- Email: [email protected]
Age and significance of Holocene valley fills in the Skeleton Coast, Namibia
Abstracts / Quaternary International 404 (2016) 174e213
dust transport? Does Saharan dust transport in interaction with changing climate develop multiple equilibria and bifurcations? The dust source model by Stanelle et al. (submitted) is applied to set up stationary state simulations for the Holocene and today's conditions. Holocene surface characteristics, including vegetation cover and hydrological quantities such as lake area and soil moisture, are derived from proxy data and the dynamical wetlands hydrology scheme by Stacke (2012). Modelling results of the amount of emitted and deposited dust for present climate and Holocene climate are evaluated and compared with measured data. Keywords: Holocene, Saharan dust dynamics, abrupt climate change References: Brovkin, V., M. Claussen, V. Petoukhov and A. Ganopolski: On the stability of the atmosphere vegetation system in the Sahara/Sahel region, J. Geophys. Res., 31, 613e31,624, 1998. Claussen, M., C. Kubatzki, V. Brovkin, A. Ganopolski, P. Hoelzmann and H. J. Pachur: Simulation of an abrupt change in Saharan vegetation in the midHolocene, Geophys. Res. Lett., 26, 2037e2040,1999. deMenocal, P, J. Ortiz, t. Guilderson, J.Adkins, M. Sarnthein, L. Baker and M. Yarusinsky: Abrupt onset and termination of the African humid period: Rapid climate response to gradual insolation forcing, Quat. Sci. Rev.,19, 347e361, 2000. zine, H. Eggermont, C. Cocquyt, P. Kroepelin, S., D. Verschuren, A.-M. Le Francus, J.-P. Cazet, M. Fagot, B. Rumes, J. M. Russell, F. Darius, D. J. Conley, M. Schuster, H. von Suchodoletz, D. E. Engstrom: Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years, Science 320(5877), 765e768, 2008. Stacke, T., S. Hagemann: Development and evaluation of a global dynamical wetlands extent scheme, Hydrology and Earth System Sciences 16(8), 2915e2933, 2012. Stanelle,T., I. Bey: Anthropogenically induced changes in the 20th century mineral dust burden and the associated impact on radiative forcing, submitted, 2014.
PALAEO-ENVIRONMENTAL GULLIES
RECONSTRUCTION
OF
THE
DIEPDRIFT
M. Evans*, Z. Jinnah, S. Mhlongo. School of Geosciences, University of the Witwatersrand, South Africa. E-mail address: [email protected].
The Diepdrift erosional site is located about 5 km north of Bela Bela in Limpopo Province, South Africa. It is an interesting geological feature as it is the only locally situated gully of its size in the Bela Bela area. No surrounding areas show a large degree of gullying as is depicted by the Diepdrift site. It is located in the heart of the Bushveld Complex surrounded by granites of the Rooiberg Suite as well as forming the drainage and catchment basin of the Waterberg Group. It is lies unconformably above these characteristic red granites, and is situated between two large granite domes that flank the site to the east and the west. These domes form a radial drainage pattern, which converges in the Diepdrift site, slightly upstream and along the length of the gully. A river meanders through the gully itself, and is fed by smaller streams flowing towards the centre of the feature. The banks of the river channel seem to incise into the walls of the gully causing a basal undercutting appearance. The river flows down slope towards the south and collects in a dam further downstream in the adjacent farm. The sediments consists of various horizons of brownish-orange to grey, fine to coarse grained sandstones and palaeosols with a vertical piping / gullying pattern forming on the exposed faces of these stratigraphic units. The faces are exposed over varying depths. In some locations the exposed face is as small as 0.5 m, while in other areas, predominantly downstream, the faces are exposed up to 6 m in height. The gully seems to weather down to the previously mentioned granite in some locations. This granite forms the basal bedrock underlying the sediments.
* Corresponding author.
