Development of the African Middle Stone Age (MSA) before 300 thousand years ago (ka) raises the question of how environmental change influenced the evolution of behaviors characteristic of early
We ...use 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 dramatically different faunal community, more pronounced erosion-deposition cycles, tectonic activity, and enhanced wet-dry variability. As early as 615 ka, aspects of Acheulean technology in this region imply that greater stone material selectivity and wider resource procurement coincided with an increased pace of land-lake fluctuation, potentially anticipating the adaptability of MSA hominins.
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.
Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which ...forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO3 waters containing little Ca (<2 mg l−1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO2 degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca2+ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.
Bicarbonate-rich source waters were needed to form the largest sodium carbonate evaporite deposits in the geologic record, the early and middle Eocene Green River trona (NaHCO3·Na2CO3·2H2O) in the ...Bridger basin, Wyoming, and nahcolite (NaHCO3) in the Piceance Creek basin, Colorado. Large modern and Pleistocene trona deposits are associated with magmatic activity and Na+-HCO3−-rich hydrothermal inflow waters, either within the depositional basin (Lake Magadi, Kenya) or at great distances (Searles Lake, California). No evidence exists for magmatic sources of CO2 near the Green River Formation. Several regional volcanic centers were active 300km or more to the north, but drainage reconstructions show that waters from these areas did not discharge into the Green River Formation lakes during evaporite deposition. Alternatively, Na+-HCO3−-rich waters could have drained northwestward from the Colorado Mineral belt to the Bridger basin via the proposed Aspen River. A river originating in the Colorado Mineral belt (Sawatch uplift) could also have provided source waters to the Piceance Creek basin. Field evidence, however, has not yet documented these flow paths, and specific Eocene volcanic centers and hydrothermal source areas have yet to be identified.
Other explanations for the elevated alkalinities needed to form thick sodium carbonate evaporites include accelerated silicate mineral weathering rates during a period of high atmospheric pCO2 and Eocene warmth. Amplified chemical weathering may explain the cluster of sodium carbonate evaporites in the USA and China that are Eocene in age. Another possible source of alkalinity to the Green River lakes is fault-controlled upward migration of a deep sedimentary source of CO2. The large amount of organic matter preserved in the evaporitic Wilkins Peak and Parachute Creek Members raises the possibility that decay of organic matter in Green River lakes could have added sufficient CO2 and alkalinity to produce waters capable of precipitating trona and nahcolite, although such degradation of organic matter has not created hyperalkalinity in modern lakes.
Opaline silica (opal‐A) has formed in marine, lacustrine and geothermal environments throughout geological time. During diagenesis opal‐A normally changes to opal‐CT, then opal‐C, and finally to ...quartz. Such changes commonly destroy the original fabrics and any fossils that opal‐A contained. The physical changes that accompany the opal‐A to opal‐CT transition, however, are known poorly. X‐ray diffraction analyses, electron microprobe analyses and high‐resolution, high‐magnification scanning electron microscope imagery of siliceous sinters from the Geysir geothermal area in Iceland show that opal‐A is formed of heterometric arrays of randomly packed microspheres (up to 5 μm diameter) with neighbouring spheres commonly being joined by small connection pads. In contrast, enlarged spheres, lepispheres, inverse opal (two types) and spindle frameworks with hexagonal motifs characterize opal‐CT. The textures in opal‐CT, which vary on a microscale, reflect the complex interplay between dissolution (e.g. inverse opal) and precipitation (e.g. enlarged spheres, spindle frameworks) that probably was mediated by groundwater in a near‐surface environment. The processes deciphered from these young rocks should, however, be applicable to sedimentary opal‐A and opal‐CT of all ages, irrespective of their origin.
Although climate change is considered to have been a large-scale driver of African human evolution, landscape-scale shifts in ecological resources that may have shaped novel hominin adaptations are ...rarely investigated. We use well-dated, high-resolution, drill-core datasets to understand ecological dynamics associated with a major adaptive transition in the archeological record ~24 km from the coring site. Outcrops preserve evidence of the replacement of Acheulean by Middle Stone Age (MSA) technological, cognitive, and social innovations between 500 and 300 thousand years (ka) ago, contemporaneous with large-scale taxonomic and adaptive turnover in mammal herbivores. Beginning ~400 ka ago, tectonic, hydrological, and ecological changes combined to disrupt a relatively stable resource base, prompting fluctuations of increasing magnitude in freshwater availability, grassland communities, and woody plant cover. Interaction of these factors offers a resource-oriented hypothesis for the evolutionary success of MSA adaptations, which likely contributed to the ecological flexibility typical of
foragers.
