Understanding the climatic drivers of environmental variability (EV) during the Plio-Pleistocene and EV’s influence on mammalian macroevolution are two outstanding foci of research in African ...paleoclimatology and evolutionary biology. The potential effects of EV are especially relevant for testing the variability selection hypothesis, which predicts a positive relationship between EV and speciation and extinction rates in fossil mammals. Addressing these questions is stymied, however, by 1) a lack of multiple comparable EV records of sufficient temporal resolution and duration, and 2) the incompleteness of the mammalian fossil record. Here, we first compile a composite history of Pan-African EV spanning the Plio-Pleistocene, which allows us to explore which climatic variables influenced EV. We find that EV exhibits 1) a long-term trend of increasing variability since ∼3.7 Ma, coincident with rising variability in global ice volume and sea surface temperatures around Africa, and 2) a 400-ky frequency correlated with seasonal insolation variability. We then estimate speciation and extinction rates for fossil mammals from eastern Africa using a method that accounts for sampling variation. We find no statistically significant relationship between EV and estimated speciation or extinction rates across multiple spatial scales. These findings are inconsistent with the variability selection hypothesis as applied to macroevolutionary processes.
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.
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.
The carbon isotopic ratios of organic matter in fish fossils from diatomites and other lake beds in the HSPDP drill core from Tugen Hills, Kenya (2.56–3.29 Ma) reflect trophic resource uses and can ...indicate the dietary habitats of fish in the paleolake. This information offers insight into how fish communities responded to lake-level fluctuations during the Plio-Pleistocene in the East African Rift Valley. We have compared this record with fish fossil isotopes from both a previously published study of a Lake Malawi drill core (139 ka - present) and core top (modern ca 1978) samples collected at the water/sediment boundary from Lake Turkana (Kenya) of known environmental provenance. Both the Lake Malawi drill core fossils (−7.2‰ to −27.5‰ VPDB) and modern Lake Turkana samples (−16‰ to −24.6‰ VPDB) have δ13C values indicating a mix of near-shore and deep-water pelagic species. In contrast, the δ13C values for the Tugen Hills core fossils vary only between −20‰ and −27‰ VPDB. The absence of δ13C values greater than −19‰ suggests none of these fossils are derived from near-shore benthic habitats. The lack of shallow water, benthic lacustrine fish fossils through the Tugen Hills lake cycles may indicate that the rate of change from low-lake stands to deeper lake phases was very rapid, and shallow water communities were not established for long enough to leave a fish fossil record at the core site. These results strongly suggest that lake-level responses to climate variability in the Baringo Basin of the East African Rift were very abrupt during the Plio-Pleistocene transition.
Two cores were recovered from the Lake Magadi and Nasikie Engida Basins in the south Kenya Rift. Core MAG14-2A (194 m) contains a middle Pleistocene to Holocene record, whereas core NAS15/19 (4.36 m) ...covers only the late Holocene. Surficial sediments from springs and shallow-water sites were sampled in both basins. MAG14-2A rests on trachyte dated at 1.08 Ma. Diatoms are rare in the oldest sediments, but well preserved after about 545 ka, documenting a trend from less to more saline water. Core MAG14-2A contains fifteen facies, five of which are diatomaceous. In contrast, NAS15/19 is dominated by two facies, each containing well-preserved diatoms. Both sequences are distinct from others of similar age in the Kenya Rift in lacking pedogenic horizons, reflecting the location of Lake Magadi and Nasikie Engida in a tectonic sump where aquatic environments were maintained by geothermal and meteoric springs. Canonical Correspondence Analysis distinguishes three assemblages in the modern surface muds of Lake Magadi and Nasikie Engida, but with no pre-Holocene counterparts. Eleven diatom zones are recognised in MAG14-2A: Zones D2 to D10 contain rare to common diatoms dominated by
Aulacoseira granulata
and its varieties,
Aulacoseira agassizii, Thalassiosira faurii
,
Thalassiosira rudolfi
and
Cyclotella meneghiniana
. Individual samples commonly include a mixture of benthic and planktonic taxa and saline and freshwater species. These assemblages indicate waters that ranged between pH 7.4 and 11.5 with conductivities of ~ 300 to > 25,000 μS cm
−1
. Correlations with the neighbouring Olorgesailie and Koora Basins indicate four major environmental phases that affected the south Kenya Rift during the last million years with fresh to moderately saline water, or land surfaces, developing during Phase I (1000 to 750 ka). These environments gave way to generally wetter conditions and freshwater lakes in all basins during Phase II (~ 750 to 500 ka). Phase III (~ 500 to 325 ka) was characterised by drier conditions with paleoenvironments becoming more variable and dry during Phase IV (325 ka to present).