197
The aim of this research project is to produce a palaeoenvironmental reconstruction for the Diepdrift gullies and to derive an understanding of gullying in the area and the factors which contributed to its formation. Through optically stimulated luminescence dating a high resolution geochronology will be derived which will allow for the correlation of the stratigraphy to recognised climatic events that were recorded for the Quaternary. Initial results suggest that the Diepdrift structure is as a result of an alluvial fan collecting material from the underlying granite bedrock in an around the area and from other sediments further upstream. The fairly regular, continuous layering and lack of prominent channel deposits places the gully in the distal sections of the fan. Possible colluvial action is likely to influence the initial sediments from the granite domes on either side of the deposit. However, this does not directly affect the deposit itself. Due to the sodic nature of the soils the gullying in the site can be attributed to cation exchange causing degradation of the soils in an arid to semi-arid climatic environment. Keywords: gullies, optically stimulated luminescence dating, Quaternary environmental reconstruction
THE DEVELOPMENT OF THE PRESENT DAY DRAINAGE PATTERNS OF SOUTH-CENTRAL AFRICA T. Flügel 1, *, F. Eckardt 2, F.P.D. Cotterill 3. 1 Department of Military Geography, University of Stellenbosch, South Africa; 2 Department of Environmental and Geographical Sciences, University of Cape Town, South Africa; 3 AEON Geoecodynamics Research Hub, c/o Department of Botany and Zoology, University of Stellenbosch, South Africa. E-mail address: tyrelfl[email protected].
One of Africa's most distinctive morphological characteristics is the occurrence of the low lying Congo Basin (mean elevation of 400 m.a.s.l) adjacent to the elevated Kalahari Plateau (mean elevation of 1100 m.a.s.l). The transition zone between low and elevated Africa also demarcates the watershed between the Congo and Okavango-Zambezi drainage systems. This south-central African watershed, here termed the Congo-Kalahari watershed, is dominated by a smooth, seasonally wet tableland, flanked by the Angola highlands in the west and the Western branch of the East African Rift System (EARS) in the east. Thus the Congo-Kalahari incorporates three types of landforms: old, macro-landforms of the Kalahari Plateau and Congo Basin; newer, meso-landforms of EARS, horst and graben structures; and newer, process landforms as evidenced by headwater capture features and channel adjustements along the watershed. Therefore a better understanding of the Congo-Kalahari watershed may provide insights into the development of the mega-geomorphology of central and southern Africa. The Congo-Kalahari watershed separates the northerly flowing tributaries of the Congo River from the southerly flowing tributaries the Zambezi and Okavango Rivers. The present day drainage patterns of these river systems were investigated using remote sensing and a geospatial information system. The observed juxtaposition of river drainage patterns suggests a multi-stage evolution of these drainage systems with several controls acting on the development of these river systems. While much of southcentral Africa's drainage has been impacted on by events of the Neogene, significant changes have occurred during the Quaternary. It is during the Quaternary that the Congo and Okavango-Zambezi drainage systems attain their modern forms, although these system have and continue to be modified throughout the Quaternary. By combining geomorphic and geologic evidence with published data from several fields, including phylogeography, an overview of drainage system evolution may be discerned. Keywords: south-central Africa, Congo, Kalahari, GIS AGE AND SIGNIFICANCE OF HOLOCENE VALLEY FILLS IN THE SKELETON COAST, NAMIBIA D.S.G. Thomas*, J.A. Durcan. School of Geography and Environment, University of Oxford, UK. E-mail address: [email protected].
198
Abstracts / Quaternary International 404 (2016) 174e213
East-west orientated valleys with extremely ephemeral modern channel flow are a significant landscape feature of northwest Namibia. Today these valleys are important sources of aeolian dust, as each year plumes during Berg wind events fertilise Atlantic waters off the Skeleton Coast. Here we provide an OSL chronology for silts from the Huab valley, demonstrating a temporally complex Holocene sequence of valley-wide infilling (with early phases associated with the African humid period?) and subsequent late Holocene entrenchment during confined channel flows. Age data affirm fluvial histories from the Hoanib system (Eitel et al 2006), but additionally provide an association with modern aeolian dynamics and dust-sourcing that points to the vital role of wet e accumulation as sourcing the aeolian systems of today's hyper-arid regime. Keywords: Holocene, valley fills, luminescence dating, aeolian dust References: Eitel, B., Kadereit, A., Blumel, W.-D., Huser, K., Lomax, J., Hilgers, A., 2006. Environmental changes at the eastern Namib Desert margin before and after the Last Glacial Maximum: new evidence from fluvial deposits in the upper Hoanib River catchment, northwestern Namibia. Palaeogeography, Palaeoclimatology, Palaeoecology 234, 201e222.
