Lakes Bogoria and Baringo lie in a semi-arid part of the Kenya Rift Valley between 0 ◦ 15′–0 ◦ 30... more Lakes Bogoria and Baringo lie in a semi-arid part of the Kenya Rift Valley between 0 ◦ 15′–0 ◦ 30’N and 36 ◦ 02′– 36 ◦ 05′E. Nevertheless, the area around these lakes contains numerous wetland systems that have been formed: along lake shorelines; along faults where hot, warm and cold springs have developed; and along river systems that cross the rift floor. Six major types of wetland are recognized: Proximal Hot Springs; Hot Spring Marshes; Blister Wetlands; Typha and Cyperus papyrus Swamps; Floodplain Marshes; Hypersaline Lake Littoral Wetlands; and Freshwater Lake Littoral Wetlands. These show significant variability in terms of geomorphic setting, water chemistry, temperature, plant communities and diatom floras. They are variously dominated by macrophytes, such as Cyperus laevigatus, Typha domingensis and Cyperus papyrus. In some cases macrophytes are absent. In hot spring settings and in hypersaline lake littoral zones bacterial mats are common. Although absent in some samples,...
ABSTRACT Hominin evolution took place in Africa during the Plio-Pleistocene and climate change is... more ABSTRACT Hominin evolution took place in Africa during the Plio-Pleistocene and climate change is thought to be a factor, with Africa experiencing a general cooling and increasing aridification over the last several million years. Today, the climate of the East African Rift Valley of Kenya is characterized as semi-arid with evapotranspiration four times precipitation. Water resources are a valuable commodity for the many millions of inhabitants of the Valley. The short instrumental record shows precipitation fluctuates at sub-decadal timeframes as a result of the ENSO cycle; while during prehistory variations in monsoonal precipitation occurred on Milankovitch timescales (i.e. African Humid Period). Both timescales exhibit significant impacts on the distribution of surface water. However, little is known regarding precipitation variability over sub-millennial timescales. Emerging paleoclimate data indicates that the near surface presence of water has also varied over century length timescales. We present paleoclimate data from multiple sites along a north-south 600 km transect of the Gregory Rift Valley (Kenya) that indicate the region experienced wetter conditions during the Little Ice Age (A.D. 1400-1850). Our reconstructions of landscape and climate during this time frame rely upon a multiproxy and interdisciplinary approach. We discuss data from a variety of environmental settings (e.g. lakes, wetlands, and springs) that indicate an overall increase in hydrologic balance. Evidence is derived from biologic microfossils such as pollen, diatom and testate amoebae assemblages as well as inorganic components of the sedimentary record and geomorphic changes. The data differs significantly from studies undertaken to the west in Uganda and the Congo, where negative hydrologic balances occurred during the Little Ice Age. While the atmospheric dynamics causing this disparity are not yet recognized, interactions between the Intertropical Convergence Zone and the Congo Air Boundary are a likely causal agent.
The East African Rift Valley is characterized as an arid to semi-arid region, with several large,... more The East African Rift Valley is characterized as an arid to semi-arid region, with several large, well studied, alkaline lakes; within the region, freshwater wetlands persist. These wetland systems, locally are important sources of freshwater. They also provide overlooked important paleoclimate archives, as the fragile ecology of these wetlands can be affected by even minor changes in hydrology and climate. Loboi Swamp is a 1.5 km2 freshwater wetland located near the equator in the Kenyan Rift Valley. The region receives approximately 700 mm of precipitation per year, while potential evaporation exceeds 2500 mm annually. Analysis of 25-years of precipitation data from local weather stations indicate that significant positive precipitation anomalies occur during El Niño years. Radiocarbon, pollen, and diatom data from Loboi Swamp indicates that the current wetland developed approximately 700 years ago. Sediment surface samples were collected for pollen, seeds and testate amoebae, alo...