Magadiite, a rare hydrous sodium-silicate mineral NaSi
O
(OH)
·4(H
O), was discovered about 50 years ago in sediments around Lake Magadi, a hypersaline alkaline lake fed by hot springs in the ...semi-arid southern Kenya Rift Valley. Today this harsh lacustrine environment excludes most organisms except microbial extremophiles, a few invertebrates (mostly insects), highly adapted fish (Alcolapia sp.), and birds including flamingos. Burrows discovered in outcrops of the High Magadi Beds (~25-9 ka) that predate the modern saline (trona) pan show that beetles and other invertebrates inhabit this extreme environment when conditions become more favourable. Burrows (cm-scale) preserved in magadiite in the High Magadi Beds are filled with mud, silt and sand from overlying sediments. Their stratigraphic context reveals upward-shallowing cycles from mud to interlaminated mud-magadiite to magadiite in dm-scale units. The burrows were formed when the lake floor became fresher and oxygenated, after a period when magadiite precipitated in shallow saline waters. The burrows, probably produced by beetles, show that trace fossils can provide evidence for short-term (possibly years to decades) changes in the contemporary environment that might not otherwise be recognised or preserved physically or chemically in the sediment record.
Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes ...that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions.
Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods.
Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.
•Progressive increase in palaeolake alkalinity and salinity over the past million years.•Middle Pleistocene to Holocene increase in aridity was interrupted by climate-induced wetter episodes.•Diatom floras indicate episodically meromictic palaeolakes.•Step-like changes in sedimentation reflect faulting and drainage diversion.•Faulting tapped deep groundwater releasing Si-enriched deep fluids and mantle CO2 to produce abundant chert and thick trona deposits.
A ~42 m section of Late Cretaceous Upper Campanian sediments in Saskatchewan Landing Provincial Park, southwestern Saskatchewan, Canada, represents the easternmost outcrop of the Dinosaur Park ...Formation in the Western Interior Basin. Herein we document a new microvertebrate locality from the upper part of this formation that shows high diversity in a mixed coastal and marine assemblage. Palynology, ichnology, sedimentology, and vertebrate palaeontology are integrated to determine paleoenvironmental and paleoecological conditions in the region. The site is interpreted as having been deposited under marginal-marine conditions near a shoreline undergoing transgression by the encroaching Bearpaw Sea. Though well studied and sampled in Alberta, the Dinosaur Park Formation is poorly exposed with little known associated vertebrate assemblages in Saskatchewan. These discoveries from the new microvertebrate site offer new insights into Late Cretaceous ecosystems near paleocoastlines, allowing for future studies of spatial diversity patterns relative to Albertan faunas. Herein is also presented the first published occurrences of several Late Campanian vertebrate taxa in Saskatchewan.
•Presents the first description of a Campanian-aged (Late Cretaceous) terrestrial microvertebrate site in Saskatchewan•Several described taxa extend the spatial range of species known from the Dinosaur Park Formation in Alberta.•Several lines of evidence suggest the site was deposited in a marginal marine environment undergoing regional transgression.•Illustrates how microvertebarte sites can be a source of as paleoenvironmental information
Located in southern Iceland, Geysir was first named in 1647 and thereafter became the eponym for geysers that are known from geothermal areas throughout the world. As noted by many early explorers, ...Geysir has a long history of erratic eruptions with periods of activity alternating with periods of dormancy. Vertical sections through the discharge apron, exposed in the valley cut by Geysisgil along its western and northern margins, reveal complex sequences of interbedded spring deposits, volcanic ashes, and soils that provide a historical record of the discharge apron development. The basal beds are formed of opal-CT whereas the higher beds are formed of opal-A. Before Geysir became active the area was covered with volcanic ashes that resulted from the eruptions of Katla and Hekla ~3300 to 2850 years ago. Once Geysir became active, precipitation of opal-A from the geothermal waters that flowed across the area around the geyser led to development of siliceous sinters. Today the triangular shaped discharge apron around Geysir is ~200 m long and ~200 m wide with the vent pool located in the southwest corner. Its surface is divided into a southern sinter area, northwest vegetated area, and northeast broken sinter area. Precipitation of the opal-A is controlled by the large volumes of water that are episodically ejected from the geyser and water flow patterns across the discharge apron. Such waters cool rapidly, especially during the winter when air temperatures may be as low as −15 °C. Facies evident in the siliceous sinter that formed on the surface of the discharge apron include thinly laminated sinter, microbial sinter, flakey sinter, and plant sinter. Microbial sinters are generally rare in proximal areas but common in distal parts of the apron. Silicified birch leaves and wood are common, especially in areas where the water flows through the vegetated areas of the discharge apron. The unique facies architecture and topography of the discharge apron around Geysir, which developed in response to the interaction between many different variables, provide a depositional model that can be used in the interpretation of ancient geothermal deposits.
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