Frances M. Williams

Reach-scale river dynamics moderate the impact of rapid Holocene climate change on floodwater farming in the desert Nile

The relationship between climate change and the development of Old World riverine civilizations is poorly understood because inadequate dating control has hindered effective integration of archaeological, fluvial, and climate records. This paper presents the most comprehensive and robustly dated archaeological and paleoenvironmental data sets yet compiled for the desert Nile. It focuses on the valley floor hinterland of the Kingdom of Kerma (2400–1450 B.C.) in northern Sudan. Kerma emerged as a rival to Egypt during Africa’s first “Dark Age” drought. In contrast to other irrigation-based agriculturists in Egypt and Asia, Kerma flourished during the environmental crisis ca. 2200 B.C. We have studied the stratigraphy and archaeological records of paleochannels across an 80 km reach of the Nile upstream of Kerma using optically stimulated luminescence to date when channels flowed and when they dried up. The dynamics of the local alluvial environment were critical in determining whether climatic fluctuations and changes in river flow represented an opportunity for floodwater farmers (5000–3500 B.C.), a hazard that could be managed (2400–1300 B.C.), or an environmental catastrophe that resulted in settlement abandonment (after 1300 B.C.).

New light on the age of the White Nile

Landsat 5 Thematic Mapper and 7 Enhanced Thematic Mapper imagery reveal lake shorelines in the White Nile valley as far south as lat 10°N. The highest shoreline is at 386 m elevation and was eroded when the White Nile formed a lake as wide as 70 km and >500 km long. Finely laminated green clays laid down on the floor of this lake are overlain by alluvial sands and clays, dated by optically stimulated luminescence as 15 ka to older than 250 ka. The alluvium was deposited during interglacial episodes of stronger summer monsoons and very high White Nile floods. The White Nile paleolake is much older than marine oxygen isotope stage 7 and may have formed ca. 400 ka, during the exceptionally long stage 11 interglacial.

‘Of droughts and flooding rains’: an alluvial loess record from central South Australia spanning the last glacial cycle

Abstract Deposits of proximal dust-derived alluvium (alluvial loess) within the catchments of the now semi-arid Flinders Ranges in South Australia record regionally synchronous intervals of fluvial entrainment, aggradation and down-cutting spanning the last glacial cycle. Today, these floodplain remnants are deeply entrenched and laterally eroded by ephemeral traction load streams. The north–south aligned ranges are strategically situated within the present-day transitional zone, receiving both topographically enhanced winter rainfall from the SW and convectional downpours from summer monsoonal incursions from the north. We develop a regional chronostratigraphy of depositional and erosional events emphasizing the Last Glacial Maximum (LGM). Based on 124 ages (94 accelerator mass spectrometry radiocarbon and 30 optically stimulated luminescence) from the most significant terrace remnants on both sides of the Ranges, we conclude that the last glacial cycle including the LGM was characterized by major environmental changes. Two pronounced periods of pedogenesis between c. 36 and 30 ka were followed by widespread erosion and reworking. A short-lived interval of climatic stability before c. 24 ka was followed by conditions in which large amounts of proximal dust (loess) were deposited across the catchments. These loess mantles were rapidly redistributed and episodically transported downstream by floods. The termination of this regime c. 18–16 ka was marked by rapid incision.

A White Nile megalake during the last interglacial period

The eastern Sahara Desert of Africa is one of the most climatically sensitive areas on Earth, varying from lake-studded savannah woodland to hyperarid desert over the course of a glacial-interglacial cycle. In currently semiarid Sudan, there is widespread evidence that a very large freshwater lake once filled the White Nile River valley. Here we present the first quantitative estimate for the dimensions of the lake and a direct age for the emplacement of its shoreline. Using a profile dating approach with the cosmogenic nuclide 10 Be, we estimate an exposure age of 109 ± 8 ka for this megalake, indicating that it probably formed during the last interglacial period. This age is supported by optically stimulated luminescence dating of Blue Nile paleochannels associated with the lake. Using a high-resolution digital elevation model, we estimate that the lake was more than 45,000 km 2 in area, making it comparable to the largest freshwater lakes on Earth today. We attribute the lake9s existence to seasonal flood pulses as a result of local damming of the White Nile by a more southern position of the Blue Nile and greatly increased precipitation associated with an enhanced monsoon.

