Orbital forcing in southern Africa: Towards a conceptual model for predicting deep time environmental change from an incomplete proxy record

Abstract

Southern Africa hosts regions of exceptional biodiversity and is rich with evidence for the presence and activities of early humans. However, few records exist of the concurrent changes in climate that may have shaped the region's ecological evolution and the development and dispersal of our ancestors. This lack of evidence limits our ability to draw meaningful inferences between important changes in the global and regional climate systems and their potential influence in shaping the region's natural and cultural history. This paper synthesises the data currently available to define a general empirically-based conceptual model of the spatio-temporal dynamics of climate change as they relate to changes in the earth's orbital configurations. The goal is to identify mechanistic links between orbital forcing, which can be calculated continuously over the past several million years, and environmental responses to related changes in the major atmospheric and oceanic circulation systems influencing southern Africa. Once identified, these relationships can be used to infer the most likely trends and patterns of climate variability for periods and regions for which proxy evidence is not available.Findings indicate that coherent patterns of change can be observed at wavelengths associated with ∼400-kyr and ∼100-kyr cycles of orbital eccentricity. In southeastern Africa, the ∼2400-kyr grand cycle in eccentricity may have had an influence long-term patterns of aridification and humidification, and the stronger ∼400-kyr eccentricity cycle has a significant influence across inter-tropical Africa, through changes in hydroclimate and monsoon circulation. The attribution of the ∼100-kyr cycle to specific orbital controls depends on location, as it can be determined by eccentricity-modulated direct insolation forcing or through the combined orbital parameters and earth system responses that drive the evolution of Pleistocene glacial-interglacial cycles.Following the onset of the mid-Pleistocene transition (c. 1250–700 ka), the increasing development of substantial polar ice sheets influence the nature of high-latitude drivers in southern Africa. In southwestern Africa, records indicate an evolution in climate and circulation systems strongly correlated with the global benthic δ18O record, suggesting a particular sensitivity to high latitude forcing. The close correlation between ∼100-kyr eccentricity and glacial-interglacial cycles makes it difficult to determine whether high- or low-latitude drivers dominate in southeastern Africa, but the spatio-temporal patterning of environmental variability in many records are generally considered to indicate a degree of high-latitude influence. Records from southeastern and southernmost Africa also indicate that the influence of low latitude forcing, expressed through the local precessional cycle, is – at least over the last glacial-interglacial cycles - dependent on eccentricity. Periods of reduced eccentricity, particularly during periods of extensive high-latitude ice sheet development, result in diminished influence in direct forcing and an increase in the expression of high latitude forcing, and an increasingly positive correlation between the northern and southern tropics at these wavelengths. In general, the records available allow for a simple conceptual model of the relationship between orbital parameters and regional climates to be defined, with the strongest relationships existing at longer timescales, such as the ∼400-kyr eccentricity cycle. At finer spatio-temporal timescales, the data indicate degrees of complexity that are not readily predicted, but the expansion of the regional dataset will continue to allow for refinements to the conceptual model described.