A stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of La... more A stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of Lake Turkana reveal four major lake level fluctuations of >50 m between ca 8.5 and 4.5 ka during the transition from the African Humid Period to Holocene aridity. We hypothesize that high stands of Lake Turkana reflect complex hydrological processes with variability in Atlantic and Indian Ocean-derived moisture into East Africa, ultimately linked to changes in sea surface temperatures (SSTs). Lake level rise was also amplified regionally by increased precipitation causing overflow from the adjacent Suguta and Chew Bahir basins, particularly during high stands at ca >8.5 ka and at 6.4 ka. The timing of transgressive and regressive events is constrained by 14C dating of carefully selected lacustrine mollusks, incorporation of previous shell ages with sufficient elevational control, and OSL dating of quartz grains from littoral and sublittoral deposits by a multiple aliquot regeneration (MAR) approach. There was a high water level up to at least 70 m at >8.5 ka that probably reached the spillover limit ∼100 m. A brief (<500 yr) high stand up to at least 50 m at ca 7 ka appears to be coincident with warm postglacial SSTs in the western Indian Ocean and thus may reflect a strengthened East African Monsoon, though some precipitation was probably derived from Atlantic sources as well. A pronounced high stand inferred at ca 6.4 ka, possibly up to 100 m, is associated with warming across the Indian Ocean and equatorward compression of the ITCZ, thus reflecting a strengthened East African Monsoon. A high stand occurred at ca 5.5–5.0 ka up to at least 80 m, with a precipitous drop in lake level by 4.6 ka. Evidence from relict Porr strand plain indicate that lake level was probably below 20 m since ca 4.5 ka, though there were two noticeable high stands up to 12–20 m at ca 830 years ago and <100 years, the latter age is consistent with the historic record.
Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for th... more Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine > 30 m oscillations in water level were documented between ca. 15 and 4 ka. The earliest lake level oscillation between ca. 14.5 and 13 ka is not well constrained with water level to at least 70 m above the present surface and subsequently fell to at least 50 m. Lake level increased to at least 90 m between ca. 11.2 and 10.4 ka, post Younger Dryas cooling. Water level fell by > 30 m by 10.2 ka, with another potential rise at ca. 8.5 ka to > 70 m above current level. Lake level regressed by > 40 m at 8.2 ka coincident with cooling in the equatorial eastern Atlantic Ocean. Two major > 70 m lake level oscillations centered at 6.6 and 5.2 ka may reflect enhanced convection with warmer sea surface temperatures in the western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5 ka and is characterized by highly variable lake level (±>40 m), rather than one monotonic fall in water level. This lake level variability reflects a complex response to variations in the extent and intensity of the East and West African Monsoons near geographic and topographic limits within the catchment of Lake Turkana. Also, for this closed lake basin excesses and deficits in water input are amplified with a cascading lake effect in the East African Rift Valley and through the Chew Bahir Basin. The final regression from a high stand of > 90 m above the present lake began at 5.2 ka and water level was below 20 m by 4.5 ka; and for the remainder of the Holocene. This sustained low stand is associated with weakening of the West African Monsoon, a shift of the mean position of Congo Air Boundary west of the Lake Turkana catchment and with meter-scale variability in lake level linked to Walker circulation across the Indian Ocean.