Lake Baringo and Lake Bogoria lie within an asymmetric half-graben in the East African Rift, Cent... more Lake Baringo and Lake Bogoria lie within an asymmetric half-graben in the East African Rift, Central Kenya. Lake Baringo to the north is fresh because of subsurface outflow (~0.8 g/l TDS; pH=8.9) whereas to the south, Lake Bogoria, which is hydrologically closed, is saline/alkaline (mixolimnion: ~40 g/l TDS; pH=10.2). The climate is semi arid (P1 km thick. The primary sources of water in the area are rivers draining the highlands, and hot, warm and cool springs and seeps of different salinities associated with the faults. Freshwater wetlands and rivers traversing the Loboi Plain represent a limited resource in this semi-arid environment and are heavily utilized for drinking water by the resident population, livestock, and wildlife, and for crop irrigation. To better understand the hydrogeochemical processes affecting the freshwater sources in the southern Loboi Plain, water samples that were collected from lakes, rivers, springs, and one of the large wetlands (Loboi Swamp) were anal...
Loboi Swamp is situated near the equator on the western fault-bounded margin of an asymmetric hal... more Loboi Swamp is situated near the equator on the western fault-bounded margin of an asymmetric half-graben within the East African Rift valley. The freshwater wetland is ~ 3km2 and developed during mid to late Holocene on the low relief floodplain of the axial Loboi River. The swamp is groundwater-fed by several springs and seeps associated with the border fault system.
STEM/AEM EVIDENCE FOR PRESERVATION OF BURIAL DIAGENETIC FABRICS IN DEVONIAN SHALES: IMPLICATIONS ... more STEM/AEM EVIDENCE FOR PRESERVATION OF BURIAL DIAGENETIC FABRICS IN DEVONIAN SHALES: IMPLICATIONS FOR FLUID/ROCK INTERACTION IN CRATONIC BASINS (USA) VICTORIA С HOVER, DONALD R. PEACOR, and LYNN M. WALTER Department of ...
Marine sediment porewaters are commonly depleted in K(+) relative to conservative mixing trends, ... more Marine sediment porewaters are commonly depleted in K(+) relative to conservative mixing trends, but the mineralogical sink for K(+) in the sediment has not been well characterized. Results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the Mississippi Delta plain estuary spanning the salinity gradient from 0-12parts per thousand. Evidence of K(+) depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy (STEM/AEM) analyses of clay components to determine possible mineralogical sinks for K(+) in the sediments. Conservative mixing between the freshwater influx from the Atchafalaya/Mississippi river systems and Gulf of Mexico seawater controls the surface-water major-element composition. Porewaters from the most saline site, however, are depleted in K(+) by up to 33% relative to overlying water, implying uptake of K(+) by the sediment. Textural characterization of the sediment...
The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by th... more The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by the Jurassic Notch Peak quartz monzonite in the central portion of the House Range, Utah. Two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km, at locations reflecting a range in metamorphic grades. The rocks were metamorphosed at ∼2 kbar, on the basis of estimates of stratigraphic overburden at the time of intrusion. The lithologies are relatively pure dolomitic limestones and impure argillaceous limestones (argillites). The progressive metamorphism has resulted in the successive appearance of talc, tremolite, scapolite, diopside, and forsterite in the meta-limestones and the appearance of biotite, tremolite, diopside, plagioclase, scapolite, vesuvianite, grossular, and wollastonite in the argillites. Mineral assemblages of the limestones compared with isobaric 2 kbar-phase equilibria in the CaO-MgO-SiO 2 -H 2 O-CO 2 system suggest that: (1) fluid buffering by metamorphic reactions has occurred; (2) domains of equilibrium are small; and (3) the limestones behaved as relatively closed systems during metamorphism. Maximum temperatures and X(CO 2 ) composition consistent with assemblages in the limestone are: (see pdf for table). Temperatures estimated by the calcite-dolomite geothermometer are consistent with these temperature estimates. Temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi-component argillite system. Compositions of fluids in equilibrium with low-grade argillites are poorly constrained but may have reached a maximum X(CO 2 ) of ∼0.75. Fluid compositions in equilibrium with medium- to high-grade argillites were more H 2 O-rich than X(CO 2 ) = 0.20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 °C. Therefore, the argillites were either initially more water-rich than limestones at the same grade, or they were more open to H 2 O-rich fluids during metamorphism.
