9 results on '"Lamb, Henry"'
Search Results
2. Diatom-derived palaeoconductivity estimates for Lake Awassa, Ethiopia: evidence for pulsed inflows of saline groundwater
- Author
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Telford, Richard J., Lamb, Henry F., and Umer Mohammed, Mohammed
- Published
- 1999
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3. Long-term resilience, bush encroachment patterns and local knowledge in a Northeast African savanna.
- Author
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Gil-Romera, Graciela, Lamb, Henry F., Turton, David, Sevilla-Callejo, Miguel, and Umer, Mohammed
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SAVANNAS ,ANIMALS ,PASTURES ,VEGETATION & climate ,BIOCLIMATOLOGY - Abstract
Abstract: Bush encroachment is a significant phenomenon in savanna environments as it affects wildlife and local livelihoods by preventing new pasture generation. In this article we present a 2000-year record of vegetation change in the Dara range of the Mago National Park, southwestern Ethiopia, an area inhabited by Mursi agro-pastoralists. We use an interdisciplinary approach to understand whether bush encroachment in this area is a recent event or a transitional state of the savanna and describe the local understanding of encroachment as a species-specific process. The vegetation record was obtained from a fossil hyrax midden, a type of sediment already used in Southern Africa but never before in East Africa. Six encroaching phases, led by Capparaceae and Grewia, were found over the last two millennia. The system proved to be resilient, with alternating open and encroached phases, and showed a non-linear response to environmental change, thereby fitting the control theory hypothesis for hysteresis loops. Determining the thresholds conditioning the system''s resilience could help to improve savanna management for both local people and National Park authorities. [Copyright &y& Elsevier]
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- 2010
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4. Oxygen and carbon isotope composition of authigenic carbonate from an Ethiopian lake: a climate record of the last 2000 years.
- Author
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Lamb, Henry F., Leng, Melanie J., Telford, Richard J., Ayenew, Tenalem, and Umer, Mohammed
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OXYGEN , *CARBON , *ISOTOPES , *CARBONATES , *METEOROLOGICAL precipitation , *EVAPORATION (Meteorology) , *RAINFALL , *CLIMATE change , *LAKES - Abstract
The oxygen and carbon isotope composition of sedimentary carbonates formed in Lake Hayq, a closed-basin lake in northern Ethiopia, suggests marked variation in the balance between precipitation and evaporation during the last 2000 years. Diatom stratigraphy shows that the lake remained deep enough to support planktonic taxa, and was never saline. The record suggests that rainfall was higher than that of the twentieth century during most of this time, especially during two short intervals centred on AD 700 and AD 1300. Climate was drier only around AD 800, and from AD 1750–1900. Similar, but slightly moister climate than today, with high interdecadal variability, prevailed from AD 800 to AD 1200, equivalent to the European ‘Mediaeval Warm Period’. A period of high effective precipitation followed, from AD 1200 to AD 1700, during the ‘Little Ice Age’. Pollen evidence suggests that increased forest cover from AD 1400 to AD 1750 was a response to higher rainfall; forest cover may have increased evaporative loss from the catchment, contributing to a subsequent trend of increasing lakewater δ18O values. The record shows similarities in the timing and direction of changes in proxy-climatic records from Lake Naivasha (Kenya) and from the Nile. [ABSTRACT FROM AUTHOR]
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- 2007
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5. Forest clearance and regrowth in northern Ethiopia during the last 3000 years.
- Author
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Darbyshire, Iain, Lamb, Henry, and Umer, Mohammed
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FOREST regeneration - Abstract
Pollen and charcoal analysis of sediment cores from two lakes in the highlands of northern Ethiopia provide evidence that the vegetation has changed in response to human impact during the last 3000 years. The natural, pre-disturbance vegetation of the area was Podocarpus-Juniperus forest. At about 500 BC, following Semitic immigration to northern Ethiopia, the forests were cleared and replaced by a secondary vegetation of Dodonaea scrub and grassland that persisted for 1800 years. Grasslands were dominant from about AD 1200 to 1400, probably as a result of further intensification of grazing, perhaps exacerbated by drought. Juniperus forest, with Olea and Celtis, then expanded from AD 1400 to 1700, possibly because of drought-induced depopulation followed by increased rainfall. Deforestation and soil erosion has again intensified during the last three centuries. Since forest regrowth was possible after 1800 years of human impact, northern Ethiopia should again be capable of supporting forest under appropriate land management. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
6. Late Quaternary climate change in the north-eastern highlands of Ethiopia: A high resolution 15,600 year diatom and pigment record from Lake Hayk.