8. DATING AND CORRELATION OF AFRICAN ARCHIVES OF ENVIRONMENTAL CHANGE AND ARCHAEOLOGY SPATIAL AND TEMPORAL VARIATION IN NORTH AFRICAN PALAEOENVIRONMENTAL AND ARCHAEOLOGICAL RECORDS DURING MARINE ISOTOPE STAGE 4 L. Farr*, S. Jones. The McDonald Institute for Archaeological Research, University of Cambridge, UK. E-mail address: [email protected].
Marine Isotope Stage 4 (MIS4) covers the timeframe of c.74e59 ka and is generally regarded as a period of sudden and rapid climatic deterioration in North Africa. MIS4 is argued to coincide with regional decreases in population size and density in North Africa, possibly concomitant with retraction into refugia in certain locales. Sitting awkwardly against this picture is recent mtDNA evidence from living human populations which suggests that, approximately c.60 ka, Homo sapiens groups exited Africa in a successful migration that resulted in colonisation of Asia, Australia and Europe. These dispersals of Homo sapiens are usually attributed to humid climatic phases in MIS5 and MIS3, when hydrological networks are argued to have been reactivated in response to pluvial events, resulting in the wetting of normally arid regions such as the Sahara. This paper reviews palaeoenvironmental and archaeological data from marine and terrestrial archives in the North African region, which are dated to the MIS4 timeframe. We evaluate spatial and temporal variation in the datasets in relation to global climate records and investigate their reliability to inform us of millennial and sub-millennial environmental change, and demographic processes in North Africa. In particular, we look at the chronometric dating of palaeoenvironmental and archaeological datasets and investigate issues regarding chronological precision, spatial resolution and geoarchaeological context. We present geoarchaeological and lithic data from part of the Haua Fteah cave sequence (Libya) which has associated MIS4 dates, and discuss how these findings fit into a wider picture of regional environment change and human population dynamics. Keywords: MIS4, North Africa, dating, environment, geoarchaeology
* Corresponding author.
DEVELOPING A HOLOCENE TEPHROSTRATIGRAPHY OF ETHIOPIA C. Martin-Jones 1,*, C.S. Lane 2, H.F. Lamb 1, N.J.G. Pearce 1, V.C. Smith 3. Department of Geography and Earth Sciences, Aberystwyth University, UK; 2 Department of Geography, University of Manchester, UK; 3 RLAHA, University of Oxford, UK. 1
E-mail address: [email protected].
Ethiopia experiences a highly variable climatic regime, subject to the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) and monsoon variability. These climatic changes are recorded in Holocene lake sediments throughout the Ethiopian Rift Valley. Synchronizing widespread regional palaeoclimate archives requires accurate and precise chronological control. Explosive volcanic eruptions generate large volumes of volcanic ash (tephra) which may be transported over continental scales and deposited in sedimentary sequences rapidly. A tephra layer represents a time-parallel marker which can be used to link sedimentary archives. Tephra from the same eruption is traced between different sites on the basis of its unique geochemical ‘fingerprint’. Ultra-distal ‘non-visible’ cryptotephra layers are increasingly used to extend the geographical range over which palaeoenvironmental archives can be correlated. Many caldera forming eruptions occurred from East African Rift volcanoes during the early Holocene to late Pleistocene. The tephras deposited provide chronological control on Pleistocene Ethiopian palaeoanthropological sites. However, Holocene tephra deposits remain largely un-documented in Ethiopia. Lacustrine cores were collected from Ethiopian Rift lakes (Ashenge, Hayk, Dendi, Hora, Tilo, Awassa and Chamo) covering 50 cm thick tephras in Lakes Tilo and Awassa during ~ 700 e 9,800 cal yr BP. These tephras have a similar major element composition to those in Lake Hora sediments (250 km NE of Lake Tilo). Distal rhyolitic tephra, possibly originating from the Afar, were deposited in Lakes Ashenge and Hayk between ~ 5,400 e 14,500 cal yr BP. Further work is now required in order to test potential correlations between these lacustrine archives. The resulting tephrostratigraphy will provide a dossier of regional Holocene volcanism and allow temporal and spatial climatic changes in this region to be investigated. Keywords: tephrochronology, geochemistry
correlation,
palaeoclimate,
Ethiopia,
U-TH BURIAL DATING OF OSTRICH EGGSHELL: A NEW GEOCHRONOMETER FOR AFRICAN ARCHAEOLOGICAL SITES W. Sharp 1,*, N.D. Fylstra 1, C.A. Tryon 2, J.T. Faith 3, D.J. Peppe 4. 1 Berkeley Geochronology Center, USA; 2 Harvard University, USA; 3 University of Queensland, Australia; 4 Baylor University, USA. E-mail address: [email protected].
