R. Bernhart Owen

ARTESIAN BLISTER WETLANDS, A PERENNIAL WATER RESOURCE IN THE SEMI-ARID RIFT VALLEY OF EAST AFRICA

A cluster of artesian springs encircled by mounds of marsh and wet meadows was discovered near the equator in Kenya, East Africa. Each spring is capped by a dense fibrous root mat that covers a mound of clayey peat with a blister of water in the center. Individual mounds are ∼15 m wide, 1–2 m high, and affect an area of ∼50 m2. The central water-blister volume is <1 m3. The arched semi-permeable vegetation cap appears to be buoyed upward by slow artesian flow that leaks through the cap and moves slowly away. Lush plant growth (Poaceae and Cyperaceae, algae, diatoms, and filamentous cyanobacteria) is supported even through the dry season (Dec.–Feb.). The term “artesian blister wetland” is proposed for this unusual marsh, which has not been described previously. Approximately 20 small, circular-to-oval artesian blister wetlands occur within a large spring and wetland complex (∼1.3 km2) that includes several freshwater Typha marshes fed by ground-water seeps. The springs discharge along a rift-related fracture system near the contact between volcanic bedrock and late Quaternary sediments. Cores (1–2 m) through the mounds revealed a dense root mat underlain by water in the center and a clayey peat that is locally pebbly at the base surrounding the blister. LOI in the clayey peat decreases from 75% at surface to ∼ 10% at 0.4–1.2 m. Blister water is cool, fresh, and dysaerobic (T=30–33 °C; pH=6.2–7.2; conductivity ∼600 ìS/cm; and DO=50%; 0.6–3.5 mg/l). The spring/wetland mounds likely form by the blanketing of the land around the spring orifice with vegetation (paludification). Plants and cyanobacteria seem to trap sediment transported by surface run-off and wind. The mound grows with time, but its height is limited by the magnitude of the hydraulic head. These ecological niches are important freshwater resources for animals and humans in semi-arid environments.

Possible Late Holocene equatorial palaeoclimate record based upon soils spanning the Medieval Warm Period and Little Ice Age, Loboi Plain, Kenya

Wetland and floodplain soils in the East African Rift of Kenya provide a record of changing palaeoclimate and palaeohydrology compatible with climate records for the mid-Holocene through the late Holocene Medieval Warm Period (~AD 800–1270) and Little Ice Age (~AD 1270–1850), documented previously in nearby lacustrine sites. Soils forming from volcaniclastic source materials in both Loboi Swamp and laterally adjacent Kesubo Marsh, two wetland systems of latest Holocene age, were investigated using micromorphology, whole-soil geochemical analysis, and stable isotope analysis of soil organic matter (SOM). Wetland formation was abrupt and possibly related to climate shift from drier conditions associated with the mid-Holocene and Medieval Warm Period, to wetter conditions associated with the Little Ice Age. Pre-wetland sediments are floodplain volcanic sandy silts comprising buried Inceptisols (SOM dC= –15x PDB) that fine upward to fine silt and clay, which are overlain by clays and organic-rich sediment (peat) (SOM dC= –26x PDB). Stable isotopes record an abrupt shift from 20 to 40% C3 vegetation (scrubland mixture of warm-season grasses and Acacia) to 100% C3 (wetland dominated by Typha) that occurred about 680F40 years BP (C-14 date from seeds). Soils developed on the periphery of the wetland show evidence for fluctuations in hydrologic budget, including siderite and redoximorphic features formed during wetter phases, and vertic (shrink–swell) and clay illuviation features developed during drier phases. Soils at Kesubo Marsh, located 2–3 km east of Loboi Swamp, consist of two buried mid-Holocene, 4000–4600 years BP (two C-14 dates from bulk SOM) Inceptisols developed from fluvially derived volcanic sand (SOM dC= –15x PDB) and separated from the latest Holocene surface soil (SOM dC= –17.5x PDB) by an unconformity and prominent stone line. Both the Loboi Swamp and Kesubo Marsh surface soils show increases in Zr, Fe, and S relative to buried soils, as well as higher leaching indices. Elevated Zr may reflect zircon

Preservation and Paleoenvironmental Significance of a Footprinted Surface on the Sandai Plain, Lake Bogoria, Kenya Rift Valley

An exhumed late Pleistocene land surface on the deltaic Sandai Plain north of Lake Bogoria, Kenya, preserves traces of bovids, suids, birds, and at least one hominid. The host sediments (Loboi Silts) are reddish brown, poorly bedded siltstones, mudstones and silty sandstones that were probably deposited in a shallow closed-basin lake. Most of the prints were impressed on exposed, moist lake-marginal mudflats. Print distribution is patchy due to a complex interaction between biogenic and sedimentological factors. The preservation of a single hominid track provides a fortuitous addition to the sparse hominid track record in East Africa. Field, petrographic, and mineralogical analyses of the fossil substrate were undertaken to determine how the footprinted surface was preserved. Comparison with modern lake-marginal processes suggests that the prints were initially stabilized by desiccation, soil-crusting, and organic films, followed by cementation of the surface sediments by calcite and analcime, with minor authigenic clay minerals and Fe-Mn-oxihydroxides. The zeolites formed by reaction of detrital silicates with saline, alkaline groundwater; calcite was precipitated from dilute runoff and fresher groundwaters. Cementation likely occurred during a prolonged period of relatively low, stable lake level. Following cementation, the surface was buried by Holocene lake sediments, then recently exhumed.