Lakes Bogoria and Baringo lie in a semi-arid part of the Kenya Rift Valley between 0 ◦ 15′–0 ◦ 30... more Lakes Bogoria and Baringo lie in a semi-arid part of the Kenya Rift Valley between 0 ◦ 15′–0 ◦ 30’N and 36 ◦ 02′– 36 ◦ 05′E. Nevertheless, the area around these lakes contains numerous wetland systems that have been formed: along lake shorelines; along faults where hot, warm and cold springs have developed; and along river systems that cross the rift floor. Six major types of wetland are recognized: Proximal Hot Springs; Hot Spring Marshes; Blister Wetlands; Typha and Cyperus papyrus Swamps; Floodplain Marshes; Hypersaline Lake Littoral Wetlands; and Freshwater Lake Littoral Wetlands. These show significant variability in terms of geomorphic setting, water chemistry, temperature, plant communities and diatom floras. They are variously dominated by macrophytes, such as Cyperus laevigatus, Typha domingensis and Cyperus papyrus. In some cases macrophytes are absent. In hot spring settings and in hypersaline lake littoral zones bacterial mats are common. Although absent in some samples,...
ABSTRACT Hominin evolution took place in Africa during the Plio-Pleistocene and climate change is... more ABSTRACT Hominin evolution took place in Africa during the Plio-Pleistocene and climate change is thought to be a factor, with Africa experiencing a general cooling and increasing aridification over the last several million years. Today, the climate of the East African Rift Valley of Kenya is characterized as semi-arid with evapotranspiration four times precipitation. Water resources are a valuable commodity for the many millions of inhabitants of the Valley. The short instrumental record shows precipitation fluctuates at sub-decadal timeframes as a result of the ENSO cycle; while during prehistory variations in monsoonal precipitation occurred on Milankovitch timescales (i.e. African Humid Period). Both timescales exhibit significant impacts on the distribution of surface water. However, little is known regarding precipitation variability over sub-millennial timescales. Emerging paleoclimate data indicates that the near surface presence of water has also varied over century length timescales. We present paleoclimate data from multiple sites along a north-south 600 km transect of the Gregory Rift Valley (Kenya) that indicate the region experienced wetter conditions during the Little Ice Age (A.D. 1400-1850). Our reconstructions of landscape and climate during this time frame rely upon a multiproxy and interdisciplinary approach. We discuss data from a variety of environmental settings (e.g. lakes, wetlands, and springs) that indicate an overall increase in hydrologic balance. Evidence is derived from biologic microfossils such as pollen, diatom and testate amoebae assemblages as well as inorganic components of the sedimentary record and geomorphic changes. The data differs significantly from studies undertaken to the west in Uganda and the Congo, where negative hydrologic balances occurred during the Little Ice Age. While the atmospheric dynamics causing this disparity are not yet recognized, interactions between the Intertropical Convergence Zone and the Congo Air Boundary are a likely causal agent.
The East African Rift Valley is characterized as an arid to semi-arid region, with several large,... more The East African Rift Valley is characterized as an arid to semi-arid region, with several large, well studied, alkaline lakes; within the region, freshwater wetlands persist. These wetland systems, locally are important sources of freshwater. They also provide overlooked important paleoclimate archives, as the fragile ecology of these wetlands can be affected by even minor changes in hydrology and climate. Loboi Swamp is a 1.5 km2 freshwater wetland located near the equator in the Kenyan Rift Valley. The region receives approximately 700 mm of precipitation per year, while potential evaporation exceeds 2500 mm annually. Analysis of 25-years of precipitation data from local weather stations indicate that significant positive precipitation anomalies occur during El Niño years. Radiocarbon, pollen, and diatom data from Loboi Swamp indicates that the current wetland developed approximately 700 years ago. Sediment surface samples were collected for pollen, seeds and testate amoebae, alo...
