A multi-proxy palaeoenvironmental reconstruction of the Homa Peninsula, Western Kenya

Abstract

The Plio-Pleistocene of East Africa marks a crucial yet poorly understood period for hominin evolution. To better understand hominin activities from this time, the environmental substrate on which they resided must first be understood, as changes in the environment most likely influenced hominin evolutionary developments. Detailed palaeoenvironmental reconstructions throughout East Africa have been implemented using multiproxy approaches to address this. Such records thus far are spatially and temporally limited. This thesis aims to rectify this by producing a multiproxy palaeoenvironmental reconstruction of two sites on the Homa Peninsula, western Kenya – a novel palaeoenvironmental setting which hosts Plio-Pleistocene sedimentary sequences containing traces of hominin activity. These sites include Nyayanga (~ 2.6 Ma) and Sare River (~ 1.77 Ma). The multi-proxy approach implemented in this research encompasses analyses of stratigraphy, particle size and phytoliths to reconstruct site sedimentary dynamics, depositional environment and palaeovegetation. End-member mixing analysis is additionally utilised to ‘unmix’ multimodal particle size distributions and provide more detailed information on sedimentary dynamics. Nyayanga is interpreted as an alluvial plain environment upon which deposition occurred via episodic hyperconcentrated flows and intermittent unconfined fluvial flows. During flow hiatuses, secondary processes including fluvial runoff and aeolian deposition occurred, as well as stable land surface development. Higher energy hyperconcentrated flows became absent from the record throughout time, whilst unconfined fluvial activity became more infrequent. This was likely caused by a migration of the active sector of the alluvial plain. Bushy grasslands and grassy bushlands with infrequent sedges and woodland characterised the landscape during this time. An alluvial plain is also identified as the depositional environment for sediments at Sare River. Intermittent unconfined fluvial activity deposited sediments on gentle slopes. This activity became more infrequent throughout time and flow hiatuses, characterised by the occurrence of secondary processes and stable land surface development, became more frequent. This could be attributed to a migration of the ii active sector of the alluvial plain, or to aridification at the site. Bushy grasslands with infrequent woodland characterised the landscape here. Cosmogenic nuclide dating suggests sediments here could be younger than previously thought. An interplay of both regional and local tectonics as well as climate drove palaeoenvironmental change at both sites. The development of the East African Rift System created extensive space for the accumulation of sediment, whilst also altering base-level through the formation and destruction of palaeolakes. A variable climate regime influenced variations in deposition at Nyayanga. Both sites experienced an overall trend towards greater aridity throughout deposition. Changes in palaeoclimate at Nyayanga are attributed to the intensification of the Northern Hemisphere Glaciation, whilst the development of an intensified Walker Circulation is identified as the cause of aridification at Sare River. Environmental preferences of Paranthropus at Nyayanga ~ 2.6 Ma are similar to those presented at other East African sites. This suggests these hominins inhabited or frequented similar depositional settings in open landscapes characterised by bushy grasslands with infrequent wooded vegetation. Evidence from Nyayanga provides support for the pulsed-climate hypothesis linking environmental change to hominin evolution, which suggests that the long term drying trend observed in East Africa was punctuated by periods of extreme climate variability in which large lakes appeared and disappeared. During these periods evolutionary changes in hominins are suggested to have occurred. Sediments from Sare River provide support that hominin activity ~ 1.5 Ma thrived in open environments characterised by alluvial/fluvial deposition much like other East African sites. A trend towards greater aridity and an increase in hominin activity throughout the sediments at this site is interpreted. This suggests that hominin activity here might provide support for the aridity hypothesis linking palaeoenvironmental change to hominin evolution, which suggests progressive aridity across Africa initiated grassland expansion and the novel adaptations associated with these environments.