Obtaining precise and accurate dates can be challenging at African archaeological sites, especially those that lie beyond the range of radiocarbon dating. We report herein a promising new approach to U-series dating of ostrich eggshell (OES) fragments and beads that occur widely in African archaeological contexts. U-Th dating has about ten times the range of radiocarbon dating ~500 ka rather than 50 ka) and ancient OES are generally geochemically suitable for the U-Th technique. U in OES is acquired after burial but the burial age may be estimated from measured apparent ages if U uptake takes place by diffusion. Using OES from three Pleistocene-eHolocene east African sites, we have: (1) measured U concentration profiles of OES normal to their exterior surface (by laser ablation ICP-MS), (2) determined apparent U-Th ages on outer and inner layers of eggshells (by solution ICP-MS), and (3) calculated U-Th burial ages using a simple model for diffusive uptake of uranium. We have assessed our method by comparing the U-Th burial ages with radiocarbon dates for OES calcite from the same shells and find good
dust transport? Does Saharan dust transport in interaction with changing climate develop multiple equilibria and bifurcations? The dust source model by Stanelle et al. (submitted) is applied to set up stationary state simulations for the Holocene and today's conditions. Holocene surface characteristics, including vegetation cover and hydrological quantities such as lake area and soil moisture, are derived from proxy data and the dynamical wetlands hydrology scheme by Stacke (2012). Modelling results of the amount of emitted and deposited dust for present climate and Holocene climate are evaluated and compared with measured data. Keywords: Holocene, Saharan dust dynamics, abrupt climate change References: Brovkin, V., M. Claussen, V. Petoukhov and A. Ganopolski: On the stability of the atmosphere vegetation system in the Sahara/Sahel region, J. Geophys. Res., 31, 613e31,624, 1998. Claussen, M., C. Kubatzki, V. Brovkin, A. Ganopolski, P. Hoelzmann and H. J. Pachur: Simulation of an abrupt change in Saharan vegetation in the midHolocene, Geophys. Res. Lett., 26, 2037e2040,1999. deMenocal, P, J. Ortiz, t. Guilderson, J.Adkins, M. Sarnthein, L. Baker and M. Yarusinsky: Abrupt onset and termination of the African humid period: Rapid climate response to gradual insolation forcing, Quat. Sci. Rev.,19, 347e361, 2000. zine, H. Eggermont, C. Cocquyt, P. Kroepelin, S., D. Verschuren, A.-M. Le Francus, J.-P. Cazet, M. Fagot, B. Rumes, J. M. Russell, F. Darius, D. J. Conley, M. Schuster, H. von Suchodoletz, D. E. Engstrom: Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years, Science 320(5877), 765e768, 2008. Stacke, T., S. Hagemann: Development and evaluation of a global dynamical wetlands extent scheme, Hydrology and Earth System Sciences 16(8), 2915e2933, 2012. Stanelle,T., I. Bey: Anthropogenically induced changes in the 20th century mineral dust burden and the associated impact on radiative forcing, submitted, 2014.
