SESS 5_16022011_edi as.pub Il Quaternario Italian Journal of Quaternary Sciences 24, (Abstract AIQUA, Roma 02/2011), Congresso AIQUA Il Quaternario Italiano: conoscenze e prospettive Roma 24 e 25 febbraio 2011 AN EARLY HOLOCENE TRANSITORY ARID PHASE IN THE CENTRAL SAHARA AT THE TIME OF THE 8.2 KY BP CLIMATE ANOMALY Andrea Zerboni Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano Corresponding author: A. Zerboni ABSTRACT: Zerboni A., An early Holocene transitory arid phase in the central Sahara at the time of the 8.2 Ky BP climate anomaly. (IT ISSN 0349-3356, 2011) The beginning of the Holocene in the Sahara is marked by increased rainfall due to the expansion of the African monsoon over the North Africa. In the central Sahara springs were activated in the mountains and shallow lakes formed between dunes. An isotopic and geochronological approach to the study of spring and lake carbonates allowed defining the beginning of the Holocene humid period; moreover, data show its abrupt interruption at c. 8 Ky BP. The arid spell is attributed to the 8.2 Ky BP event and its significance is discussed in the frame of Holocene climate changes recorded in North Africa and in the Mediterranean. RIASSUNTO: Zerboni A., Una breve fase arida registrata nel Sahara centrale in corrispondenza dell’anomalia climatica di 8.2 Ky BP. (IT ISSN 0349-3356, 2011) L’inizio dell’Olocene nel Sahara è marcato da un incremento delle precipitazioni causato dall’espansione dell’area di influenza monsonica sul Nord Africa. Nel Sahara centrale furono attivate sorgenti nelle aree montane e si formarono laghi tra le dune. Lo studio isotopico e geocronologico di carbonati lacustri e di sorgente ha permesso di definire l’inizio della fase umida olocenica e di identificare un evento arido a 8 Ky BP. Tale evento è posto in relazione con l’anomalia climatica di 8.2 Ky BP e il suo significato viene discusso nell’ambito delle variazioni climatiche registrate in Nord Africa e nel bacino del Mediterraneo. Key words: climate changes, palaeohydrological records, 8.2 event, African monsoon, Sahara. Parole chiave: cambiamenti climatici, archivi paleoidrologici, evento 8.2, monsone africano, Sahara. 1. INTRODUCTION The palaeoenvironmental research carried out in the hyperarid central Sahara (SW Libya) permitted to elucidate the time and steps of the Holocene climatic changes occurred in the region (CREMASCHI & ZERBONI, 2009). The isotopic and geochronological study of carbonatic sediments related to highly sensitive palaeohydrological archives (CREMASCHI et al., 2010) allowed to date the beginning of humid phase and evidenced the role played by the 8.2 Ky BP climate anomaly. 2. METHODS Spring and lake sediments were discovered and sampled during a geoarchaeological survey. Spring tufa were dated using multicollector ICP-MS techniques (details in FLEITMANN et al., 2007). Organic lake sediments were radiocarbon dated and ages calibrated (2σ) using the INTCAL04 calibration curve (REIMER et al., 2004). The stable C- and O-isotope values were measured using a ThermoFinngan DeltaplusXl isotope ratio mass spectrometer and results are reported in ‰, relative to the Vienna PeeDee Belemnite (VPDB).  3. RESULTS AND DISCUSSION Tufa formation typically required abundant water and a continuous soil cover in order to account for CO2 input into the groundwater system. According to U-Th data such conditions lasted from 9.6 to 7.6 Ky BP (Fig. 1a). Stable isotopes recorded a trend towards wetter conditions between 9.6 and 8.8 Ky BP, whereas, after a hiatus, less negative δ18O values suggest a reduction in precipitation (Fig. 1b). A major hydrological change is also evident from the δ13C versus δ18O cross plot, differentiated on the basis of U/Th ages: tufa deposited before 9 Ky BP shows δ18O values ranging from -12 to -9‰, while younger tufa shows values ranging from -9 to -2‰ (Fig. 1c), suggesting higher rain supply to springs and soil metabolism at the beginning of the period, followed by a progressive decline (CREMASCHI et al., 2010). Conditions suitable for tufa formation were substantially reduced c. 8 Ky BP, whereas the occasional persistence of local tufa deposition until c. 7.6 Ky BP may be related to the resilience of water saturation in parts of the mountain. Carbonatic lake deposits discovered in the sand seas indicates former, large groundwater-fed water bodies, which origin was directly linked to the 225 - 227 226 amount of rainfall feeding confined, surface aquifers. More than 70 samples of organic lake sediment have been dated, reflecting Holocene age. Groundwater seepage is recorded from 8.9 to 1.6 uncal. Ky BP, but most of the results are concentrated in the interval 10.5–5.5 cal. Ky BP. Limnic activity is well dated in the edeyen of Murzuq, due to the existence of deep organic to carbonate-rich lacustrine sections; two distinct water high stands were identified, interrupted by an arid phase between c. 8.2 and 7.8 cal. Ky BP. Hydrological changes recorded in the central