The geological history of Geysir, Iceland: a tephrochronological approach to the dating of sinter

The vertical succession through the discharge apron of Geysir, up to 3.5 m thick, is formed of five lithological units, A–E. Units A, B, C, and E are formed of siliceous sinters whereas Unit D is a brown ash bed. Tephra grains in units C, D, and E came from Katla, Hekla, and Veiðivötn. Katla tephra in Unit D has been largely altered to palagonite. Integration of tephrochronological ages, historical records, and geological and geomorphological relationships shows that the Geysir succession developed in four main phases. During Phase I, c. 10 000–4000 years ago, laminated sinters formed from hot spring waters. Phase II, initiated c. 3300 years ago, saw the termination of sinter formation and blanketing of the area with tephra from Katla and Hekla. Phase III, c. 3000–900 years ago, represented by the boundary between units D and E, was characterized by weathering and little sinter deposition. Phase IV was initiated with the (re)birth of Geysir 800–900 years ago. Sinter deposited from the discharged waters has, however, been largely confined to the southern part of the discharge apron. Only limited opal-A precipitation has occurred since 1916 because of a decline in Geysirs eruptions.

Environmental dynamics during the onset of the Middle Stone Age in eastern Africa

The Middle Stone Age in Africa The Olorgesailie basin in the southern Kenya rift valley contains sediments dating back to 1.2 million years ago, preserving a long archaeological record of human activity and environmental conditions. Three papers present the oldest East African evidence of the Middle Stone Age (MSA) and elucidate the system of technology and behavior associated with the origin of Homo sapiens. Potts et al. present evidence for the demise of Acheulean technology that preceded the MSA and describe variations in late Acheulean hominin behavior that anticipate MSA characteristics. The transition to the MSA was accompanied by turnover of large mammals and large-scale landscape change. Brooks et al. establish that ∼320,000 to 305,000 years ago, the populations in eastern Africa underwent a technological shift upon procurement of distantly sourced obsidian for toolmaking, indicating the early development of social exchange. Deino et al. provide the chronological underpinning for these discoveries. Science, this issue p. 86, p. 90, p. 95 Changes in fauna, landscapes, and climate were associated with novel adaptive behaviors in the earliest Homo sapiens. Development of the African Middle Stone Age (MSA) before 300,000 years ago raises the question of how environmental change influenced the evolution of behaviors characteristic of early Homo sapiens. We used temporally well-constrained sedimentological and paleoenvironmental data to investigate environmental dynamics before and after the appearance of the early MSA in the Olorgesailie basin, Kenya. In contrast to the Acheulean archeological record in the same basin, MSA sites are associated with a markedly different faunal community, more pronounced erosion-deposition cycles, tectonic activity, and enhanced wet-dry variability. Aspects of Acheulean technology in this region imply that, as early as 615,000 years ago, greater stone material selectivity and wider resource procurement coincided with an increased pace of land-lake fluctuation, potentially anticipating the adaptability of MSA hominins.

Diversity and distribution of macrophytes in a freshwater wetland, Loboi Swamp (Rift Valley) Kenya

ABSTRACT An inventory of Loboi swamp was undertaken to determine the macrophyte diversity and distribution. A total of 36 vascular plant species in 13 families were recorded, with Cyperaceae forming over 30% of macrophytes. Two vegetation zones were observed, characterised by the presence of Typha and papyrus. The Typha zone, comprising over 70% of the swamp, is dominated by T. domingensis and is species rich with 35 plant species whereas the papyrus zone includes the dominant Cyperus papyrus and only one other macrophyte species. Distribution of macrophytes is correlated with depth and period under water, with the Typha zone seasonally flooded while the papyrus zone is permanently under water at depths over 0.5m. Water chemistry has little influence on the distribution of macrophytes in the swamp, but at the edges there is predominance of Cyperus laevigatus in high alkalinity soils. Current uses of the swamp include dry season grazing, harvesting of papyrus and other plant material for mat making and house thatching, and use of the swamp water for domestic and irrigation agriculture. Further monitoring is needed to evaluate the effect of the resource uses on the swamp.

Late Holocene Environmental Reconstructions from Lake Solai, Kenya

Playa lake systems tend to be overlooked archives of paleoenvironmental change due to the likelihood of a short and intermittent record of deposition. Groundwater-fed wetlands associated with these climate-sensitive playas, however, preserve changes in hydrologic budget and are thus valuable archives for semiarid regions. This study examines the paleoecological record of a groundwater-fed wetland from Lake Solai, Kenya. Biological proxies are used to reconstruct paleoenvironmental change and climate impacts over the past millennium. Dry conditions persisted between CE 1115 and 1490, followed by wetter conditions during the Little Ice Age. Near surface sediments indicate increasing anthropogenic impact through pastoralism.

Progressive aridification in East Africa over the last half million years and implications for human evolution

Significance Previous research hypotheses have related hominin evolution to climate change. However, most theories lack basin-scale evidence for a link between environment and hominin evolution. This study documents continental, core-based evidence for a progressive increase in aridity since about 575 ka in the Magadi Basin, with a significant change from the Mid-Brunhes Event (∼430 ka). Intense aridification in the Magadi Basin corresponds with faunal extinctions and changes in toolkits in the nearby Olorgesailie Basin. Our data are consistent with climate variability as an important driver in hominin evolution, but also suggest that intensifying aridity may have had a significant influence on the origins of modern Homo sapiens and the onset of the Middle Stone Age. Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.