U13A-01 13:45h
Complex Records of Environmental Character and Change in Neogene African Rift Basins
The Neogene rift basins of East Africa preserve complex environmental records encompassing the effects of tectonics and climate along with the influences of volcanism, landscape succession and biotic change. Detailed reconstruction of individual basin evolution, along with the integration of parallel basin histories can allow the isolation of controlling factors, particularly when they are represented by distinctive signatures. Major Neogene basins, such as the Turkana and Middle Awash systems, preserve long and detailed records, with critical correlation links based on tephrostratigraphic ties, isotopic age determinations and magnetic polarity stratigraphy. Smaller basinal systems, such as Olduvai, Konso and Olorgesailie often reflect shorter accumulation histories, but may add crucial data for distinguishing the influence of global climate, regional tectonics, and local environmental effects. Miocene sedimentary records provide important baseline data on environmental conditions across the African continent, including major tectonic effects initiating subsidence along with strong local climatic influences. Plio-Pleistocene records are far more complete, and demonstrate both short-term (seasonal) as well as long-term (Milankovitch scale) climatic influences superimposed on regional tectonic controls and episodically overwhelmed by explosive volcanic signals. Depositional and post-depositional features of these sedimentary sequences directly reflect habitat character and change through crucial intervals of development in African terrestrial ecosystems. These continental archives preserve direct evidence of the environmental patterns which influenced crucial steps in hominin evolution, the appearances, adaptation, behavioral innovation and extinctions of the hominin clade.
U13A-02 INVITED 14:05h
Late Cenozoic Moisture History of East Africa
Evidence from fluvio-lacustrine sediments in ten separate basins in the Ethiopian and Kenya rifts suggests there were three protracted humid periods during the Late Cenozoic; at 2.7 - 2.5, 1.9 - 1.7, and 1.1 - 0.9 million years before present. These wet periods are coeval with known increases of aridity in parts of North West and North East Africa, indicating significant regional shifts in African climate. These three East African wet periods correspond to major global climatic changes as well as maxima in eccentricity and thus precession, suggesting a combined global and local causation. These climatic changes were important for the speciation and dispersal of mammals and hominids in East Africa as it implies that key steps in human evolution occurred during relatively humid periods in a region containing extensive deep lakes.
U13A-03 14:25h
Wetlands as a Record of Climate Change and Hydrological Response in Arid Rift Settings
Of all the terrestrial depositional settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. Lake deposits were the only rift component studied for records of long-term climatic change and for testing hypotheses of orbital forcing. Recently, the continuing quest for the paleontological and cultural records of human origins entombed in the sedimentary rocks of the East African Rift System raised questions concerning hydrologic and biologic response to climatic change. Additional issues are the impact of climate on paleolandscapes and the environmental stresses that might have affected human evolution. Other important indicators of rift hydrology, such as springs and wetlands are now emerging as viable records of climate change. Rift valley basins are shallow, hydrologically closed systems that are responsive to shifts in climate, and specifically sensitive to changes in the hydrologic budget (P-ET). Long term wet-dry cycles in the low latitudes are thought to be astronomically controlled, i.e. Milankovitch precession cycles (19-23 ka). In the tropics, precipitation (P) varies with changes in solar insolation which fluctuates <8-10 % over a cycle. Stronger insolation drives stronger summer monsoon maxima increasing P. Mean annual temperatures are high, but evapo-transpiration, ET (~ 2500 mm/yr) varies little. Consequently, during wetter periods regional groundwater reservoirs enlarge, the water table rises and springs and wetlands increase in number and in size compared to drier periods. Lake levels are known to fluctuate in response to change in hydrologic budget and wetlands appear to respond similarly. Springs and groundwater-fed wetlands are common, however the sources and sustainability of water or what geologic factors lead to the formation and longevity of wetlands is not well established. It appears that rainfall is trapped on topographic highs (rift fault blocks and volcanoes). This meteoric water infiltrates quickly through porous volcanic rocks and is stored in aquifers and released slowly. As a component of the rift hydrologic system, wetlands appear to be reliable indicators of rainfall fluctuations on both Milankovitch and sub-Milankovitch time scales. Wetland sediments are commoner in the geologic record during times of higher rainfall and are less common during drier periods. Modern arid rift wetland records are peats and organic-rich clay deposits that contain eolian-transported mineral matter, plant remains (e.g. roots, stems) pollen, phytoliths, diatoms, root casts, charcoal, carbonate and manganese-rich nodules, as well as copious evidence of bioturbation (plants and invertebrates to large vertebrate trampling). Older (Pleistocene) deposits that retain little original organic matter and plant remains are generally silicified, but otherwise the record is similar to modern wetlands. Records from Olduvai Gorge (1.85-1.75 Ma) contain springs and wetlands associated with stone tools. On shorter time-frames, a drought occurred during the Medieval Warm Period (MWP) and higher rainfall in East Africa during the Little Ice Age (700 BP) led to higher lake levels of Lake Naivasha and Lake Turkana (Mohamed et al. 1995; Verschuren et al. 2000) and the simultaneous initiation and expansion of Loboi Swamp in Baringo-Bogoria basin (Ashley et al. 2004). Therefore, a wetland as an indicator of climate change even on millennial-scale cycles is a viable option to lakes as paleoclimatic indicators in arid, low-latitude continental settings. Groundwater reservoirs provide a perennial water source for plants and animals (including hominins) in what might otherwise be a parched environment.
