Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision:Examples from the South African Interior

Resistant bedrock outcrops can exert control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. To examine how variation in lithologic resistance impacts landscape development in the post-orogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and Schoonspruit are widely variable, with channel bed denudation rates ranging from 14-255 m/Myr and valley side and top denudation rates ranging from 11-50 m/Myr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (~20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 kyr). These results suggest that in the post-orogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics

Late Quaternary climatic changes revealed by luminescence dating, mineral magnetism and diffuse reflectance spectroscopy of river terrace palaeosols: a new form of geoproxy data for the southern African interior

AbstractThe nature, spatial patterns and forcing mechanisms of Quaternary climatic changes across southern Africa remain unresolved and contentious, principally due to the scarcity of continuous and robustly-dated proxy records. We present what we interpret to be a broadly continuous record of late Quaternary climatic change based on optically stimulated luminescence (OSL) dating, and mineral magnetic and diffuse reflectance spectroscopy (DRS) analyses of stacked palaeosols within an overbank alluvial succession along the Modder River, central South Africa. The OSL ages indicate that alluvial sedimentation occurred at a fairly steady rate, averaging ∼0.15 mm/yr from at least 44 ka until ∼0.83 ka. This suggests that the palaeosols are accretionary, having formed contemporaneously with sedimentation. Climate is identified as the key soil-forming factor controlling the intensity of pedogenesis and is reflected in the changing concentration of pedogenic ferrimagnetic minerals (magnetite/maghemite) of single domain and superparamagnetic dimensions, and by variations in the amount of hematite compared to goethite. These data indicate that the climate was generally dry (rainfall ∼200–400 mm/yr) from ∼46 to 32 ka, except for a brief peak in humidity at ∼42 ka. There was then a period of greater humidity (rainfall ∼400–600 mm/yr) from ∼32 to 28 ka, possibly reflecting enhanced moisture supply from the Atlantic Ocean associated with the equatorward migration and intensification of westerly storm tracks. Although the precise mechanism remains unresolved, this climatic change may have been linked to an obliquity minimum at ∼29 ka. After ∼28 ka, the climate became progressively cooler and drier, especially between ∼18 and 15.5 ka when rainfall was as low as ∼100–200 mm/yr. Temperatures and rainfall then increased from ∼15.5 ka onwards, with the latter possibly linked to rising sea-surface temperatures in the SW Indian Ocean and enhanced moisture supply from easterly circulation. At ∼0.83 ka, a time corresponding with part of the Medieval Climatic Anomaly (MCA, ∼900–1300 AD), rainfall reached ∼600–700 mm/yr and was higher than at present (∼400–500 mm/yr). Fluvial landforms have previously been overlooked as a source of palaeoenvironmental information in southern Africa, but this study clearly demonstrates the potential to extract robust palaeoenvironmental data from alluvial-palaeosol successions in the arid to semi-arid interior where other forms of proxy record are scarce

New investigations at Kalambo Falls, Zambia:Luminescence chronology, site formation, and archaeological significance

AbstractFluvial deposits can provide excellent archives of early hominin activity but may be complex to interpret, especially without extensive geochronology. The Stone Age site of Kalambo Falls, northern Zambia, has yielded a rich artefact record from dominantly fluvial deposits, but its significance has been restricted by uncertainties over site formation processes and a limited chronology. Our new investigations in the centre of the Kalambo Basin have used luminescence to provide a chronology and have provided key insights into the geomorphological and sedimentological processes involved in site formation. Excavations reveal a complex assemblage of channel and floodplain deposits. Single grain quartz optically stimulated luminescence (OSL) measurements provide the most accurate age estimates for the youngest sediments, but in older deposits the OSL signal from some grains is saturated. A different luminescence signal from quartz, thermally transferred OSL (TT-OSL), can date these older deposits. OSL and TT-OSL results are combined to provide a chronology for the site. Ages indicate four phases of punctuated deposition by the dominantly laterally migrating and vertically aggrading Kalambo River (∼500–300 ka, ∼300–50 ka, ∼50–30 ka, ∼1.5–0.49 ka), followed by deep incision and renewed lateral migration at a lower topographic level. A conceptual model for site formation provides the basis for improved interpretation of the generation, preservation, and visibility of the Kalambo archaeological record. This model highlights the important role of intrinsic meander dynamics in site formation and does not necessarily require complex interpretations that invoke periodic blocking of the Kalambo River, as has previously been suggested. The oldest luminescence ages place the Mode 2/3 transition between ∼500 and 300 ka, consistent with other African and Asian sites where a similar transition can be found. The study approach adopted here can potentially be applied to other fluvial Stone Age sites throughout Africa and beyond

Moderate agreement between self-reported stroke and hospital-recorded stroke in two cohorts of Australian women: a validation study

Background: Conflicting findings on the validity of self-reported stroke from existing studies creates uncertainty about the appropriateness of using self-reported stroke in epidemiological research. We aimed to compare self-reported stroke against hospital-recorded stroke, and investigate reasons for disagreement. Methods: We included participants from the Australian Longitudinal Study on Women's Health born in 1921-26 (n = 1556) and 1946-51 (n = 2119), who were living in New South Wales and who returned all survey questionnaires over a defined period of time. We determined agreement between self-reported and hospitalised stroke by calculating sensitivity, specificity and kappa statistics. We investigated whether characteristics including age, education, area of residence, country of birth, language spoken at home, recent mental health at survey completion and proxy completion of questionnaire were associated with disagreement, using logistic regression analysis to obtain odds ratios (ORs) with 95% confidence intervals (CIs). Results: Agreement between self-report and hospital-recorded stroke was fair in older women (kappa 0.35, 95% CI 0.25 to 0.46) and moderate in mid-aged women (0.56, 95% CI 0.37 to 0.75). There was a high proportion with unverified self-reported stroke, partly due to: reporting of transient ischaemic attacks; strokes occurring outside the period of interest; and possible reporting of stroke-like conditions. In the older cohort, a large proportion with unverified stroke had hospital records of other cerebrovascular disease. In both cohorts, higher education was associated with agreement, whereas recent poor mental health was associated with disagreement. Conclusion: Among women who returned survey questionnaires within the period of interest, validity of self-reported stroke was fair to moderate, but is probably underestimated. Agreement between self-report and hospital-recorded stroke was associated with individual characteristics. Where clinically verified stroke data are unavailable, self-report may be a reasonable alternative method of stroke ascertainment for some epidemiological studies