U21A-0701 0800h
The Lake Tanganyika Accommodation Zone Structural Highs: Probable Archive of Continuous Miocene to Recent Paleoenvironmental and Paleoclimatic Information for East Africa
Continental drilling of lake deposits has proven an important source of high-resolution paleoclimatic and paleoenvironmental information. The large rift lakes of East Africa react dramatically to climatic perturbations, recording responses in sedimentary records of lake level, chemistry, and biota. No continuous continental paleoclimate records covering the full period of hominid evolution, especially the critical transitions of the late Miocene through the Plio-Pleistocene, are currently available or forthcoming. However, approximately 1000 km of sparker and air gun reflection seismic profiles collected during a number of field campaigns on Lake Tanganyika demonstrate the existence of three major mid-lake isolated structural highs: from N-S the Ubwari, Kavala Island, and Kalya horsts, whose sedimentary cover may provide records of this critical interval. Several coring campaigns demonstrated sedimentation rates over the last 100 ka much slower than adjacent basinal settings, in some cases as low as 0.1 mm/year. Sequence stratigraphic analyses of sediments on the shallower (300-500 m) of these horsts (Ubwari and Kavala Island), and on other structural platforms have shown the presence of numerous unconformities related to lake level fluctuations and paleoclimatic variability. During the Last Glacial Maximum, for example, features such as prograding delta lobes and paleochannels indicate water levels may have fallen by as much as 360 meters. Erosional unconformities at depths of as much as 600 m have been noted at basinal sites adjacent to these relatively shallow horsts. The northeastern edge of the Kalya horst, however, lies at sufficient water depths ($>$ 600 m) to have escaped these major erosional truncations. Furthermore, this site is located in a depositional environment of relative tectonic quiescence, apparently undisturbed by faulting, unlike the northern structural highs. Preliminary seismic stratigraphic analysis of the Kalya horst shows the presence of at least 300 m of continuously accumulated sediment. Cores from the region, spanning the late Pleistocene and Holocene, have shown that the sediments are partially laminated and contain a wealth of geochemical and paleoecological indicators of glacial-interglacial to millennial-scale hydroclimatic fluctuations. Given the age of Lake Tanganyika (10-12 Ma) and the highly continuous nature of sedimentation on the deep accommodation zone, the Kalya horst has the potential to provide a continuous and readily-interpretable record of paleoclimate history over much, if not all, of the critical phases of hominid evolution in East Africa.
U21A-0702 0800h
Extensional Tectonics and Paleoclimate of the Albertine and Edward Rifts: Constraints from Integrated Seismic Reflection and Topographic Studies
Lakes Albert and Edward are among the largest lakes in Africa and form substantial reservoirs for tropical precipitation within the Upper Nile Watershed (UNW), the equatorial headwaters supporting annual discharge of the main Nile River into the Mediterranean Sea. Extensional processes forming Lakes Albert and Edward have driven landscape evolution, manifested in significant topography and reversed drainages networks. Consequently, the UNW, including Lakes Albert, Edward, Victoria, and Kyoga, and their riverine connections, provide an important case study for drainage basin evolution in actively extending continental lithosphere. In addition to multiple scales of tectonic deformation, high frequency climate change affects the basins in the UNW, which record moisture signals from both the Indian and Atlantic Oceans. Paleoclimate proxy data indicate the desiccation of Lake Victoria and cessation of Victoria Nile flow during the Last Glacial Maximum (22 - 18 ka). However, the response of Lakes Albert and Edward during this time period is not fully resolved. In 2003, 1600 line km of multichannel seismic reflection and high resolution echo-sounder data were collected from Lake Albert, while 200 km of 1 kHz seismic reflection data were collected from Lake Edward. In conjunction with recently released digital elevation models, we investigate both tectonic and climatic processes operative in the basins. In Lake Albert, we are able to map depth to pre-rift basement, basin boundary fault as well as intrabasinal fault geometries that control both bathymetry and sediment distribution. Shallow reflections suggest desiccation of the lake, probably during the LGM; these results tie well with published sediment core data. Acoustic data from Lake Edward record two regression events, although this lake apparently escaped LGM desiccation, and may contain a complete, high-resolution archive of Quaternary climate change in its deep-water sequences. Our interpretations provide another incremental step toward a more complete understanding of Lakes Albert and Edward. These acoustic data represent a new opportunity to resolve important questions regarding the long term (ca. 10 Ma) tectono-climatic evolution of region, which can be used as a baseline for constraining environmental conditions during hominid evolution in East Africa.
U21A-0703 0800h
$^{40}$Ar/$^{39}$Ar Age Calibration of the Lacustrine Sediments at Kariandusi, Central Kenya Rift
The Kariandusi site in the Nakuru-Elmenteita Basin, central Kenya Rift, contains a 31 m thick sequence of well-stratified diatomites and tuffs. It is significant in the overall late Cenozoic climate picture of East Africa in that Kariandusi is one of three sites (along with Olorgesailie, southern Kenya, and Afar, Ethiopia) that record a major wet period at $\sim$1000 ka. The diatomite assemblage records the complete history of a large, deep lake, from lowermost units containing benthic-epiphytic flora, followed by deep-lake genera, and succeeded in the higher parts of the section by an assemblage that indicates lake regression with higher alkalinity. The diatomite rests directly on the widespread Gilgil trachyte lava flows, dated at Kariandusi by laser $^{40}$Ar/$^{39}$Ar total-fusion of anorthoclase phenocrysts at 0.985 $\pm$ 0.003 Ma (1$\sigma$).The main diatomite is conformably overlain by a 0.7 m thick tephra horizon that apparently represents a direct airfall into the lake. The age of this unit by single-crystal $^{40}$Ar/$^{39}$Ar dating of anorthoclase is 0.973 $\pm$ 0.003 Ma. It is succeeded by an intercalated sequence of diatomites and tephra about 4 m thick. Previous $^{40}$Ar/$^{39}$Ar dating (Deino and Potts, 1990, JGR 95:B6: 8453-8470) indicates that the Kariandusi diatomite sequence was deposited at approximately the same time as lacustrine sediments in the lower part of the Olorgesailie Formation (bracketed between 1000-980 ka), 135 km to the south within the Kenya Rift. Limited tephra correlation based on trace-element analysis of glass separates suggests that the Olorgesailie lacustrine sequence may be equivalent to the upper part of the Kariandusi diatomite sequence, or slightly younger. Further work, including dating, tephra correlation, and diatomite assemblage studies, may eventually permit precessional-level assignment of the lacustrine phases at the two sites.
U21A-0704 0800h
Late Pleistocene Paleoclimate Record From the SW African Margin
Late Pleistocene sediments recovered from ODP Leg 175, Site 1085 are used to generate a high-resolution (500 yr) record of continental climate change in Southern Africa. Sedimentological, geochemical, and clay mineralogical variations of the Cape Basin sediments for the last 200 k.y. are used to determine transport pathways of terrigenous load and continental climate change in southern Africa. The location of Site 1085, the SW African continental slope, provides a continuous hemipelagic section with a significant terrigenous component. Terrigenous sediments are either fluvial or eolian, with fluvial load possibly being controlled by monsoonal circulation and insolation (Christensen et al., 2002; Murray et al., 2002) and eolian by glacial intensification of trade winds. Analyses, including grain size (transport), color reflectance (terrigenous input), biogenic sediment geochemistry including %CaCO3, % organic carbon, and C:N (terrigenous input, productivity), bulk sediment geochemistry (continental climate, terrigenous sediment source, and productivity), benthic foraminiferal stable isotopes (chronology), and clay mineralogy (source, continental climate, transport pathways), are used to identify continental climate conditions in southern Africa, and to address possible forcing mechanisms of climate change in the region during the last 200 k.y. Analyses indicate significant glacial and interglacial variation. Color reflectance and %CaCO3 covary down section, with highest values during MIS1, while there is an inverse correlation between color reflectance, median grain size and C:N ratios, with greatest terrigenous carbon during MIS 1. Peaks in median grain size are associated with boundaries between MIS 1 and 2 as well as MIS 2 and 3. Our results differ from Stuut et al. (2002), who find increased eolian sediments from grain size within glacial stages on the Walvis Ridge, north of the Cape Basin, rather than at the boundaries. Clay mineralogy, determined after removal of calcite, indicates the presence of smectite, different from the modern regional maps of Petschick et al. (1996). Presence of smectite downcore may attest to increased continental weathering and possibly a more humid climate in Southern Africa.
U21A-0705 0800h
Plio-Pleistocene climate-driven facial morphology in southern African australopithecines
Intensification of Northern Hemispheric Glaciation (INHG) has been cited as the driving force for climate change in southern Africa, despite the fact that rare and fragmentary continental records for the region allow for only a loose correlation between local faunal events and global climate change. Determining climatic influence from the limited climate history is exacerbated by the difficult chronology for the cave sites. The depositional history of the caves nonetheless reveals a turnover of southern African mammals, and variability, among other forcing factors, have implicated the INHG to explain the evolution of these novel forms. We suggest that evolutionary changes in southern African fauna may have also been driven by a subsequent climate event, the Onset of Walker Circulation (OWC) at ~ 2 Myr. The OWC, with enhanced high frequency climate variability, may have been more dominant than INHG in driving southern African mammalian evolution. For example, Pleistocene Australopithecus robustus, but not Pliocene Australopithecus africanus, exhibits relatively broad palates, postcanine megadonty and deep mandibular corpora that correspond to a dietary niche involving heavy mastication. These adaptations may have been selected for in unstable Pleistocene environments initiated by the OWC. Moreover, the temporal context of Australopithecus robustus, found in the Pleistocene dated caves of Swartkrans, Kromdraai and Drimolen, coincides more closely with the OWC than with the INHG.
U21A-0706 0800h
Connecting Local Environmental Sequences to Global Climate Patterns: Evidence From the Hominin-Bearing Hadar Formation, Ethiopia.
Central to the debate surrounding global climate change and Plio-Pleistocene hominin evolution is the degree to which large-scale climate patterns influence low-latitude continental ecosystems and how these global influences can be distinguished from regional tectono-volcanic events and local environmental effects. These isolated factors must then be compared to trends or events in the hominin fossil or archaeological record. The Pliocene Hadar Formation preserves a high-resolution record of hominin paleoenvironments from roughly 3.5 to 2.3 Ma. The chronostratigraphic framework at Hadar, often divisible into sub-50,000 year intervals, provides temporal resolution relevant to evolutionary change within hominins and other taxa. Combined with a high sedimentation rate and abundant faunal remains, this tightly controlled stratigraphic framework allows Hadar to be compared to detailed climate proxies such as marine core isotope, dust, and sapropel records. Preliminary analyses from Hadar suggest that these comparisons can be used to distinguish local versus regional and global environmental change, determine the overall sensitivity of the Hadar system and its fauna to such change, and how this level of sensitively compares to other East African localities. Consistent cycling observed both between and within fluvial and lacustrine depositional environments prior to 2.9 Ma at Hadar appears to be dominantly climatic in nature. However, a significant change in depositional facies after 2.9 Ma to sequences dominated by conglomerate cut-and-fill cycles indicates a strong tectonic signature related to regional developments in the Main Ethiopian Rift (MER). While specific events seen in marine proxy records may have parallels in the Hadar environmental archive, their overall patterns of high versus low variability may be even more relevant. For example, periods of relatively high amplitude climate oscillations between 3.15 and 2.95 Ma may be linked to noted morphological changes within the Hadar australopithecine lineage. Similarly, high-resolution records such as those preserved at Hadar can be used to test theories of hominin evolution such as habitat-specific versus variability selection.
U21A-0707 0800h
Discriminating Between Tectonic and Climatic Controls on Early Hominin Paleoenvironments From the Koobi Fora Region, Northeastern Turkana Basin, Kenya: Part II
Global climate is often elected as a catalyst for environmental change and used to characterize selective pressures acting on Plio-Pleistocene African hominins. Vrba's Habitat Theory (1992) and Pott's Variability Selection (1998) credit mammalian evolutionary pattern and process to global climate regulated by orbital forcing. Feibel (1999: 276) argues the need for a middle ground, tethering the "global-scale climatic phenomena" to "environmental change, habitat shift, and biotic evolution" and offers the basin as a scale for analysis. Feibel suggests that all basins are not created equal, and will respond to climate change with different sensitivities and thresholds. As such, interpretations of climate proxies must account for differences in basin size, climatic regime(s), topography, geology, and water availability when drawing relationships to global phenomena. Here we examine pedogenic carbonate isotopes (d13C, d18O) from the Plio-Pleistocene Koobi Fora Region to elucidate the differential influences of climate, tectonics, and deposition on ecological factors of early hominin evolution in the northeastern Turkana Basin of Kenya. One of the richest Plio-Pleistocene fossil localities in Africa, Koobi Fora has served as a setting for hominin evolution between 4.0 and 1.0 Ma. Numerous paleosols, stratigraphically controlled by tuffaceous marker beds, are preserved in the Plio-Pleistocene sediments of the Koobi Fora Formation. Cerling and others (1988) and Wynn (2000) conducted isotopic studies of pedogenic carbonates from the Plio-Pleistocene Omo Group deposits of the Turkana Basin. With these data Wynn (2004) demonstrates stepwise d13C shifts over the last 4.0 Ma, with marked events at 2.5 and 1.8 Ma, and interprets increased aridity on a basin scale due to comparable records on the east and west side of present Lake Turkana. In this study, we increased the sample size of the current database and conducted widespread sampling of synchronous lateral horizons in the Koobi Fora Formation emphasizing 2.0 to 1.5 Ma. With attention to paleohydrology, we partitioned the basin by proximity to the ancestral Omo River system, which was the main control on basin-wide hydrology and deposition during the Plio-Pleistocene (e.g. Feibel et al., 1991). We interpret the high variability of our isotopic results from synchronous units, spanning the C3-C4 range, as indicative of ecotonal variability complemented by the paleogeographic reconstructions. Change in mean isotopic ratios across members, interpreted as increased aridity (Wynn, 2004), occurred during a period of depositional reorganization due to a major tectonic event and a diachronic decrease in basinal accommodation space. This reorganization is recorded in the Koobi Fora Formation as an up-section progression from deep-lake, delta, shallow-lake, to river and floodplain depositional facies. We conclude that the tectonic regime of the Plio-Pleistocene Turkana Basin is equally important as global climate change for the distribution of vegetation on the hominin landscape in the Koobi Fora Region. The preservation of ecotonal gradients is highly dependent on the ability of environmental perturbations to emplace a dynamic depositional geography at the basin-wide, regional, and landscape scale.
U21A-0708 0800h
Discriminating Between Tectonic and Climatic Controls on Early Hominin Paleoenvironments From the Koobi Fora Region, Northeastern Turkana Basin, Kenya: Part I
Understanding controls on environmental records from Plio-Pleistocene Africa is critical for interpreting human origins. Recent geological studies from East Africa have focused on the relationship between Plio-Pleistocene patterns of hominin evolution, environmental change, and climate preserved in stratigraphic records of sedimentary basins (e.g. deMenocal, 2004; Wynn, 2004). Despite the fact that tectonics is a primary control on sedimentation in East African basins (e.g. Baker, 1986; Frostick, 1997), relatively few studies have either investigated its potential influence on early hominin evolution or attempted to discriminate between tectonic and climate controls on paleoenvironmental change. Presented is a study that explores these issues. Within the Koobi Fora Formation, between 4.0 and 2.5 Ma, environmental change is related to an overall trend of linear rates of tectonic subsidence. However, smaller-scale fluctuations in subsidence rates established lakes during times of increased subsidence followed by the transition to rivers during times of decreased subsidence and basin infilling (Feibel, 1994a, 2000). In contrast, environmental change during the period between 2.5 and 1.5 Ma was forced by changes in half-graben propagation, fault movement, and subsidence. This change is recorded within a stratigraphic sequence that is defined by major (erosional) boundary surface unconformities. The sequence is internally comprised of stable-lacustrine; stable-lacustrine, delta, and ephemeral-lacustrine; and fluvial environments of deposition. This environmental progression defines lowstand, transgressive, and highstand systems tracts respectively. Transition between systems tracts and depositional environments was controlled by rates of tectonic subsidence. The formation of stable-lacustrine environments of deposition during the lowstand systems tract was due to subsidence rates out-pacing sedimentation rates that was associated with a major tectonic event, characterized by uplift in Ethiopia and associated down-warping of the Turkana Basin in Kenya. While the formation of ephemeral-lacustrine and fluvial environments of deposition during the transgressive and highstand systems tract was due to sedimentation rates out-pacing subsidence rates that was associated with the infilling of the stable lake by deltaic environments. Infilling and depletion of accommodation space were primary steps needed to increase the sensitivity of the basin to climate change. Relatively shallow East African sedimentary basins are more sensitive to climate-related changes in evapo-transpiration than relatively deeper basins (e.g. Feibel, 1999). Transgressive and regressive lake margin facies that are associated with the transgressive systems tract suggest that the base level of the ephemeral-lacustrine system periodically oscillated during 2.0 and 1.5 Ma (Feibel, 1994b). The hominin record of Koobi Fora preserves relatively primitive forms between 4.0 and 2.5 Ma and relatively derived forms after about 2.5 Ma (cf. Walker, 2002). Australopithecines are associated with paleoenvironmental change that was forced by linear subsidence rates. The appearance of paranthropines in the region at about 2.5 Ma (cf. Wood, 1991) is associated with an episode of major faulting and paleoenvironmental change that was forced by an exponential decrease in subsidence rates. The appearance of early African Homo erectus in the Koobi Fora Region at 1.8 Ma (e.g. Wood, 1992; Collard and Wood, 1999) coincides with decreased subsidence rates but an increase in the sensitivity of paleoenvironmental organization in the region to climate change.