The extraordinarily durable concretes of Imperial Age (27 B.C. through 3rd century A.D.) monument construction in Rome contain scoriaceous, highly potassic, altered volcanic ash from the Pozzolane Rosse ignimbrite, erupted at 456±3ka from the Alban Hills volcano as pozzolanic mortar aggregate. Stratigraphic, micromorphological, and chemical investigations demonstrate that during the relatively warm, humid period preceding marine isotope stage 11, intensive hydrolytic pedogenesis produced dense illuvial clay coatings in the upper horizon of the ignimbrite. Moderately altered ash of the Pozzolane Rosse transitional Bt to Bw soil horizon has opal coatings overlain by limpid, illuvial halloysite coatings. Less weathered ash from a lower C horizon apparently was altered in ground water. Where the ignimbrite filled certain paleovalleys, zeolitic alteration produced phillipsite and chabazite textures. Builders selected ash from these intermediate and least altered horizons for the highest‐quality mortars of the Imperial Age, as for the Forum and Markets of Trajan (A.D. 107 to 113). The alumina‐ and alkali‐rich compositions of cementitious complexes give some preliminary insights as to why the reaction of hydrated lime with the altered, alkali‐rich Pozzolane Rosse ash produced pozzolanic cements that have remained resistant to decay for nearly 2000 years. The results of the geological analyses fully confirm empirical observations made by Esther B. Van Deman in 1912 regarding the durability of the ancient mortars and the technical choices of Roman builders. © 2009 Wiley Periodicals, Inc.

Mineralogical and geochemical analyses of ultrafine (<0.1m) extracts of Pliocene clays from Olduvai Gorge reveal the complexities of clay diagenesis in saline, alkaline paleo-waters. Multiple authigenic phases are routinely present; these may be distinguished and quantified by decomposition of XRD (060) peaks coupled with geochemical (microprobe, HRTEM-AEM) and crystallographic (IR) investigation. By assigning geochemistry based on XRD results, we identify three principal phases by octahedral composition: dioctahedral Al-rich, dioctahedral relatively Fe-rich, and Mg-rich with a trioctahedral component. Average octahedral formulae are Al 1.30 Fe 0.57 Mg 0.13 (Al-rich), Al 0.56 Fe 0.80 Mg 0.62 (Fe-rich), and Al 0.09 Fe 0.23 Mg 2.09 (Mg-rich) per half formula unit. IR analyses support these octahedral structures. The Mg-rich phase either has coexisting dioctahedral and trioctahedral domains within individual sheets, or a homogeneous octahedral sheet of intermediate composition. This is, to our knowledge, the first report of an intermediate octahedral occupancy in a 2:1 phyllosilicate.HRTEM observations indicate both solid state octahedral and illitization reactions and dissolution-precipitation. Layer charge and interlayer cations suggest that during illitization, layer charge increase was due to interaction with Na-rich brines, but later K uptake may have been with either saline or fresher fluids with higher K/Na ratios.All three phases occur throughout the basin; relative proportions and octahedral compositions vary. Total Mg content of "bulk" <0.1m clay fractions, (a paleochemical indicator in this and other basins) is the product of both the relative abundance and the Mg content of the Mg-rich phase. In addition to shedding light on the crystal chemistry of authigenic 2:1 phyllosilicates, these results demonstrate the paleolimnologic importance of discriminating the presence of multiple authigenic clay phases in lacustrine deposits.

Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.

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