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Abstract

Geochemical differences in bulk rock geochemistry, mass balance weathering calculations and carbon isotope ratios of palaeosol organic matter between early Miocene palaeosols from the Kisingiri volcano are the result of both changes in the depositional regime and a shift in climatic conditions. Wayondo Formation palaeo-Alfisols located on Rusinga Island (Gumba Beds) and at nearby Karungu, which were deposited prior to the commencement of the main phase of volcanic activity (17.8 Ma), possess well developed clay horizons, moderate base cation contents and strong degrees of weathering. These palaeosols, when compared to their unweathered granitic parent exhibit collapse and loss of 80–90% of base cations and silica suggesting that their formation was during a period of landscape stability, when climatic conditions were humid to sub-humid. In comparison the overlying Kiahera Formation palaeosols, which were deposited during hydromaginatic activity at Kisingiri, exhibit a wide range of dilation and addition, together with individual elemental losses of up to 50%. Together, these weathering trends suggest that there was a climatic shift from wetter conditions to drier, semi-arid conditions, during the initial stages of the volcanoe's growth. This is supported by δ13C values of soil organic matter from A-horizons of the Wayondo Formation palaeosols (Rusinga Island −25.9‰ and Karungu −25.0‰) and the volcanically derived Hiwegi Formation (−23.2‰) and Kiahera Formation (−24.2‰) palaeosols. While all δ13C values are indicative of vegetation using the C3 synthetic pathway, δ13C values of the palaeosols that formed during the eruption (Hiwegi and Kiahera) show more 13C-enriched values compared with the pre-volcanic palaeosols (Wayondo). The reason for the relatively 13C-enriched values is suggested to be due to reduced fractionation due to seasonal or year-round water stress in an environment that had changed from humid (Wayondo) to sub-humid/semi-arid with the onset of volcanic activity. The greater abundance of calcareous material within these syn-eruptively formed fossil soils, compared with the pre-volcanic ones, also supports a change towards greater aridity. δ13C values of organic matter from the Kulu Lake Beds (in the Hiwegi Formation) have average δ13C value of −25.4‰. However, it is not possible to determine whether these values reflect the δ13C values of the terrestrial vegetation of the watershed or whether they are dominated by lacustrine-produced organic matter. Our data indicate that the well-developed Gumba and Karungu fossil soils of the Wayondo Formation represent part of a sustained stable alluvial system and had formed under humid conditions. With the onset of volcanic activity and the creation of a rapidly aggrading pyroclastic system, on which the Kiahera, Kulu and Hiwegi Formations of the Rusinga Group formed, changes occurred in the weathering regime and in the δ13C values of organic matter, suggesting a change towards greater aridity in the pre-versus syn-volcanically formed palaeosols.