Traces from the past: the Cenozoic regolith and intraplate neotectonic history of the Gun Emplacement, a ferricreted bench on the western margin of the Mt Lofty Ranges, South Australia

The Gun Emplacement is a small but distinctive bench on the Eden–Burnside Fault Escarpment near Anstey Hill, in the northeastern suburbs of Adelaide, South Australia, occurring at an elevation of ∼210–220 m asl. It is underlain by Middle Eocene North Maslin Sand and is capped by resistant, ferricreted colluvium. Paleomagnetic dating of hematitic mottles in the ferricreted colluvium, immediately underlying the emplacement, returned a Pliocene/Early Pleistocene age. This age is equivalent to that obtained for summit surface weathering. Fault scarps and exposures, including slickensides and fault gouge material, suggest that the Eden–Burnside Fault at this location has a strong en échelon pattern developed in response to reverse-sinistral oblique-slip faulting, reflecting continental stress fields. Remnants of ferricrete cappings forming stranded benches on the Eden–Burnside Fault Escarpment at elevations up to 25 m above the Gun Emplacement demonstrate recurrent tectonism of the South Mt Lofty Ranges related to intraplate deformation. There are at least four distinct ferricrete benches preserved on the eastern side of the active fault leading up from the Gun Emplacement surface. These benches demonstrate alternating periods of stability and tectonic activity disrupting and uplifting the ferricreted surfaces. A fresh surface rupture occurs and may be related to a recent seismic event.

Lake Tana and the Blue Nile

The Blue Nile, or Abay, is Ethiopia’s best-known and most revered river. It rises in the Western Highlands and turns back on itself in a great loop before descending to the Sudan plains and meeting with the While Nile at Khartoum.

Combining geochemistry and geochronology of transported regolith to reveal bedrock-hosted mineralization in the arid east Wongatha area of south central Western Australia

Metal anomalies in transported regolith that overlie bedrock-hosted mineralization indicate that a component of mineralization (the exogenic component) can migrate through regolith. In the east Wongatha area of Western Australia, the exogenic component in the fine fraction of sandplain deposits is spatially linked to known and/or inferred bedrock-hosted Au mineralization. In three regolith profiles, the concentration of Au in aqua regia and deionized water Au are correlated, and in two of the profiles Au varies in concentration independent of changes in regolith composition. In one profile, the Au concentration in chemically-mature regolith dominated by quartz sand decreases from 31 ppb at c. 180 cm depth to 7.3 ppb at 15 cm. Optically stimulated luminescence (OSL) ages of stratigraphically-controlled regolith samples ranging from 166.9 ± 46.6 ka to 5.4 ± 1.1 ka show a strong correlation with depth (r2 = 0.99) over a three-metre interval, indicating a sandplain accumulation rate of c. 17 mm/1000 years. The decrease in Au concentration in east Wongatha regolith can be related to the migration rate of the exogenic component and the rate of sandplain accumulation. Supplementary material: Screening and analytical data for the < 50-µm fraction of regolith from three profile sites in the east Wongatha area are available at https://doi.org/10.6084/m9.figshare.c.4074413

Introduction to the Rift Valley

We have seen in previous chapters that Ethiopia is cut diagonally by the Ethiopian Rift Valley, a low-lying strip of land that separates the Western and Southeastern Highlands, and it is now time to take a closer look at this remarkable feature. Together with its extension northwards into Afar, it is probably the most exciting and significant geological feature in Ethiopia.

The Southeastern Highlands and the Ogaden

This chapter covers the region to the southeast of the Main Ethiopian Rift and Afar. It looks a big area to cover in a single chapter—about one third that of all Ethiopia—but a great deal of it is difficult to access and thus cannot be described in any detail. However, the parts which can be visited offer a variety of geological features and scenery which are not found elsewhere in Ethiopia. The people too are distinctive. Predominant over much of the region are the Somali people, their vibrant and colourful dress in contrast to the gentle white fabrics favoured by the inhabitants of the Western Highlands, and in the mountains bordering the Rift Valley are the Oromo, known in this region for their brilliant horsemanship.

Some Notes About Rock Names

There are literally hundreds of names for different types of rocks. This chapter introduces just those few that you will come across while reading this book. To begin with, rocks are classed into three main groups: sedimentary, igneous and metamorphic. Sedimentary rocks are formed of material that has been deposited by some kind of physical medium such as water, ice or wind, or by chemical precipitation, and then hardened by becoming cemented or compressed. The main sedimentary rocks that we will come across in this book are sandstone, which is formed of cemented grains of sand, and limestone, which is formed of the chemical calcium carbonate.