- Author
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Loakes, Katie L., Ryves, David B., Lamb, Henry F., Schäbitz, Frank, Dee, Michael, Tyler, Jonathan J., Mills, Keely, and McGowan, Suzanne
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INTERGLACIALS - Abstract
Abstract Multi-proxy analyses of an 8 m sediment core from Lake Hayk, a closed, freshwater lake in the north-central highlands of Ethiopia, provide a record of changing lake level and inferred regional climatic change for the last 15.6 cal ka years. Between ca. 15.6–15.2 cal ka BP, a lowstand was synchronous with Heinrich Event 1 and an intense drought across Eastern Africa. At ca. 15.2 cal ka BP a lake began to develop at the core site in response to wetter conditions, at the onset of the African Humid Period (AHP). However, in contrast to other lakes in eastern Africa, Hayk lake level fell around ca. 14.8 cal ka BP, indicating a climate shift towards aridity. The lake began filling again at ca. 12.3 cal ka BP and reached maximum water depth between ca. 12.0–10.0 cal ka BP. Lake level declined slowly during the Holocene, culminating in the termination of the AHP at Hayk between ca. 5.2–4.6 cal ka BP. In the late Holocene, ca. 2.2–1.3 cal ka BP, Lake Hayk was again deep and fresh with some evidence of short-term lake level variability. The palaeo-record from Lake Hayk indicates that while it experienced, to a broad degree, the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from eastern Africa, there are offsets in timing and rate of response. These differences reflect chronological discrepancies between records, as well as the varying climate sensitivities and site-specific factors of individual lake basins. This record highlights the different responses by lakes in a climatically vulnerable area of Ethiopia. Highlights • Discrepancies in the timing of the Younger Dryas Stadial proper. • No evidence for the catastrophic shift in climate centred at 8.2 ka. • Diatom evidence suggesting relatively rapid AHP termination. • High-resolution evidence of millennial to multi-decadal variability at Lake Hayk. • Regional discrepancies in the timing and expression of climatic events. [ABSTRACT FROM AUTHOR]
- Published
- 2018
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7. Palaeolimnology of high and low altitude sites in Ethiopia : diatom and geochemistry records from Garba Guracha and Lake Babogaya
- Author
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Grady, David Anton Ieuan, Davies, Sarah, and Lamb, Henry
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551.48 ,Ethiopia ,Garba Guracha ,Lake Babogaya ,palaeolimnology ,diatoms ,XRF geochemistry - Abstract
Palaeoenvironmental studies at high altitude in eastern Africa are rare. As a result, our understanding of contemporary climate change in the region is limited. This study presents high-resolution, multi-proxy (diatoms and core geochemistry) palaeolimnological reconstructions at sites located at contrasting altitudes: Garba Guracha a cirque lake >3950 m above sea level spanning the last 16,000 years, and since the mid-Holocene at Lake Babogaya a crater lake on the western escarpment of the Main Ethiopian Rift (1800 m above sea level). These palaeo-records broadly exhibit the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from across eastern Africa. However, these shifts are more muted at Garba Guracha. Furthermore, the precise timing and expression of these climatic events is not always synchronous between the Lake Babogaya and Garba Guracha records as well as other records across eastern Africa. The Garba Guracha sediment geochemistry and diatom record is typical of a dry, post-glacial and sparsely vegetated environment with the dominance of Fragilarioid taxa and high terrigenous input. Due to the poorly developed postglacial landscape at Garba Guracha, any Younger Dryas-like drying signal would have been superimposed on to high soil erosion and a poorly productive lake environment. The retreat of a nearby ice mass may also have over-ridden any catchment development related to the start of the African Humid Period. However, in response to increased moisture availability and warming at Garba Guracha after 10,250 cal BP, catchment productivity considerably increases with a large increase in diatom productivity and organic matter content and remains high for the majority of the remaining African Humid Period interval. The termination of the African Humid Period is clear in the Garba Guracha diatom record at around 5,500 cal BP, with a second phase of change to an environment similar to the modern day after 4,500 cal BP, potentially representing the Meghalayan. Drier conditions around the Meghalayan is also documented in the Lake Babogaya record with the deposition of a Gastropod shell layer. A reduction in organic matter content following the deposition of this layer likely represents reduced productivity and a destabilising catchment under a drier climate. These drier conditions culminate in the deposition of coarse grained, terrigenous material between 3,600 and 3,300 cal BP. Following high lake nutrient content and deepening of the lake between 3,300 cal BP and 1,750 cal BP, the Lake Babogaya record suggests a relatively stable, deep lake, with only brief intervals of shallower waters, until the top of the core (~600 cal BP). The diatom record after 560 cal BP at Garba Guracha is characterised by a dominance of aerophilous taxa such as Nitzschia amphibia and Cymbelloid taxa driven by an increased growing season and nutrient loading under a drier climate. These records further reinforce the reported heterogeneous pattern of climate across the region and the significance of site-specific dynamics in the responses of catchments to regional drivers. The data produced in this study may be used to inform future climate modelling and perform more complete regional downscaling, while also furthering our understanding of palaeoenvironmental change in eastern Africa, at a variety of altitudes.
