Preface

Southern hemisphere tropical climate over the past 145ka: Results of the Lake Malawi Scientific Drilling Project, East Africa

Current evidence suggests that all of the major events in hominin evolution have occurred in East Africa. Over the last two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of East Africa has varied in the past. The landscape of East Africa has altered dramatically over the last 10 million years. It has changed from a relatively flat, homogenous region covered with mixed tropical forest, to a varied and heterogeneous environment, with mountains over 4 km high and vegetation ranging from desert to cloud forest. The progressive rifting of East Africa has also generated numerous lake basins, which are highly sensitive to changes in the local precipitation-evaporation regime. There is now evidence that the presence of precession-driven, ephemeral deep-water lakes in East Africa were concurrent with major events in hominin evolution. It seems the unusual geology and climate of East Africa created periods of highly variable local climate, which, it has been suggested could have driven hominin speciation, encephalisation and dispersal out of Africa. One example is the significant hominin speciation and brain expansion event at ∼1.8 Ma that seems to have been coeval with the occurrence of highly variable, extensive, deep-water lakes. This complex, climatically very variable setting inspired first the variability selection hypothesis, which was then the basis for the pulsed climate variability hypothesis. The newer of the two suggests that the long-term drying trend in East Africa was punctuated by episodes of short, alternating periods of extreme humidity and aridity. Both hypotheses, together with other key theories of climate-evolution linkages, are discussed in this paper. Though useful the actual evolution mechanisms, which led to early hominins are still unclear and continue to be debated. However, it is clear that an understanding of East African lakes and their palaeoclimate history is required to understand the context within which humans evolved and eventually left East Africa.

Saline lakes are important environmental features in continental basins worldwide. They are critical for water and mineral resources, economic activity, and paleoenvironmental studies. Water and solute sources for saline lake brines are direct precipitation, surface flow, local or regional meteoric groundwaters, interstitial water of sediments, or deep basinal fluids. Solute concentrations are controlled by the weathering of source rocks, evaporation, and solute losses to minerals and biota. Aquifer residence times, rock dissolution rates, leakage ratios, and dissolution–precipitation fractionation are also important factors. With evaporation, inflow waters generally reach supersaturation first with respect to alkaline-earth carbonates, the first of several major ‘chemical divides.’ Subsequent chemical divides occur for gypsum, magnesite, and sulfates of magnesium and sodium, as shown in the revised brine evolution flowchart. Brine evolution along different flow paths is therefore controlled by the molar ratio of solutes, eventually producing diagnostic mineral assemblages. Idealized brine evolution paths and the associated evaporite mineral assemblages are complicated by local variation in hydrology, biotic activity, mineral kinetics, and diagenesis.

The ‘Spencer Triangle’ is a valuable tool for assessing solute behavior in the system Ca–SO₄–HCO₃. Such plots show that the initial solution chemistry is a good predictor of the resulting mineral assemblage. A similar triangle in which the calcium vertex is replaced with magnesium shows that the initial Ca/Mg ratio and the timing of calcium depletion relative to magnesium are critical factors in determining whether the subsequent brine will be dominated by sulfate or carbonate. These two triangles together demonstrate the principal major ion behaviors responsible for solution chemistries in saline lakes worldwide. In addition, normative calculations on the major ion concentrations of representative saline lake waters can provide specific measures of their hydrochemical evolution.