Interpreting geoproxies of late Quaternary climate change in African drylands: Implications for understanding environmental change and early human behaviour

摘要

Climate and environmental change in the Late Quaternary are widely regarded as key drivers of early human development and dispersal. In Africa, robust records of change are required to assess the nature of potential impacts. Today's late Quaternary palaeoenvironmental/palaeoclimate reconstructions from the interior southern African basin (Kalahari) are more spatially detailed, have a stronger chronometric underpinning, and span a longer time range, than those of 10-20 years ago, but are no less controversial. Reconstructions for the last ～150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today, and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid-latitude westerlies. With more data than ever before, why then is the record not yet delivering a more coherent picture of regional climate change? This paper deliberately addresses current challenges and controversies, including issues often ignored or neglected, with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future. Sources of palaeo-data from the interior are predominantly 'geoproxies': reconstructions not based on highly resolved organic or isotope records, but on interpretation of spatially extensive geomorphological proxies of past environments. Despite being increasingly datable using OSL, their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources. Consequently, they are sometimes removed from regional syntheses of climate change. These omissions are often replaced by spatially-exaggerated interpretations of'higher quality' local records allowing more spatially-coherent timeslice interpretations to be presented. Yet this is counter-intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies, or in climatic and environmental systems today. In addition, the quest to link regional records to 'master records' from ice and ocean cores can 'force' interpretation within a temporal scale that may be inappropriate for their analysis, underplaying the reality of complexity within the environmental system. There are not necessarily 'good' or bad' records of terrestrial palaeoenvironments in interior southern Africa. Rather, there are records that contain different suites of information about past land surface, hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable. Consideration of these factors may resolve apparent conflicts between datasets, resulting in a better understanding of past environmental dynamics, and revealing the nature of environmental variability, which may be a key facet to which human development has responded.