Lake Baringo and Lake Bogoria lie within an asymmetric half-graben in the East African Rift, Cent... more Lake Baringo and Lake Bogoria lie within an asymmetric half-graben in the East African Rift, Central Kenya. Lake Baringo to the north is fresh because of subsurface outflow (~0.8 g/l TDS; pH=8.9) whereas to the south, Lake Bogoria, which is hydrologically closed, is saline/alkaline (mixolimnion: ~40 g/l TDS; pH=10.2). The climate is semi arid (P1 km thick. The primary sources of water in the area are rivers draining the highlands, and hot, warm and cool springs and seeps of different salinities associated with the faults. Freshwater wetlands and rivers traversing the Loboi Plain represent a limited resource in this semi-arid environment and are heavily utilized for drinking water by the resident population, livestock, and wildlife, and for crop irrigation. To better understand the hydrogeochemical processes affecting the freshwater sources in the southern Loboi Plain, water samples that were collected from lakes, rivers, springs, and one of the large wetlands (Loboi Swamp) were anal...
Loboi Swamp is situated near the equator on the western fault-bounded margin of an asymmetric hal... more Loboi Swamp is situated near the equator on the western fault-bounded margin of an asymmetric half-graben within the East African Rift valley. The freshwater wetland is ~ 3km2 and developed during mid to late Holocene on the low relief floodplain of the axial Loboi River. The swamp is groundwater-fed by several springs and seeps associated with the border fault system.
STEM/AEM EVIDENCE FOR PRESERVATION OF BURIAL DIAGENETIC FABRICS IN DEVONIAN SHALES: IMPLICATIONS ... more STEM/AEM EVIDENCE FOR PRESERVATION OF BURIAL DIAGENETIC FABRICS IN DEVONIAN SHALES: IMPLICATIONS FOR FLUID/ROCK INTERACTION IN CRATONIC BASINS (USA) VICTORIA С HOVER, DONALD R. PEACOR, and LYNN M. WALTER Department of ...
Marine sediment porewaters are commonly depleted in K(+) relative to conservative mixing trends, ... more Marine sediment porewaters are commonly depleted in K(+) relative to conservative mixing trends, but the mineralogical sink for K(+) in the sediment has not been well characterized. Results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the Mississippi Delta plain estuary spanning the salinity gradient from 0-12parts per thousand. Evidence of K(+) depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy (STEM/AEM) analyses of clay components to determine possible mineralogical sinks for K(+) in the sediments. Conservative mixing between the freshwater influx from the Atchafalaya/Mississippi river systems and Gulf of Mexico seawater controls the surface-water major-element composition. Porewaters from the most saline site, however, are depleted in K(+) by up to 33% relative to overlying water, implying uptake of K(+) by the sediment. Textural characterization of the sediment...
The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by th... more The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by the Jurassic Notch Peak quartz monzonite in the central portion of the House Range, Utah. Two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km, at locations reflecting a range in metamorphic grades. The rocks were metamorphosed at ∼2 kbar, on the basis of estimates of stratigraphic overburden at the time of intrusion. The lithologies are relatively pure dolomitic limestones and impure argillaceous limestones (argillites). The progressive metamorphism has resulted in the successive appearance of talc, tremolite, scapolite, diopside, and forsterite in the meta-limestones and the appearance of biotite, tremolite, diopside, plagioclase, scapolite, vesuvianite, grossular, and wollastonite in the argillites. Mineral assemblages of the limestones compared with isobaric 2 kbar-phase equilibria in the CaO-MgO-SiO 2 -H 2 O-CO 2 system suggest that: (1) fluid buffering by metamorphic reactions has occurred; (2) domains of equilibrium are small; and (3) the limestones behaved as relatively closed systems during metamorphism. Maximum temperatures and X(CO 2 ) composition consistent with assemblages in the limestone are: (see pdf for table). Temperatures estimated by the calcite-dolomite geothermometer are consistent with these temperature estimates. Temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi-component argillite system. Compositions of fluids in equilibrium with low-grade argillites are poorly constrained but may have reached a maximum X(CO 2 ) of ∼0.75. Fluid compositions in equilibrium with medium- to high-grade argillites were more H 2 O-rich than X(CO 2 ) = 0.20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 °C. Therefore, the argillites were either initially more water-rich than limestones at the same grade, or they were more open to H 2 O-rich fluids during metamorphism.
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