Researchers have studied the rocks of the Mweru Rift System to trace millions of years of landscape evolution. They made use of certain elements formed by the influence of cosmic rays, so-called cosmogenic nuclides. The Mweru Rift System is the southwestern extension of the East African Rift System and once housed a huge lake. The researchers have determined its extent for the first time using cosmogenic nuclides.
The East African Rift System (EARS) is one of the best-studied active intracontinental rifts on Earth. It is more than 3,000 kilometers long and has been active throughout the Neogene, i.e., for at least about 25 million years. There, researchers have a unique opportunity to study the evolution of a continental rift valley: from the initial breakup of the Earth's crust to the onset of oceanic rifting and eventually the opening of future oceans.
Until now, little was known about the evolution of the landscape of the western branch of the East African Rift and the lakes that formed there, which stretch for more than a thousand kilometers. The study by Spiros Olivotos and his colleagues, which has just been published in the scientific journal "Geomorphology", investigated key mechanisms of landscape evolution there. The researchers focused on the present-day lakes Mweru and Mweru Wantipa, which lie between the Northern Province of Zambia and the southeastern Katanga Province of the Democratic Republic of Congo.
It has long been scientifically assumed that the original Lake Mweru, the subject of this research, once formed a large reservoir. However, no scientific fieldwork and/or geomorphological analysis has yet explored the exact extent and duration of a major paleolake Mweru. Some large and deep paleolakes during the Plio-Pleistocene are well attested in the region, for example, the Turkana, Edward-Albert, Obweruka, Bangweulu, Magadi, Thamalakane, and Tanganyika paleolakes.
The research team now worked with so-called cosmogenic nuclides as part of their investigations. They used beryllium (10Be) and aluminum (26Al), which are among the six most commonly used nuclides: 3He, 10Be, 14C, 21Ne, 26Al and 36Cl. These are produced by the action of cosmic rays, a high-energy particle radiation from space, in rocks on the Earth's surface and therefore make it possible to to date the exposure of rock surfaces. With this method one can look back in time from about 100 up to 10 million years ago.
The in situ terrestrial cosmogenic nuclide method has revolutionized the way scientists can study the evolution of landscapes over long geological timescales. Single or multiple nuclides from a rock surface can be measured using accelerator or noble gas mass spectrometry. In this way, the ratio of a (radio)isotope to another - stable - isotope of the same element is measured. Different half-lives are utilized for the method: 10Be, for example, has a half-life of 1.39 million years, 26Al aluminum has a half-life of 705,000 years.
Thus, researchers learn what the erosion rates were on boulder and rock surfaces, at what rate rivers cut into the landscape, how quickly individual landforms or entire catchments were exposed (denudation rates). It can be illuminated how rock surfaces and sediments were buried, but also how and at what rate landforms retreated, one learns about faulting rates and receives information about paleoseismology and paleoaltimetry. In short, cosmogenic nuclides open up unimagined possibilities.
Minimum exposure age of the knickpoints (waterfalls)
In this study by Spiros Olivotos, by interpreting a network of surface exposure data of important landforms, the age of formation of the paleo lake Mweru and its extent was inferred. Samples were collected from each of two plateaus, the "Mporokoso Plateau" and the "Kundelungu Plateau", which surround and border the present lake. The researchers took a large number of rock samples at waterfalls, geologists speak of knickpoints.
Such knickpoints control quite significantly the geographic distribution of life (biota) because they act as natural barriers to the dispersal of species, especially fish. Populations that live upstream or downstream of such natural barriers develop differently. Geologically, such points are particularly informative because it is possible to determine the minimum exposure age of the knickpoint at its current location. Knickpoint retreat rates and maximum denudation rates can also be determined. A total of twenty-six samples of quartzitic bedrock were collected. Other important landforms for sampling included riverbeds and fault lines.
Results of the study
Fault analysis indicates that movements along the active Mweru-Mweru Wantipa Fault System have been responsible for restructuring the landscape and drainage patterns in the western branch of the EARS since the Miocene. Matching the observed burial patterns with morphotectonic and stratigraphic analysis indicates the existence of an extensive paleolake during the Pleistocene. Hypsometric analysis of the now dated knickpoints has constrained the water level of the paleolake to ~1200 m asl. High denudation rates of up to ~40 millimeters per 1000 years along the eastern Kundelungu Plateau indicate that uplift, induced by normal faulting, caused rapid incision of the river, controlling paleolake drainage.
This now-identified paleolake far exceeded the size of present-day Lakes Mweru and Mweru Wantipa. Formation about 2.6 million years ago is likely. The existence of the lake is also consistent with phylogenetic analyses of the molecular clock of endemic fish species, which also place the origin of the paleolake in the late Pliocene - early Pleistocene.
Spiros Olivotos, Samuel Niedermann, Tyrel Flügel, Vasiliki Mouslopoulou, Silke Merchel, Fenton Cotterill, Bodo Bookhagen, Andreas Gärtner, Georg Rugel, Andreas Scharf, Marie-Josée Nadeau, Régis Braucher, Martin Seiler (2021): Quaternary landscape evolution in a tectonically active rift basin (paleo-lake Mweru, south-central Africa), Geomorphology, Volume 381, 2021, 107669. Link.
The study is part of the project: "Exploiting the Genomic Record of Living Biota to Reconstruct the Landscape Evolution of South Central Africa" (Volkswagen Stiftung)