Three studies were performed in order to ascertain how climate variability in Africa may have influenced the speciation, extinction and dispersals of our early hominin ancestors since the ...Plio-Pleistocene. δ13C values from fish fossils from a Hominin Sites and Paleolakes Drilling Project Core (HSPDP) from Tugen Hills, Kenya were used to determine that local responses to climate shifts between 3.29 – 2.57 Ma were abrupt enough to limit the habitats of fish in the paleolake. Singular spectral analysis and correlation analysis on five paleoenvironmental records from Africa and one from NE Russia indicate that orbital parameters can explain most of the variance in climate records and that there are far reaching high-latitude teleconnections to African climate. An evolutionary algorithm run on reconstructed time-series from western, eastern and southern Africa allowed for environmental context of hominin evolutionary events to be made. Comparisons between the evolutionary model and the paleoanthropological record showed that during a period of relative stability in eastern Africa, 2.9 – 2.7 Ma, a drastic change in environment at 2.8 Ma would have elicited a response from a population of increased plasticity. This event coincides with a major turnover in the faunal assemblages as well as the first appearance of the genus Homo. New methods were developed in order to aggregate and compare paleorecords of different type and thereby quantify periods of high and low climate variability. Both the Standard Deviation Variability Method and the Accumulated Weighted Rate of Change Variability Method proved valuable in determining environmental variability in and around Africa over the last 400-ka. When the new methods were applied to environmental records and compared to the genetic, archaeological and fossil record, they indicated that the earliest appearance of the MSA occurred during a period of high variability in northern and eastern Africa. Furthermore, these methods showed that migrations within and out of Africa occurred after the source region switched from a period of high variability to a stable environment.
The multiproxy examination of ten soft sediment cores extracted from Triangle Pond was used in order to establish alterations in the northwestern coastal environment of San Salvador Island, The ...Bahamas over the last 3,500 years. Multiple paleolimnological techniques such as sediment identification through composition and color, grain-size analysis, Loss-on-Ignition, X-ray fluorescence and macrofossil identifications were used to identify eight separate depositional units. The different units represent alterations in the insular and surrounding environment of Triangle Pond through changes in coastal morphology, climate and sea level, anthropogenic activity and catastrophic events such as major storms and tsunamis. Triangle Pond began as a tidal creek when sea level rose and inundated an interdunal swale approximately 3,500 yr B.P. The tidal creek became increasingly influenced by both fresh water input from increased storm activity and seawater input from rising sea level over the next 1,500 years. As sea level regressed to present day levels, the protected marine environment of the tidal creek became overrun by mangrove swamp. The mangrove swamp was destroyed in one depositional event which closed the tidal inlet and created the current hypersaline to brackish algal dominated lake. The sedimentary record of Triangle Pond indicates that the driving factors behind the evolution of the coastal environment of San Salvador Island were fluctuations in Holocene climate and sea level changes.
Between 1977 and 1988, 22 patients underwent definitive repair for pulmonary atresia with intact ventricular septum. Fifteen underwent biventricular repair (mean age 24 months). All had mild to ...moderate right ventricular hypoplasia at the time of definitive repair. Repair consisted of closure of the atrial septal defect with enlargement of the right ventricular cavity and outflow tract with a patch in eight, insertion of a valved homograft in three, and superior vena cava-pulmonary artery connection in four. There was one operative death (7%). Seven patients had severe right ventricular hypoplasia at the time of definitive repair and underwent a Fontan procedure (mean age 46 months). In two a valved connection was made to the right ventricle and in five a right atrial-pulmonary artery nonvalved connection. There were two operative deaths (29%). Three patients had right ventricular-coronary sinusoids: Two of them underwent a Fontan procedure and one a biventricular repair. We conclude that with adequate early palliation a biventricular repair may be successfully performed for patients with mild or moderate right ventricular hypoplasia, and the Fontan procedure may be used for those with severe right ventricular hypoplasia.
PDF