Sahara agree well with records of climate changes in the surrounding region (Fig. 2); for instance, a gap in sedimentation and erosion under arid conditions are registered by anthropogenic cave deposits from the Tadrart Acacus massif (CREMASCHI et al., 2010). The attribution of the dry phase recognized in the study area to the 8.2 event is consistent with a palaeoclimatic model proposed by WIERSMA & RENSSEN (2006). An anomaly at 8.2 Ky BP was identified in many proxy records from the African monsoon region (MAYEWSKI et al., 2004). A drop in precipitation was recorded by summer-monsoon-fed lakes between 8.5 and 7.8 cal. Ky BP (Fig. 2) and was reported from several continental records from the circum- Mediterranean region (BERGER & GUILAINE, 2009). Finally, the arid spell was largely coeval with the relapse within sapropel S1 in the Mediterranean Sea, supporting the hypothesis concerning a strong correlation between weakening in the African monsoon intensity and interruption of anoxic events in the Eastern Mediterranean (ROHLING et al., 2002). Fig. 1, a) Duration of tufa deposition in each locality. (b) O-isotope profile of sample 02/99. (c) δ13C versus δ18O data differentiated on the base of U/Th ages (from CREMASCHI et al., 2010). (a) Durata della deposizione di calcareous tufa in ogni località. (b) Curva isotopica del campione 02/99. (c) Dati isotopici (δ13C versus δ18O) differenziati sulla base delle datazioni U/Th (da CREMASCHI et al., 2010). A. Zerboni 227 Ms. received: Testo ricevuto il 4.CONCLUSION Deposition of carbonates in today’s hyperarid central Sahara records a prolonged period of wet climate which prevailed during the early Holocene in response to a northward expansion of the SW African Monsoon system, controlled by the insolation maximum. The available data suggest a gap in sedimentation between 8.2 and 7.8 Ky BP. Probably triggered by the abrupt drainage of Laurentide lakes and subsequent slowdown of the North Atlantic thermohaline circulation (THOMAS et al., 2007), this event led to a temperature reduction in the Guinea Gulf and a strong decrease in evaporation. As a consequence, the strength of the African monsoon decreased, with the sudden shutdown of water supply to the central Sahara and reduction of runoff to the Mediterranean Sea. REFERENCES BERGER J.-F. & GUILAINE G. (2009) - The 8200 cal Bp abrupt environmental change and the Neolithic transition: A Mediterranean perspective. Quaternary International, 200, 31-49. CREMASCHI M. & ZERBONI A. (2009) - Early to middle Holocene landscape exploitation in a drying environment: two case compared from the central Sahara (SW Fezzan, Libya). Comptes Rendus Geoscience, 341, 689-702. CREMASCHI M., ZERBONI A., SPÖTL C. & FELLETTI F. (2010) - The calcareous tufa in the Tadrart Acacus Mt. (SW Fezzan, Libya). An early Holocene paleoclimate archive in the central Sahara. Palaeogeography, Palaeoclimatology, Palaeoecology, 287, 81-94. FLEITMANN D., BURNS S.J., MANGINI A., MUDELSEE M., KRAMERS J., VILLA I., NEFF U., AL-SUBBARYE A., BUETTNER A., HIPPLER D. & MATTER A. (2007) - Holocene ITCZ and Indian monsoon dynamics recorded in stalagmites from Oman and Yemen (Socotra). Quaternary Science Reviews, 26, 170-188. MAYEWSKI P.A., ROHLING E., STAGER J.C., KARLEN W.K., MAASCH K.A., MEEKER L.D., MEYERSON E.A., GASSE F., VAN KREVELD S., HOLMGREN K., LEE-THORP J., ROSQVIST G., RACK F., STAUBWASSER M., SCHNEIDER R. & STEIG E.J. (2004) - Holocene climate variability. Quaternary Research, 62, 243-255. REIMER P.J., BAILLIE M.G.L., BARD E., BAYLISS A., BECK J.W., BERTRAND C.J.H., BLACKWELL P.G., BUCK C.E., BURR G.S., CUTLER K.B., DAMON P.E., EDWARDS R.L., FAIRBANKS R.G., FRIEDRICH M., GUILDERSON T.P., HOGG A.G., HUGHEN K.A., KROMER B., MCCORMAC G., MANNING S., BRONK RAMSEY C., REIMER R.W., REMMELE S., SOUTHON J.R., STUIVER M., TALAMO S., TAYLOR F.W., VAN DER PLICHT J. & WEYHENMEYER C.E. (2004) - IntCal04 terrestrial radiocarbon age calibration, 0–26 cal Ky BP. Radiocarbon, 46, 1029- 1058. ROHLING E.J., CANE T.R., COOKE S., SPROVIERI M., BOULOUBASSI I., EMEIS K.C., SCIEBEL R., KROON D., JORISSEN F.J., LORRE A & KEMP A.E.S. (2002) - African monsoon variability during the previous interglacial maximum. Earth and Planetary Science Letters, 202, 61-75. THOMAS E.R., WOLFF E.W., MULVANEY R., STEFFENSEN J.P., JOHNSEN S.J., ARROWSMITH C., WHITE J.W.C. VAUGHN B. & POPP T. (2007) - The 8.2 ka event from Greenland ice cores. Quaternary Science Reviews, 26, 70-81. WIERSMA A.P & RENSSEN H. (2006) - Model–data comparison for the 8.2 ka BP event: confirmation of a forcing mechanism by catastrophic drainage of Laurentide Lakes. Quaternary Science Reviews, 25, 63-88. An early Holocene transitory arid phase in the ... January 14, 2011 14 gennaio 2011 Fig. 2, Palaeohydrological records from the central Sahara compared with other archives from North Africa and the Mediterranean region; the grey bar indicates the 8.2 event (THOMAS et al., 2007). 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