Throughout the Holocene, Lake Turkana has been subject to drastic changes in lake levels and the ... more Throughout the Holocene, Lake Turkana has been subject to drastic changes in lake levels and the subsistence strategies people employ to survive in this hot and arid region. In this paper, we reconstruct the position of the lake during the Holocene within a paleoclimatic context. Atmospheric forcing mechanisms are discussed in order to contextualize the broader landscape changes occurring in eastern Africa over the last 12,000 years. The Holocene is divided into five primary phases according to changes in the strand-plain evolution, paleoclimate, and human subsistence strategies practiced within the basin. Early Holocene fishing settlements occurred adjacent to high and relatively stable lake levels. A period of high-magnitude oscillations in lake levels ensued after 9,000 years BP and human settlements appear to have been located close to the margins of the lake. Aridification and a final regression in lake levels ensued after 5,000 years BP and human communities were generalized pastoralists-fishers-foragers. During the Late Holocene, lake levels may have dropped below their present position and subsistence strategies appear to have been flexible and occasionally specialized on animal pastoralism. Modern missionary and government outposts have encouraged the construction of permanent settlements in the region, which are heavily dependent on outside resources for their survival. Changes in the physical and cultural environments of the Lake Turkana region have been closely correlated, and understanding the relationship between the two variables remains a vital component of archaeological research.
Water level in Lake Turkana, Kenya in the past ca. 150 years is controlled primarily from the bia... more Water level in Lake Turkana, Kenya in the past ca. 150 years is controlled primarily from the biannual passage of the East and West African monsoon, with rainfall volume related partially to sea surface temperatures (SSTs) in the Western Indian and East Atlantic oceans. Empirical orthogonal function analyses show significant correlation between Eastern Atlantic or Western Indian SSTs and lake level anomalies, with the first mode accounting for 66% and 55% of the variability. The primary geographic loadings are consistent with a Gulf of Guinea moisture source and positive Indian Ocean Dipole (IOD) state. The second mode explains 10% of variability, and reflects the westward extension of an Indian Ocean cool pool, potentially indicative of a normal to a negative IOD state. There is significant spatial correlation between basin rainfall anomalies associated with Eastern Atlantic SSTs and a low in the continental divide between the Kenyan and the Ethiopian highlands, which is a passage for moisture from the Congo Basin. Linear regression analysis with Bootstrap sampling and Monte Carlo simulations define numeric relations between Western Indian and Eastern Atlantic SSTs and lake level change for AD 1992 to 2013. The monthly and yearly lake level reconstructions based on this numeric analysis capture the decadal-scale variability and the 15 m drop in water level in the early 20th century. Meter-scale variability in lake level since ca. AD 1930 is associated with precipitation sourced from the Western Indian Ocean with IOD variability, whereas the 15 m drop in water level in the early 20th century may reflect a profound decrease in moisture from Atlantic/Congo Basin source. These numerical solutions are poised to reconstruct water level variations in the past ca. 300 years for Lake Turkana with new proxy records of SSTs from the Western Indian Ocean and the Gulf of Guinea.
Relict beaches adjacent to Lake Turkana, Kenya provide a high-fidelity record of water level vari... more Relict beaches adjacent to Lake Turkana, Kenya provide a high-fidelity record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine > 30 m oscillations in water level were documented between ca. 15 and 4 ka. This variability in lake level probably reflects varying rainfall contributions from the West and East African monsoons, amplified by cascading lake systems in the East African Rift Valley and the Ethiopian Plateau that empty into Lake Turkana. Lake level rose to at least 70 m above the present surface between ca. 14.5 and 13 ka associated with an enhanced eastern flux of moisture with the migration of the Congo Air Boundary, possibly reflecting an increase in sea surface temperatures in the Eastern Equatorial Atlantic Ocean. Lake level increased to at least 90 m between ca. 11.2 and 10.4 ka, post Younger Dryas cooling. Water level fell by > 30 m by 10.2 ka, with another potential rise at ca. 8.5 ka to > 70 m above current level. Lake level regressed by > 40 m at 8.2 ka coincident with cooling in the equatorial Eastern Atlantic Ocean. Two major >70 m lake level oscillations centered at 6.6 and 5.2 ka and may reflect enhanced convection with warmer sea surface temperatures in the Western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5 ka and is characterized by highly variable lake level (± > 40 m), rather than one monotonic fall in water level. The final regression from a high stand of > 90 m above the present lake began at. 5.2 ka and water level was below 20 m by 4.5 ka; and for the remainder of the Holocene.
A stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of La... more A stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of Lake Turkana reveal four major lake level fluctuations of >50 m between ca 8.5 and 4.5 ka during the transition from the African Humid Period to Holocene aridity. We hypothesize that high stands of Lake Turkana reflect complex hydrological processes with variability in Atlantic and Indian Ocean-derived moisture into East Africa, ultimately linked to changes in sea surface temperatures (SSTs). Lake level rise was also amplified regionally by increased precipitation causing overflow from the adjacent Suguta and Chew Bahir basins, particularly during high stands at ca >8.5 ka and at 6.4 ka. The timing of transgressive and regressive events is constrained by 14C dating of carefully selected lacustrine mollusks, incorporation of previous shell ages with sufficient elevational control, and OSL dating of quartz grains from littoral and sublittoral deposits by a multiple aliquot regeneration (MAR) approach. There was a high water level up to at least 70 m at >8.5 ka that probably reached the spillover limit ∼100 m. A brief (<500 yr) high stand up to at least 50 m at ca 7 ka appears to be coincident with warm postglacial SSTs in the western Indian Ocean and thus may reflect a strengthened East African Monsoon, though some precipitation was probably derived from Atlantic sources as well. A pronounced high stand inferred at ca 6.4 ka, possibly up to 100 m, is associated with warming across the Indian Ocean and equatorward compression of the ITCZ, thus reflecting a strengthened East African Monsoon. A high stand occurred at ca 5.5–5.0 ka up to at least 80 m, with a precipitous drop in lake level by 4.6 ka. Evidence from relict Porr strand plain indicate that lake level was probably below 20 m since ca 4.5 ka, though there were two noticeable high stands up to 12–20 m at ca 830 years ago and <100 years, the latter age is consistent with the historic record.
Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for th... more Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine > 30 m oscillations in water level were documented between ca. 15 and 4 ka. The earliest lake level oscillation between ca. 14.5 and 13 ka is not well constrained with water level to at least 70 m above the present surface and subsequently fell to at least 50 m. Lake level increased to at least 90 m between ca. 11.2 and 10.4 ka, post Younger Dryas cooling. Water level fell by > 30 m by 10.2 ka, with another potential rise at ca. 8.5 ka to > 70 m above current level. Lake level regressed by > 40 m at 8.2 ka coincident with cooling in the equatorial eastern Atlantic Ocean. Two major > 70 m lake level oscillations centered at 6.6 and 5.2 ka may reflect enhanced convection with warmer sea surface temperatures in the western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5 ka and is characterized by highly variable lake level (±>40 m), rather than one monotonic fall in water level. This lake level variability reflects a complex response to variations in the extent and intensity of the East and West African Monsoons near geographic and topographic limits within the catchment of Lake Turkana. Also, for this closed lake basin excesses and deficits in water input are amplified with a cascading lake effect in the East African Rift Valley and through the Chew Bahir Basin. The final regression from a high stand of > 90 m above the present lake began at 5.2 ka and water level was below 20 m by 4.5 ka; and for the remainder of the Holocene. This sustained low stand is associated with weakening of the West African Monsoon, a shift of the mean position of Congo Air Boundary west of the Lake Turkana catchment and with meter-scale variability in lake level linked to Walker circulation across the Indian Ocean.
Throughout the Holocene, Lake Turkana has been subject to drastic changes in lake levels and the ... more Throughout the Holocene, Lake Turkana has been subject to drastic changes in lake levels and the subsistence strategies people employ to survive in this hot and arid region. In this paper, we reconstruct the position of the lake during the Holocene within a paleoclimatic context. Atmospheric forcing mechanisms are discussed in order to contextualize the broader landscape changes occurring in eastern Africa over the last 12,000 years. The Holocene is divided into five primary phases according to changes in the strand-plain evolution, paleoclimate, and human subsistence strategies practiced within the basin. Early Holocene fishing settlements occurred adjacent to high and relatively stable lake levels. A period of high-magnitude oscillations in lake levels ensued after 9,000 years BP and human settlements appear to have been located close to the margins of the lake. Aridification and a final regression in lake levels ensued after 5,000 years BP and human communities were generalized pastoralists-fishers-foragers. During the Late Holocene, lake levels may have dropped below their present position and subsistence strategies appear to have been flexible and occasionally specialized on animal pastoralism. Modern missionary and government outposts have encouraged the construction of permanent settlements in the region, which are heavily dependent on outside resources for their survival. Changes in the physical and cultural environments of the Lake Turkana region have been closely correlated, and understanding the relationship between the two variables remains a vital component of archaeological research.
