Abigail Stone

Last Glacial Maximum conditions in southern Africa: Are we any closer to understanding the climate of this time period?

The Last Glacial Maximum (LGM) (21±2 ka) is an important period for which to understand past climatic and environmental conditions. In particular it is a key time-slice for evaluating the performance of numerical climate model simulations of glacial palaeoclimates using palaeoenvironmental data sets. However, our palaeoenvironmental data sets and reconstructions of climatic conditions at the LGM are still debated in certain regions. This is the case for southern Africa, despite more than half a century of research since early conceptual models of palaeoclimate were proposed. The greatest debates are about the spatial patterning of relatively wetter and drier conditions than present and the position of the mid-latitude westerlies at the LGM. Different patterns emerge from: separate syntheses of palaeoenvironmental proxies, from different numerical model simulations and from comparisons of the two. In this review of the progress over half a century of research in southern Africa: (1) a brief historical review of key conceptual models is given, (2) key points of conflict that emerge in synthesis of palaeoenvironmental proxy records are outlined and (3) numerical model simulations are considered. From these, some points for future progress are suggested.

Controls on sand ramp formation in southern Namibia

Sand ramps have the potential to provide rich palaeoenvironmental information in dryland regions wherein proxy records are typically scarce. However, current knowledge of the geomorphic controls and processes of sand ramp formation is limited. This study provides a data-rich examination of the key factors controlling sand ramp formation. The location and morphology of 75 sand ramps in southern Namibia are examined. The sediments and chronologies of 10 sand ramps are studied in detail using 51 OSL dates and 83 grain-size and LOI samples. Heavy mineral assemblages are used to determine the provenance of 10 samples and OSL sensitivity is used to explore geomorphic processes of 8 samples. Sand ramp morphology can be grouped into one of four classes of increasing size and complexity and is closely linked to the available accommodation space. Heavy mineral assemblages indicate local sediment sources and all 75 studied sand ramps are within 4 km of a large ephemeral river channel or within 5.5 km of a dune field. Therefore, accommodation space and sediment supply are identified as the primary controls of sand ramp formation. Sedimentology and OSL sensitivity suggest a complex interplay of aeolian, fluvial and colluvial processes contribute to sand ramp formation with large variability observed between ramps. Three of the ten dated sand ramps have been present in the Namibian landscape for >100 ka. Eight sand ramps show episodic deposition between >75-12 ka and five show evidence of surface reworking over the past 2 ka. Environmental sensitivity is likely to be linked to the size and availability of the accommodation space. Therefore, individual sand ramps are expected to reflect local environmental conditions, recording when an abundant sediment supply coincided with available accommodation space, whilst a regional analysis of multiple sand ramps with chronometric data offers the potential to identify larger scale palaeoenvironmental controls of sediment supply.

Squeezing a rainfall record out of desert sand dunes

Reconstructing past rainfall fluctuations in drylands is challenging. the deposition of sedimentary archives, such as sand dunes, are mediated by a number of factors, of which changing moisture availability is just one. Former river courses, and former lakes fed by rivers, often record changes imported from a climatic zone outside their dryland location (exogenous systems). speleothems are restricted to limestone terranes and tend to be discontinuous records in drylands. Isotopic records within hyrax middens have growing potential, but are also spatially restricted to areas with rock-ledge habitats. therefore, developing further proxies capable of recording changes in moisture availability is extremely valuable for dryland Quaternary environmental reconstruction.