Abstract
The Lefika La Noka (LLN) tufa site is located at the point of confluence of the spring-fed Grootvlei River and the Skeerpoort River. It is located on the boundary of the North West and Gauteng provinces within the Cradle of Humankind (Cradle) UNESCO World Heritage Site, South Africa. This tufa site is located north of the Malapa and Gladysvale fossil sites in the Cradle. The tufa deposits cover an area of at least 170 m by 85 m to a depth of 12.5 m. The LLN tufa is characterized by calcium carbonate precipitates from freshwater and indicative of past aqueous environments and atmospheric conditions. Despite its extensive nature and location near key hominin-bearing fossil sites, Malapa and Gladysvale respectively, the tufa deposit yet remains understudied.
The aim of the present study is to establish the timing, syn- and post-depositional evolution of the LLN tufa deposits. The study employed in situ cosmogenic beryllium-10 and aluminium-26 (26Al/10Be) burial dating of quartz from tufa drill cores. In addition, the study aims to assess erosion rates of rock outcrop and river sediments to help infer the tufas geomorphic setting. This study used cosmogenic 10Be to determine exposure ages and erosion rates of bedrock and river sediment samples. There are five tufa drill cores processed and analysed for cosmogenic burial ages. There are two river sediments (LLNR1 and LLNR2), two bedrock samples (LLNB1 and LLNB3), and one soil sample (LLNS1) analysed to determine exposure ages and erosion rates. Pre-cleaning and physical sample processing for this study was conducted at the SPECTRUM Analytical Facility at the University of Johannesburg, South Africa. Complete quartz dissolution and chemical separation of Al and Be isotopes was carried at Helmholtz Zentrum Dresden Rossendorf (HZDR), Germany.
The five tufa core samples reveal old burial ages, ranging from 2.24 to 3.51 Ma. These burial ages are inconsistent with other LLN burial ages from different sources such as U-Th ages. The bedrock, river sediments and soil sample revealed exposure ages range between 25.7 to 433 ka. The soil sample exhibits exposure age that is in agreement with U-Th ages of ~245 ka from another study. The soil sample was collected close to the tufa deposits, is therefore considered to be sourced from the tufa. The exposure age of the soil sample (~265 ka) suggests approximation timing of the LLN tufa. The average exposure ages of the river sediments and single soil sample suggest a Middle Pleistocene age for the tufa deposits.
Two rock samples and river sediment samples as well as the single soil sample yielded erosion rates ranging from 2.08 to 26.9 m/Ma. The soil sample and river sediments yielded narrow
VI
erosion rates, ranging between 2.08 to 2.25 m/Ma. These rates obtained are similar to other erosion rates previously documented from the Cradle. Bedrock samples yielded large range in erosion rates than the stream sediments. The range difference particularly in bedrock samples strongly reflect how lithology influences rates and landscape dynamics across various tectonic and climatic settings.
By integrating in situ cosmogenic burial data and geomorphological data, this study seeks to help and refine our understanding of environmental changes of the Greater Cradle and their implications for paleoenvironmental reconstruction and the hominin origin contexts. Moreover, this research provides aids to both conceptual and methodological advancements in cosmogenic geochronology demonstrating the strengths and advantages of using in situ cosmogenic nuclides in diverse carbonate settings. In addition to geochronology advancements, this study provides insights to landscape evolution and geomorphic changes in karstic environments enhancing our understanding of how the underlying geology and environmental processes exert controls on the surrounding landscape.