The West African Stone Age

In terms of artefacts present, West Africa is not short of evidence relating to human occupation during the Quaternary. The problem hitherto has been one of context and dating; there has been some progress in this regard but poor preservation conditions still restrict the presence of organic remains prior to the beginning of the Late Stone Age (LSA). Nonetheless, an excellent climatic record for the last 520 kya has been established on the basis of cores obtained from Lake Bosumtwi. Stratified Acheulean sites have been excavated at Sansandé and Ravin Blanc on the Falémé River in eastern Senegal. The succeeding Sangoan is an entity for which a consistent and reliable classification remains to be achieved. Despite this, excavations at Anyama in the Ivory Coast have produced a sizeable quantity of material, with a terminus post quem thermoluminescence (TL) date of 254 ± 51 kya. Our knowledge of the Middle Stone Age (MSA) has been transformed by the work carried out at Ounjougou in Mali. More than twenty-five distinct archaeological occurrences have been detected, extending from about 75 to 25 kya. The MSA elsewhere is abundant, and at Adrar Bous is in place beneath the Aterian, but much of it lacks a good stratigraphic context. The following dry period, the Ogolian, must have had a dramatic effect on human settlement, and the majority of LSA sites postdate this episode. There is no apparent link between them and the MSA. Nonetheless, the LSA at Shum Lake in Cameroon does have 14C dates in the range 32,700–12,800 BP. The most significant LSA site is Iwo Eleru, notable for the presence of modern human remains with “archaic” characteristics. A parallel situation has been detected at Ishango in the eastern Democratic Republic of Congo both indicating a hitherto unsuspected “deep substructure” in Late Pleistocene African populations.

Using past interglacial temperature maxima to explore transgressions in modern Maldivian coral and Amphistegina bleaching thresholds

Tropical corals and Amphistegina, an example genus of symbiont-bearing larger benthic foraminifera, are presently living close to their thermal bleaching thresholds. As such, these essential reef-building organisms are vulnerable to the future prospect of more frequent sea surface temperature (SST) extremes. Exploring the earth’s paleo-climatic record, including interglacials warmer than present, may provide insights into future oceanographic conditions. We analyse foraminiferal shell geochemical compositions, from Recent surface sediments and Marine Isotope stage (MIS) 9e and MIS11c aged sediments, from the International Ocean Discovery Program Expedition 359 Site U1467 drilled in the Inner Sea of the Maldives. We illustrate through traditional (pooled) geochemical analysis (δ18O, Mg/Ca) that tropical temperatures were indeed marginally warmer during MIS9e and MIS11c in comparison to the modern ocean. Individual foraminiferal analysis (IFA) from the Recent (representing the last few hundred years) and MIS9e samples shows SSTs occasionally breached the coral bleaching threshold similarly to the modern-day. Significantly, the number of transgressions was four times higher during MIS11c, a recognised analogue for a warmer modern world. This new knowledge and novel IFA insight and application is invaluable given thermal stress is already obvious today with an increasing number of bleaching events over the last few decades.

Climatology: Methods

Environmental constraints have large impacts on populations, especially in semi-arid regions such as Africa. Climate and weather have long affected African societies, but unfortunately the traditional climatic record for the continent is relatively short. For that reason, historical information has often been used to reconstruct climate of the past. Sources of historical information include reports and diaries of explorers, settlers, and missionaries; government records; reports of scientific expeditions; and historical geographical and meteorological journals. Local oral tradition is also useful. It is reported in the form of historical chronicles compiled centuries later. References to famine and drought, economic conditions, floods, agriculture, weather events, and the season cycle are examples of useful types of information. Some of the records also include meteorological measurements. More recently chemical and biological information, generally derived from lake cores, has been applied to historical climate reconstruction. Early works provided in most cases qualitative, discontinuous information, such as drought chronologies. However, a statistical method of climate reconstruction applied to a vast collection of historical information and meteorological data allowed for the creation of a two-century, semi-quantitative “precipitation” data set. It consists of annual indices related to rainfall since 1800 for ninety regions of the African continent. This data set has served to illustrate several 19th-century periods of anomalous rainfall conditions that affected nearly the entire continent. An example is widespread aridity during several decades early in that century.

Inferring palaeo-accumulation records from ice-core data by an adjoint-based method: application to James Ross Island's ice core

Ice cores contain a record of snow precipitation that includes information about past atmospheric circulation and mass imbalance in the polar regions. We present a novel approach to reconstruct a climatic record – by both optimally dating an ice core and deriving from it a detailed accumulation history – that uses an adjoint-based method. The motivation of our work is the recent application of phase-sensitive radar which measures the vertical velocity of an ice column. The velocity is dependent on the history of subsequent snow accumulation, compaction and compression; in our inverse formulation of this problem, measured vertical velocity profiles can be utilized directly, thereby reducing the uncertainty introduced by ice-flow modelling. We first apply our method to synthetic data in order to study its capability and the effect of noise and gaps in the age–depth observations. The method is then applied to the ice core retrieved from James Ross Island, Antarctica. We show that the method is robust and that the results depend on the quality of the age–depth observations and the derived flow regime around the core site. The method facilitates the incorporation of increasing detail provided by ice-core analysis together with observed full-depth velocity in order to construct a complete climatic record of the polar regions.