Isotopic and Trace Element Evidence from Lavas, Bearing on Mantle Heterogeneity Beneath Kenya

Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease at different rates. The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed

Holocene environmental changes in the lower Thames Valley, London, UK: implications for our understanding of the history of Taxus woodland

A radiocarbon-dated multiproxy palaeoenvironmental record from the Lower Thames Valley at Hornchurch Marshes has provided a reconstruction of the timing and nature of vegetation succession against a background of Holocene climate change, relative sea level movement and human activities. The investigation recorded widespread peat formation between c. 6300 and 3900 cal. yr BP (marine ‘regression’), succeeded by evidence for marine incursion. The multiproxy analyses of these sediments, comprising pollen, Coleoptera, diatoms, and plant and wood macrofossils, have indicated significant changes in both the wetland and dryland environment, including the establishment of Alnus (Alder) carr woodland, and the decline of both Ulmus (Elm; c. 5740 cal. yr BP) and Tilia (Lime; c. 5600 cal. yr BP, and 4160–3710 cal. yr BP). The beetle faunas from the peat also suggest a thermal climate similar to that of the present day. At c. 4900 cal. yr BP, Taxus (L.; Yew) woodland colonised the peatland forming a plant community that has no known modern analogue in the UK. The precise reason, or reasons, for this event remain unclear, although changes in peatland hydrology seem most likely. The growth of Taxus on peatland not only has considerable importance for our knowledge of the vegetation history of southeast England, and NW Europe generally, but also has wider implications for the interpretation of Holocene palaeobotanical records. At c. 3900 cal. yr BP, Taxus declined on the peatland surface during a period of major hydrological change (marine incursion), an event also strongly associated with the decline of dryland woodland taxa, including Tilia and Quercus, and the appearance of anthropogenic indicators