High-resolution 14C dating of a 25,000-year lake-sediment record from equatorial East Africa

We dated a continuous, ~22-m long sediment sequence from Lake Challa (Mt. Kilimanjaro area, Kenya/Tanzania) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change in equatorial East Africa over the past 25,000 years. The age model is based on a total of 168 AMS 14C dates on bulk-organic matter, combined with a 210Pb chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i) pairing bulk-organic 14C dates with either 210Pb-derived time markers or 14C dates on grass charcoal, and ii) wiggle-matching high-density series of bulk-organic 14C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from ~450 yr during glacial and late glacial time to ~200 yr during the early and mid-Holocene, and increasing again to ~250 yr today. The screened and corrected 14C dates were calibrated sequentially, statistically constrained by their stratigraphical order. As a result their constrained calendar-age distributions are much narrower, and the calibrated dates more precise, than if each 14C date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of ~50–230 yr during the Holocene and ~250–550 yr in the glacial section of the record. The d13C values of paired bulk-organic and grass-charcoal samples, and additional 14C dating on selected turbidite horizons, indicates that the old-carbon age offset in Lake Challa is caused by a variable contribution of old terrestrial organic matter eroded from soils, and controlled mainly by changes in vegetation cover within the crater basin.

Adsorption and photocatalytic bleaching of acid orange 7 on P25 titania

The observed adsorption of acid orange 7, AO7(-), on P25 titania over a range of pH values (pH 2-8) gives a good fit to data generated using a charge distribution, multisite complexation, i.e. CD-MUSIC, model, modified for aggregated dye adsorption. For this system the model predicts that both the apparent dark Langmuir adsorption constant. K-L, and the number of adsorption sites, n(o), increase with decreasing pH, and are negligible above pH 6. At pH 2 the CD-MUSIC model predicts the fraction of singly co-ordinated sites occupied by the dye,f(AO7), is ca. 32% under the in situ monitoring experimental conditions used in this work to study the photocatalytic bleaching of AO7(-) under UV light illumination ([TiO2] = 20 mgdm(-3); [AO7(-)](total) = 4.86 x 10(-5) M). Although AO7(-) adsorption on P25 titania is insignificant above pH 6 and increases almost linearly and markedly below this pH, the measured initial rate of bleaching of AO7(-), photocatalysed by titania using UV appears to only increase modestly (

FACTORS AFFECTING THE KINETICS OF METHYL-ORANGE REDUCTION PHOTOSENSITIZED BY COLLOIDAL CDS

The kinetics of photoreduction of methyl orange by ascorbic acid sensitized by colloidal CdS has been studied. Different experimental factors such a [O2], pH and temperature, as well as the presence of potential competitive species like MV2+ and Cd2+ have been taken as variables in this study. O2 and Cd2+ clearly inhibit the photoreduction but the presence of MV2+ increases the reaction rate. The pH greatly influences the kinetics and temperature (T) has little effect. The results are interpreted using a reaction scheme proposed in earlier papers where dispersions of crystalline CdS were used as the photocatalyst and EDTA as the hole scavenger.

METHYL-ORANGE AS A PROBE OF PHOTOELECTROCHEMICAL REACTIONS SENSITIZED BY COLLOIDAL CDS

The kinetics of photoreduction of methyl orange by ethylenediaminetetraacetic acid (EDTA) sensitized by colloidal CdS are reported as a function of [methyl orange], [O2] and [EDTA]. The results are interpreted using a reaction scheme which was proposed in an earlier paper for the same reaction sensitized by a powdered dispersion of highly crystalline CdS. An analysis of the results for the CdS colloid based on this reaction scheme shows that the rate of dye reduction by photogenerated electrons is approximately 50 times greater than the rate of oxygen reduction and the rate of scavenging of the photogenerated holes is approximately 7000 times greater than the rate of recombination. These findings are discussed in the light of similar observations reported for powdered CdS.