PALAEO-ENVIRONMENTAL GULLIES
RECONSTRUCTION
OF
THE
DIEPDRIFT
M. Evans*, Z. Jinnah, S. Mhlongo. School of Geosciences, University of the Witwatersrand, South Africa. E-mail address: [email protected].
The Diepdrift erosional site is located about 5 km north of Bela Bela in Limpopo Province, South Africa. It is an interesting geological feature as it is the only locally situated gully of its size in the Bela Bela area. No surrounding areas show a large degree of gullying as is depicted by the Diepdrift site. It is located in the heart of the Bushveld Complex surrounded by granites of the Rooiberg Suite as well as forming the drainage and catchment basin of the Waterberg Group. It is lies unconformably above these characteristic red granites, and is situated between two large granite domes that flank the site to the east and the west. These domes form a radial drainage pattern, which converges in the Diepdrift site, slightly upstream and along the length of the gully. A river meanders through the gully itself, and is fed by smaller streams flowing towards the centre of the feature. The banks of the river channel seem to incise into the walls of the gully causing a basal undercutting appearance. The river flows down slope towards the south and collects in a dam further downstream in the adjacent farm. The sediments consists of various horizons of brownish-orange to grey, fine to coarse grained sandstones and palaeosols with a vertical piping / gullying pattern forming on the exposed faces of these stratigraphic units. The faces are exposed over varying depths. In some locations the exposed face is as small as 0.5 m, while in other areas, predominantly downstream, the faces are exposed up to 6 m in height. The gully seems to weather down to the previously mentioned granite in some locations. This granite forms the basal bedrock underlying the sediments.
* Corresponding author.
197
The aim of this research project is to produce a palaeoenvironmental reconstruction for the Diepdrift gullies and to derive an understanding of gullying in the area and the factors which contributed to its formation. Through optically stimulated luminescence dating a high resolution geochronology will be derived which will allow for the correlation of the stratigraphy to recognised climatic events that were recorded for the Quaternary. Initial results suggest that the Diepdrift structure is as a result of an alluvial fan collecting material from the underlying granite bedrock in an around the area and from other sediments further upstream. The fairly regular, continuous layering and lack of prominent channel deposits places the gully in the distal sections of the fan. Possible colluvial action is likely to influence the initial sediments from the granite domes on either side of the deposit. However, this does not directly affect the deposit itself. Due to the sodic nature of the soils the gullying in the site can be attributed to cation exchange causing degradation of the soils in an arid to semi-arid climatic environment. Keywords: gullies, optically stimulated luminescence dating, Quaternary environmental reconstruction
THE DEVELOPMENT OF THE PRESENT DAY DRAINAGE PATTERNS OF SOUTH-CENTRAL AFRICA T. Flügel 1, *, F. Eckardt 2, F.P.D. Cotterill 3. 1 Department of Military Geography, University of Stellenbosch, South Africa; 2 Department of Environmental and Geographical Sciences, University of Cape Town, South Africa; 3 AEON Geoecodynamics Research Hub, c/o Department of Botany and Zoology, University of Stellenbosch, South Africa. E-mail address: tyrelfl[email protected].
