Detrital zircons in modern African rivers: does the Sink truly reflect the Source?

Detrital zircon geochronology is becoming increasingly popular in provenance studies relying on the growing availability of accurate techniques and extensive datasets. This large amount of data is currently employed in ancient settings to describe source-to-sink scenarios deriving tectonic and (eventually) climatic implications. The study of modern sediments of African river systems offers the opportunity to observe how the zircon signal is propagated along these vast sediment highways and how the mere interpretation the zircon signal in the deep sea may lead to a significant misfit of interpretations in the source-to-sink system.
One of the factors linking major river systems is the highly segmented nature of sediment transport. Even though this phenomenon is currently enhanced by effect of dams, geomorphic barriers prove to be highly effective. In Niger River, for example, the exclusively Archean-Paleoproterozoic age signal from Leo-Man Shield in Guinea is unable to cross the natural barrier of sand dunes of the Inner Delta. Similarly, old zircons characterising the sands of the Victoria Nile and Albert Nile sink in Lake Victoria and the Sudd marshes. The recent evolution of the Zambezi River demonstrates that zircon geochronology alone is insufficient for depicting the routing system. Paired observations of bulk petrography, elemental, and isotope geochemistry effectively reveals that provenance signals are extremely segment along the river course.
Undoubtedly, the high durability of zircon through sedimentary cycles is effective in indicating the major episodes of crustal growth at continental scale. However, it simultaneously highlights the pitfall of not considering recycling as one of the main features for zircon provenance. In the Orange River, the Pan-African age cluster is dominant from the headwaters all the way to the mouth and transported by ocean currents as far as Angola. This occurs even though the Pan-African basement is only minimally exposed in the catchment, whereas zircon signal is stored in the Karoo Supergroup sandstones.