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Despite being one of the world’s oldest deserts, and the subject of decades of research, evidence of past climate change in the Namib Desert is extremely limited. As such, there is significant debate regarding the nature and drivers of climate change in the low-latitude drylands of southwestern Africa. Here we present data from stratified accumulations of rock hyrax urine that provide the first continuous high-resolution terrestrial climate record for the Namib Desert spanning the past 50,000 yr. These data, spanning multiple sites, show remarkably coherent variability that is clearly linked to orbital cycles and the evolution and perturbation of global boundary conditions. Contrary to some previous predictions of southwestern African climate change, we show that orbital-scale cycles of hydroclimatic variability in the Namib Desert region are in phase with those of the northern tropics, with increased local summer insolation coinciding with periods of increased aridity. Supported by climate model simulations, our analyses link this to variations in position and intensity of atmospheric pressure cells modulated by hemispheric and land-sea temperature gradients. We conclude that hydroclimatic variability at orbital time scales is driven by the combined influence of direct low-latitude insolation forcing and the influence of remote controls on the South Atlantic anticyclone, with attendant impacts on upwelling and sea-surface temperature variations.

n their comment to our recent paper (Jacques et al., 2008), Zazzo et al. (2009-this issue) provide a correction regarding some technical features of one of the isotopic mass spectrometers used in our comparative study. It appears that the system used to purify CO2 in Saskatoon was a dual inlet (DI) and not a continuous flow (CF) as mistakenly stated. As this system was considered as reliable as the other CF used in the comparative study, Zazzo et al. (2009-this issue) infer that the isotopic ratios previously measured on heavily contaminated material from a late Miocene Chadian site, Toros Menalla (TM), with another DI are reliable as well. Our reply will re-evaluate the comparison between CF and DI systems in the light of the correction for the system used in Saskatoon, and discuss the claim that previous isotopic measurements made on Chadian material reflect biogenic signatures and were not affected by oxide contamination.

High resolution chemical and textural investigation of Upper Miocene mammalian tooth enamel from Chad using electron microprobe and scanning electron microscope techniques provide direct evidence of extensive diagenetic alteration when compared with some modern and contemporaneous fossil specimens. Chemical and textural changes in Chadian fossil enamel during fossilization include large amounts of secondary Fe, Si, Mn, and Al-bearing oxides - both as coarse precipitation in cracks and permeating the apatite - high fluorine contamination (i.e., apatite alteration), Ca, P, Cl, Na and Mg depletion, and sulphur contamination. Such diagenetic contamination significantly affects the oxygen and carbon isotope analysis of carbonate in apatite, altering the delta O-18 and delta C-13 biogenic signals towards more positive values. Isotopic alteration is however greater for oxygen than for carbon. Indeed, isotopic analysis of the heavily oxide and S-contaminated Chadian fossil tooth enamel samples on a classical Dual Inlet-MultiPrep system versus three Continuous Flow-IRMS mass spectrometers show great differences in both the delta O-18 and delta C-13 values. While the MultiPrep-Dual Inlet approach provides carbon and oxygen isotopic results that are indubitably altered and generally inconsistent with any biological values, isotopic analysis on Continuous Flow systems - which automatically filter out contaminating gases - allow to attain or at least greatly approach primary isotopic signals. Some of the Chadian fossil enamel samples analysed in this work come from the very important Upper Miocene TM266 fossil locality that yielded Sahelanthropus tchadensis a.k.a. "Toumai", the oldest member of the hominid clade known so far. At this site, as in others, a few or no other environmental proxies than isotopic analysis of biomaterials are available for palaeoenvironmental and palaeoecological reconstructions. Therefore, to obtain reliable environmental information from the time of this ancient Hominid, recovering uncontaminated biogenic isotopic signals is crucial and from these results it is now technically possible. (c) 2008 Elsevier B.V. All rights reserved.