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- 2020
8. A Phytolith Supported Biosphere-Hydrosphere Predictive Model for Southern Ethiopia: Insights into Paleoenvironmental Changes and Human Landscape Preferences since the Last Glacial Maximum.
- Author
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Fischer, Markus L., Bachofer, Felix, Yost, Chad L., Bludau, Ines J. E., Schepers, Christian, Foerster, Verena, Lamb, Henry, Schäbitz, Frank, Asrat, Asfawossen, Trauth, Martin H., and Junginger, Annett
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PREDICTION models , *LANDSCAPE changes , *CLIMATE change , *PLEISTOCENE Epoch , *VEGETATION dynamics , *LAST Glacial Maximum , *GLACIAL landforms - Abstract
During the past 25 ka, southern Ethiopia has undergone tremendous climatic changes, from dry and relatively cold during the Last Glacial Maximum (LGM, 25–18 ka) to the African Humid Period (AHP, 15–5 ka), and back to present-day dry conditions. As a contribution to better understand the effects of climate change on vegetation and lakes, we here present a new Predictive Vegetation Model that is linked with a Lake Balance Model and available vegetation-proxy records from southern Ethiopia including a new phytolith record from the Chew Bahir basin. We constructed a detailed paleo-landcover map of southern Ethiopia during the LGM, AHP (with and without influence of the Congo Air Boundary) and the modern-day potential natural landcover. Compared to today, we observe a 15–20% reduction in moisture availability during the LGM with widespread open landscapes and only few remaining forest refugia. We identify 25–40% increased moisture availability during the AHP with prevailing forests in the mid-altitudes and indications that modern anthropogenic landcover change has affected the water balance. In comparison with existing archaeological records, we find that human occupations tend to correspond with open landscapes during the late Pleistocene and Holocene in southern Ethiopia. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
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9. Using multiple chronometers to establish a long, directly-dated lacustrine record: Constraining >600,000 years of environmental change at Chew Bahir, Ethiopia.
- Author
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Roberts, Helen M., Ramsey, Christopher Bronk, Chapot, Melissa S., Deino, Alan L., Lane, Christine S., Vidal, Céline, Asrat, Asfawossen, Cohen, Andrew, Foerster, Verena, Lamb, Henry F., Schäbitz, Frank, Trauth, Martin H., and Viehberg, Finn A.
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OPTICALLY stimulated luminescence , *CHRONOMETERS , *MESOLITHIC Period , *STONE Age , *MASTICATION - Abstract
Despite eastern Africa being a key location in the emergence of Homo sapiens and their subsequent dispersal out of Africa, there is a paucity of long, well-dated climate records in the region to contextualize this history. To address this issue, we dated a ∼293 m long composite sediment core from Chew Bahir, south Ethiopia, using three independent chronometers (radiocarbon, 40Ar/39Ar, and optically stimulated luminescence) combined with geochemical correlation to a known-age tephra. The site is located in a climatically sensitive region, and is close to Omo Kibish, the earliest documented Homo sapiens fossil site in eastern Africa, and to the proposed dispersal routes for H. sapiens out of Africa. The 30 ages generated by the various techniques are internally consistent, stratigraphically coherent, and span the full range of the core depth. A Bayesian age-depth model developed using these ages results in a chronology that forms one of the longest independently dated, high-resolution lacustrine sediment records from eastern Africa. The chronology illustrates that any record of environmental change preserved in the composite sediment core from Chew Bahir would span the entire timescale of modern human evolution and dispersal, encompassing the time period of the transition from Acheulean to Middle Stone Age (MSA), and subsequently to Later Stone Age (LSA) technology, making the core well-placed to address questions regarding environmental change and hominin evolutionary adaptation. The benefits to such studies of direct dating and the use of multiple independent chronometers are discussed. • Four independent dating methods applied to ∼293 m lake core from southern Ethiopia. • Reveals 620 ka high-resolution sedimentary record near key fossil hominin sites. • Mean accumulation rate of 0.47 mm/a comparable to other African lacustrine sediments. • Accumulation rate fell to 0.1 mm/a during MIS 2, likely due to reduced sediment supply. • Use of multiple independent chronometers is a powerful approach in lake settings. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
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