et al., 2006; O'Brien et al., 2006). 2. Geological and hydrogeological background The Tadrart Acacus Mountains (Fig. 2) covers an area of 4800 km 2 with a maximum elevation of 1100 m a.s.l. in the western part (Desio, 1936, 1937; Bartolomei, 1966). As a whole, the mountain system consists of a monocline gently tilted toward the E–NE and forming a cuesta. The monocline is composed of Palaeozoic sedimentary rocks (Desio, 1936, 1937; Bartolomei, 1966; Goudarzi, 1971; Marcolongo, 1987; Kalefa El-Gahali, 2005) dissected by a fossil drainage network (Cremaschi, 1998), whose NE–SW, E–W, and N–S trending patterns are controlled by the tectonic structure (Galeĉiĉ, 1984; Jakovljeciĉ; 1984; Klett, 2000; Klitzsch, 2000). A sharp scarp delimits the massif towards the W, while it progressively passes through a pediment to the dunefield of the Erg Uan Kasa to the E. The gentle eastward dip (2–10°) is interrupted by many normal faults and fractures tens of kilometres in length mostly trending SW–NE, NS, and, less frequently, NW–SE, that dissect the sedimentary succession into numerous up- lifted fault blocks (Fig. 2). Displacement along these faults is variable but small. Most of the fault systems dip steeply eastward, showing vertical displacements ranging from a few up to 30 m (Galeĉiĉ, 1984; Jakovljeciĉ; 1984). Palaeogeography, Palaeoclimatology, Palaeoecology 287 (2010) 81–94 ⁎ Corresponding author. Tel.: +39 02 50315523; fax: +39 02 50315494. E-mail address: andrea.zerboni@unimi.it (A. Zerboni). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.01.019 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo