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The structural and environmental history of the Suguta Valley, Northern Kenya Rift

Fachliche Zuordnung Paläontologie
Förderung Förderung von 2007 bis 2015
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 46753714
 
Erstellungsjahr 2015

Zusammenfassung der Projektergebnisse

We have proposed a comprehensive and multidisciplinary program to investigate the structural and environmental history of the Suguta Valley, Northern Kenya Rift in order to close an important gap in knowledge on the long-term history of the East African Rift. The valley is located in the transition zone between equatorial and monsoonal climate domains and therefore provides the possibility for important insights into spatio-temporal shifts in the relative influence of these domains on the East Africa moisture regime. The northern Kenya Rift also links the well-studied central Kenya Rift tectonic domain with the Omo-Turkana Rift and allows to expand the hypothesis of a major change in the tectonic stress field during the past three million years. Our middle Miocene to Recent structural record of the Suguta Valley shows two distinct phases of extension: a first phase characterized by monoclinal warping on the eastern rift flank in response to slip along a major, rift-bounding fault located along the western flank, and a second phase that has been active until the present day as documented by normal fault scarps affecting volcanic and sedimentary units that post-date the African Humid Period (15-5 kyr BP) lake highstand along both rift flanks as well as along its axial zone. By balancing the offsets of the highstand shoreline across multiple groups of faults in the Suguta Valley and South Turkana we were able to estimate extension rates along this segment of the Kenya Rift that range between 4.2 and 5.8 mm/yr. We furthermore investigated the relationships between tectonically created relief, rainfall, vegetation cover and the voracity of erosion by measuring cosmogenic 10Be-derived catchment-mean denudation rates across a range of hillslope gradients in the rift-shoulder environments, where quartz-rich lithologies are exposed. We find that millennial-scale denudation rates from sparsely vegetated parts are up to 0.13 mm/yr, while those from humid and more densely vegetated rift sectors reach a maximum of 0.08 mm/yr, despite higher mean hillslope gradients. These results imply that changing vegetation cover under present-day conditions can contribute to complex erosional responses to climate and that future comparisons with paleo-erosion rates will potentially yield rich future insights into the role of climate change and its impact on sediment production (Objective #1). The analysis of Holocene sediments and shorelines and their correlation with other East African chronologies clearly indicates the distinct difference between the summer monsoonal (in the north, as recorded by the Turkana and Ethiopian lakes) and tropical equatorial climate regime (as recorded by the Suguta Valley and lakes further to the south). The only explanation for this sharp spatial difference is a strong influence of topography on the seasonal migration of the Intertropical Convergence Zones (ITCZ) and the Congo Air Boundary (CAB) in East Africa. Our chronology of paleo-shorelines and the sedimentary record in combination with modeling results suggest that the termination of the AHP was gradual and not abrupt, in contrast to what has been suggested by others. During the AHP, however, we observe rapid variations of the lake level on time scales of a few decades or centuries, when the Congo Air Boundary, largely blocked by the East African and Ethiopian Plateaus today, longitudinally shifted across these plateaus multiple times. The Mid Pleistocene sequences of the Suguta Valley indicate predominatly lacustrine conditions in the Suguta Valley between ca. 900 and 600 kyrs ago, e.g., slightly after the Early-Mid Pleistocene Transition, but correlating with other lake records in the East African Rift System. During this time interval we found several episodes of a wetter climate lasting for about 10,000–20,000 years, as reflected by the occurrences of laminated undisturbed diatomites with abundant fish fossils with soft tissue preserved, indicating deep freswater lacustrine conditions. These humid periods were interrupted by episodes of a drier climate with approximately the same length. At this time scale, also tectonic processes shaping the basin's geometry has played an important role in eroding, transporting and depositing sediment in the Suguta Valley and therefore needs to be considered when interpreting the Mid Pleistocene lacustrine record, as the results of a provenance analysis based on strontium isotopes and XRF-based element compositions of the sediments suggest. The project received much attention in the media, for example: "Feuchtes Klima begünstigt die menschliche Evolution", by Rüdiger Braun, MAZ, July 2014. – "In der Welt der ersten Menschen", by Lars Abromeit, Jörn Auf dem Kampe, Christian Ziegler, GEO, March 2010. – "Wiege der Menschheit", by Ina Matthes, Märkische Oderzeitung, June 2009. – "Suche nach der Wiege der Menschheit", by Mario Beltschak, Badische Neuste Nachrichten, December 2008. – "In die Wiege der Menschheit geschaut", by Elke Partovi, Rheinpfalz, November 2008. – "Die große Stunde der Generalisten", original text by Jan Kixmüller, Potsdamer Neueste Nachrichten, September 2008. – "Rocking the cradle of humanity", original text by Beth Christensen, M. Maslin, Geotimes, January 2008. – "Knochenjob bei sengender Sonne", original text by by Bettina Micka, PORTAL, October 2007. We have also contributed to three TV and one radio productions: "Das Geheimnis des aufrechten Gangs", by Jo Siegler, ARTE, March 2012. – "Geological Journey II", by Nick Eyles and Kenton Vaughn, CBC Television Documentary, October 2010. – "Becoming Human", by Grahman Townsley, PBS/NOVA Televison Documentary, November 2009. – "Knochenjob im Norden Kenias", RadioEins Interview by Jörg Thadeusz, June 2009.

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