One of Africa's most distinctive morphological characteristics is the occurrence of the low lying Congo Basin (mean elevation of 400 m.a.s.l) adjacent to the elevated Kalahari Plateau (mean elevation of 1100 m.a.s.l). The transition zone between low and elevated Africa also demarcates the watershed between the Congo and Okavango-Zambezi drainage systems. This south-central African watershed, here termed the Congo-Kalahari watershed, is dominated by a smooth, seasonally wet tableland, flanked by the Angola highlands in the west and the Western branch of the East African Rift System (EARS) in the east. Thus the Congo-Kalahari incorporates three types of landforms: old, macro-landforms of the Kalahari Plateau and Congo Basin; newer, meso-landforms of EARS, horst and graben structures; and newer, process landforms as evidenced by headwater capture features and channel adjustements along the watershed. Therefore a better understanding of the Congo-Kalahari watershed may provide insights into the development of the mega-geomorphology of central and southern Africa. The Congo-Kalahari watershed separates the northerly flowing tributaries of the Congo River from the southerly flowing tributaries the Zambezi and Okavango Rivers. The present day drainage patterns of these river systems were investigated using remote sensing and a geospatial information system. The observed juxtaposition of river drainage patterns suggests a multi-stage evolution of these drainage systems with several controls acting on the development of these river systems. While much of southcentral Africa's drainage has been impacted on by events of the Neogene, significant changes have occurred during the Quaternary. It is during the Quaternary that the Congo and Okavango-Zambezi drainage systems attain their modern forms, although these system have and continue to be modified throughout the Quaternary. By combining geomorphic and geologic evidence with published data from several fields, including phylogeography, an overview of drainage system evolution may be discerned. Keywords: south-central Africa, Congo, Kalahari, GIS AGE AND SIGNIFICANCE OF HOLOCENE VALLEY FILLS IN THE SKELETON COAST, NAMIBIA D.S.G. Thomas*, J.A. Durcan. School of Geography and Environment, University of Oxford, UK. E-mail address: [email protected].
198
Abstracts / Quaternary International 404 (2016) 174e213
East-west orientated valleys with extremely ephemeral modern channel flow are a significant landscape feature of northwest Namibia. Today these valleys are important sources of aeolian dust, as each year plumes during Berg wind events fertilise Atlantic waters off the Skeleton Coast. Here we provide an OSL chronology for silts from the Huab valley, demonstrating a temporally complex Holocene sequence of valley-wide infilling (with early phases associated with the African humid period?) and subsequent late Holocene entrenchment during confined channel flows. Age data affirm fluvial histories from the Hoanib system (Eitel et al 2006), but additionally provide an association with modern aeolian dynamics and dust-sourcing that points to the vital role of wet e accumulation as sourcing the aeolian systems of today's hyper-arid regime. Keywords: Holocene, valley fills, luminescence dating, aeolian dust References: Eitel, B., Kadereit, A., Blumel, W.-D., Huser, K., Lomax, J., Hilgers, A., 2006. Environmental changes at the eastern Namib Desert margin before and after the Last Glacial Maximum: new evidence from fluvial deposits in the upper Hoanib River catchment, northwestern Namibia. Palaeogeography, Palaeoclimatology, Palaeoecology 234, 201e222.
8. DATING AND CORRELATION OF AFRICAN ARCHIVES OF ENVIRONMENTAL CHANGE AND ARCHAEOLOGY SPATIAL AND TEMPORAL VARIATION IN NORTH AFRICAN PALAEOENVIRONMENTAL AND ARCHAEOLOGICAL RECORDS DURING MARINE ISOTOPE STAGE 4 L. Farr*, S. Jones. The McDonald Institute for Archaeological Research, University of Cambridge, UK. E-mail address: [email protected].
Marine Isotope Stage 4 (MIS4) covers the timeframe of c.74e59 ka and is generally regarded as a period of sudden and rapid climatic deterioration in North Africa. MIS4 is argued to coincide with regional decreases in population size and density in North Africa, possibly concomitant with retraction into refugia in certain locales. Sitting awkwardly against this picture is recent mtDNA evidence from living human populations which suggests that, approximately c.60 ka, Homo sapiens groups exited Africa in a successful migration that resulted in colonisation of Asia, Australia and Europe. These dispersals of Homo sapiens are usually attributed to humid climatic phases in MIS5 and MIS3, when hydrological networks are argued to have been reactivated in response to pluvial events, resulting in the wetting of normally arid regions such as the Sahara. This paper reviews palaeoenvironmental and archaeological data from marine and terrestrial archives in the North African region, which are dated to the MIS4 timeframe. We evaluate spatial and temporal variation in the datasets in relation to global climate records and investigate their reliability to inform us of millennial and sub-millennial environmental change, and demographic processes in North Africa. In particular, we look at the chronometric dating of palaeoenvironmental and archaeological datasets and investigate issues regarding chronological precision, spatial resolution and geoarchaeological context. We present geoarchaeological and lithic data from part of the Haua Fteah cave sequence (Libya) which has associated MIS4 dates, and discuss how these findings fit into a wider picture of regional environment change and human population dynamics. Keywords: MIS4, North Africa, dating, environment, geoarchaeology
* Corresponding author.
