Palaeoenvironments of early Miocene Kisingiri Volcano Proconsul sites; evidence from carbon isotopes, palaeosols and hydromagmatic deposits

Early Miocene ( approximately 18 m.y.) hominoid sites from Rusinga Island, Lake Victoria, occur in palaeosols and volcaniclastic strata deposited in a semi-arid, seasonal climate on the flanks of the active, low-relief Kisingiri volcano, Kenya. Palaeosol organic matter and pedogenic carbonate from the Rusinga group have average delta (super 18) C values of -23.8 + or - 0.8 and -7.7 + or - 1.1 0/00, respectively, considerably heavier than average C3 vegetation. These isotopic values are attributed to reduce photosynthetic fractionation under water-stressed conditions. Pyroclastic surge features interpreted as the deposits of pyroclastic surge dune bedforms and ballistically emplaced volcanic ejecta, were both produced by powerful hydromagmatic explosions from Kisingiri volcano. The initial stages of this carbonatite-nephelinite volcano are thought to have had repeated episodes of hydromagmatic eruptions which built a large (15-20 km radius), low-relief tuff ring or maar volcano.