Water level in Lake Turkana, Kenya in the past ca. 150 years is controlled primarily from the bia... more Water level in Lake Turkana, Kenya in the past ca. 150 years is controlled primarily from the biannual passage of the East and West African monsoon, with rainfall volume related partially to sea surface temperatures (SSTs) in the Western Indian and East Atlantic oceans. Empirical orthogonal function analyses show significant correlation between Eastern Atlantic or Western Indian SSTs and lake level anomalies, with the first mode accounting for 66% and 55% of the variability. The primary geographic loadings are consistent with a Gulf of Guinea moisture source and positive Indian Ocean Dipole (IOD) state. The second mode explains 10% of variability, and reflects the westward extension of an Indian Ocean cool pool, potentially indicative of a normal to a negative IOD state. There is significant spatial correlation between basin rainfall anomalies associated with Eastern Atlantic SSTs and a low in the continental divide between the Kenyan and the Ethiopian highlands, which is a passage for moisture from the Congo Basin. Linear regression analysis with Bootstrap sampling and Monte Carlo simulations define numeric relations between Western Indian and Eastern Atlantic SSTs and lake level change for AD 1992 to 2013. The monthly and yearly lake level reconstructions based on this numeric analysis capture the decadal-scale variability and the 15 m drop in water level in the early 20th century. Meter-scale variability in lake level since ca. AD 1930 is associated with precipitation sourced from the Western Indian Ocean with IOD variability, whereas the 15 m drop in water level in the early 20th century may reflect a profound decrease in moisture from Atlantic/Congo Basin source. These numerical solutions are poised to reconstruct water level variations in the past ca. 300 years for Lake Turkana with new proxy records of SSTs from the Western Indian Ocean and the Gulf of Guinea.
Relict beaches adjacent to Lake Turkana, Kenya provide a high-fidelity record of water level vari... more Relict beaches adjacent to Lake Turkana, Kenya provide a high-fidelity record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine > 30 m oscillations in water level were documented between ca. 15 and 4 ka. This variability in lake level probably reflects varying rainfall contributions from the West and East African monsoons, amplified by cascading lake systems in the East African Rift Valley and the Ethiopian Plateau that empty into Lake Turkana. Lake level rose to at least 70 m above the present surface between ca. 14.5 and 13 ka associated with an enhanced eastern flux of moisture with the migration of the Congo Air Boundary, possibly reflecting an increase in sea surface temperatures in the Eastern Equatorial Atlantic Ocean. Lake level increased to at least 90 m between ca. 11.2 and 10.4 ka, post Younger Dryas cooling. Water level fell by > 30 m by 10.2 ka, with another potential rise at ca. 8.5 ka to > 70 m above current level. Lake level regressed by > 40 m at 8.2 ka coincident with cooling in the equatorial Eastern Atlantic Ocean. Two major >70 m lake level oscillations centered at 6.6 and 5.2 ka and may reflect enhanced convection with warmer sea surface temperatures in the Western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5 ka and is characterized by highly variable lake level (± > 40 m), rather than one monotonic fall in water level. The final regression from a high stand of > 90 m above the present lake began at. 5.2 ka and water level was below 20 m by 4.5 ka; and for the remainder of the Holocene.
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Papers by Chris Bloszies