DEVELOPING A HOLOCENE TEPHROSTRATIGRAPHY OF ETHIOPIA C. Martin-Jones 1,*, C.S. Lane 2, H.F. Lamb 1, N.J.G. Pearce 1, V.C. Smith 3. Department of Geography and Earth Sciences, Aberystwyth University, UK; 2 Department of Geography, University of Manchester, UK; 3 RLAHA, University of Oxford, UK. 1
E-mail address: [email protected].
Ethiopia experiences a highly variable climatic regime, subject to the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) and monsoon variability. These climatic changes are recorded in Holocene lake sediments throughout the Ethiopian Rift Valley. Synchronizing widespread regional palaeoclimate archives requires accurate and precise chronological control. Explosive volcanic eruptions generate large volumes of volcanic ash (tephra) which may be transported over continental scales and deposited in sedimentary sequences rapidly. A tephra layer represents a time-parallel marker which can be used to link sedimentary archives. Tephra from the same eruption is traced between different sites on the basis of its unique geochemical ‘fingerprint’. Ultra-distal ‘non-visible’ cryptotephra layers are increasingly used to extend the geographical range over which palaeoenvironmental archives can be correlated. Many caldera forming eruptions occurred from East African Rift volcanoes during the early Holocene to late Pleistocene. The tephras deposited provide chronological control on Pleistocene Ethiopian palaeoanthropological sites. However, Holocene tephra deposits remain largely un-documented in Ethiopia. Lacustrine cores were collected from Ethiopian Rift lakes (Ashenge, Hayk, Dendi, Hora, Tilo, Awassa and Chamo) covering 50 cm thick tephras in Lakes Tilo and Awassa during ~ 700 e 9,800 cal yr BP. These tephras have a similar major element composition to those in Lake Hora sediments (250 km NE of Lake Tilo). Distal rhyolitic tephra, possibly originating from the Afar, were deposited in Lakes Ashenge and Hayk between ~ 5,400 e 14,500 cal yr BP. Further work is now required in order to test potential correlations between these lacustrine archives. The resulting tephrostratigraphy will provide a dossier of regional Holocene volcanism and allow temporal and spatial climatic changes in this region to be investigated. Keywords: tephrochronology, geochemistry
correlation,
palaeoclimate,
Ethiopia,
U-TH BURIAL DATING OF OSTRICH EGGSHELL: A NEW GEOCHRONOMETER FOR AFRICAN ARCHAEOLOGICAL SITES W. Sharp 1,*, N.D. Fylstra 1, C.A. Tryon 2, J.T. Faith 3, D.J. Peppe 4. 1 Berkeley Geochronology Center, USA; 2 Harvard University, USA; 3 University of Queensland, Australia; 4 Baylor University, USA. E-mail address: [email protected].
Obtaining precise and accurate dates can be challenging at African archaeological sites, especially those that lie beyond the range of radiocarbon dating. We report herein a promising new approach to U-series dating of ostrich eggshell (OES) fragments and beads that occur widely in African archaeological contexts. U-Th dating has about ten times the range of radiocarbon dating ~500 ka rather than 50 ka) and ancient OES are generally geochemically suitable for the U-Th technique. U in OES is acquired after burial but the burial age may be estimated from measured apparent ages if U uptake takes place by diffusion. Using OES from three Pleistocene-eHolocene east African sites, we have: (1) measured U concentration profiles of OES normal to their exterior surface (by laser ablation ICP-MS), (2) determined apparent U-Th ages on outer and inner layers of eggshells (by solution ICP-MS), and (3) calculated U-Th burial ages using a simple model for diffusive uptake of uranium. We have assessed our method by comparing the U-Th burial ages with radiocarbon dates for OES calcite from the same shells and find good