U13A-04 14:45h
Is orbital forcing of the local environment the forgotten cause of human evolution?
Orbital Forcing has two major influences on African climate: first it modulates global climate and second it directly influences the local climate. A lot of work has focused on orbital forcing of global climate for example glacial-interglacial cycles. Indeed the interaction between the different orbital parameters and the climate system are implicit in key global climate transitions; such as the onset of Northern Hemisphere Glaciation (3.2-2.5 Ma), alteration of the Walker circulation (~2.0 Ma) and the Mid-Pleistocene Revolution (~1.0-0.6 Ma). Less attention has been paid to the dominant role precession (cyclicity of 23 kyrs and 19 kyrs) has on the local African climate. For example over the last 8 million years eccentricity modulated precession has varied the insolation on the 21st March at the equator from minimum of 390 to a maximum 490 W/m2, a variation of nearly 25 percent. Within the East African Rift valley the long and short rainy seasons are strongly controlled by April-March and October-November insolation, respectively. Hence without any other influences orbital variations can have a huge effect on the moisture availability of East Africa. Because orbital parameters are cyclic the rate of change derivative contains two periods of very rapid change. 60 percent of all the variation within a precessional cycle is concentrated into two 2 kyrs periods. If this is then combined with the localised threshold effect of the Rift Valley on rainfall, then large-scale localised climate variations can occur on the time-scale of an individual. We suggest that eccentricity modulated precession provides the regularly rapid large environmental changes in Eastern Africa which may have had a strong influence on human evolution. This mechanism may provide a repetitive forcing essential for brain expansion and behavourial flexibility implicit in the Variability Hypothesis.
U13A-05 15:05h
Understanding Chad Basin Evolution Since Miocene: Climate and Vegetation Simulations, Roles of Orbital Parameters and East African Rift.
Since the discovery of the earliest hominid known, Chad basin is a major place to study paleoclimates and hominid evolution. This discovery implies to re-evaluate the "East Side Story" paradigm for early hominids. To achieve this goal, we have performed numerical simulations to quantify the climatic and vegetation response of the Rift Uplift. We used a zoomed (144 X 108) AGCM (LMDz from IPSL). On the one hand, offline continental biosphere model (ORCHIDEE) has been used to simulate the vegetation response over western and eastern parts of the rift. On the other hand, since geomorphologic evidences have shown that from Upper Miocene to mid-Holocene Lake Chad had known several level oscillations leading to a huge lake known as Mega Lake Chad (MLC), we also ran atmospheric simulations to demonstrate, with boundary conditions at 6 000 BP, that orbital forcing allowed the existence of a MLC. Volume and surface of the lake have been calculated using an adapted lake model. These simulations have shown that the ITCZ shift induced by the mid-Holocene orbital parameters drives the existence of a MegaChad. Our model result having been tested successfully for the last occurrence of the MLC, we will apply it to Upper Miocene accounting for topographic changes, in order to reconstruct as accurately as possible the first hominids environments.