42-2 Evidence for Evolving Provenance at Lake Malawi from Detrital Zircon U-Pb Geochronology

Session: Integrating 20 Years of Scientific Drilling in the East African-Syrian Rift: A Session In Honor of Andrew Cohen, Part II

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Drill cores from Lake Malawi hold an unparalleled record of environmental change with implications for evolutionary mechanisms and human evolution; however, paleoenvironmental interpretations of the Lake Malawi Drilling Project core 1B (MAL05-1B) have varied with respect to both chronology and environmental forcing mechanisms.  One hypothesis suggested that changes in Lake Malawi’s outlet location and elevation shaped deepwater depositional patterns and therefore signals of climate change recorded in MAL05-1B prior to 700,000 years BP. A central tenet of this hypothesis links to the tectonic evolution of the Ruhuhu River watershed, found on the eastern margin of the lake. While the westward flowing Ruhuhu River builds a delta in Lake Malawi today, in the past, the river may have flowed eastward to the Indian Ocean.  If accurate, this hypothesis provides a mechanism not directly related to climate change that helps to explain relatively shallow conditions at Lake Malawi prior to the Mid-Pleistocene Transition, in evidence from aquatic pollen taxa and marsh ostracods, as well as genetic similarities among fish species from Lake Malawi and the Indian Ocean.  Few datasets have emerged to test this hypothesis, however. Here, we present new results from a detrital zircon U-Pb geochronological analysis of silty and sandy beds (n=11) in core MAL05-1B.  All samples evaluated contain zircons with U-Pb ages of ~2 Ga, which is consistent with extensive Paleoproterozoic bedrock in the adjacent watersheds.   Most of the zircons found at the deepest stratigraphic levels of MAL05-1B fall into three age groupings (~2 Ga, ~1100-1000 Ma, ~580-540 Ma), suggesting a provenance region with Paleoproterozoic, Mesoproterozoic, and latest Neoproterozoic rocks.  Samples at intermediate stratal levels capture a transition, in which the ~1100-1000 Ma and ~580-540 Ma zircon populations disappear, and a ~730 Ma zircon population first appears. At the youngest stratal levels available for zircon dating, only two age populations are prominent: ~730 Ma and ~2 Ga.  The Ruhuhu River watershed is a likely source for these zircons, owing to the presence of the Cryogenian Txitonga Group and Paleoproterozoic Irumide Belt lithologies.  While the precise timing of the change remains challenging to pin down, the zircon data make clear that either (a) a shift in provenance region occurred or (b) geomorphic processes drove evolution of one source region.                               

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5464

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