Peritidal stromatolites as indicators of stepping-stone freshwater resources on the Palaeo-Agulhas Plain landscape

Abstract

Natural sources of freshwater are a limiting feature to the distribution of terrestrial species that rely on regular hydration, especially in coastal environments. Early modern humans would similarly be restricted by water supply and likely would have congregated or settled in close proximity to permanent water bodies such as beyond the estuarine extent of large rivers. However, along the modern south coast of southern Africa, for example, distances between freshwater from rivers can exceed what would be convenient daily walking limits. In the absence of alternative water supply, or a means of water storage, these distances would preclude human settlement beyond river borders. Recent discovery of living peritidal stromatolites forming along the southern South African coastline suggests a potential alternative water source. These unique ecosystems form only at locations where groundwater rich in calcium carbonate meets periodic marine intrusion along the coastal margin. Calcium carbonate is precipitated by cyanobacteria and sediment is trapped by diatoms, primarily, to form layered deposits that preserve into the fossil record. Critical to the formation and growth of these stromatolites is a fresh, uninterrupted supply of groundwater. During their growth, the peritidal stromatolites accumulate and retain this freshwater through the formation of rimstone dams and barrage pools at the seaward margin of neocoastal geological outcrops. Recognising the presence of permanent water sources other than rivers for early humans is challenging; however, these stromatolites provide a clear signature of an alternative water resource. Here we review the modern prevalence of peritidal stromatolites in this region, of which approximately 540 are known along a 200 km stretch of South Africa's southern coastline featuring only 11 estuaries. This is linked to features (cf. rimstone dams and barrage pools) resembling peritidal stromatolites in the adjacent Palaeo-Agulhas Plain landscape using multibeam echosounder and side-scan sonar data and contextualised as a possible water resource for early modern humans.