Unveiling 4500 years of environmental dynamics and human activity at Songo Mnara, Tanzania

Coastal East Africa has undergone massive transformations through the Late Holocene, with a combination of changes in sea level, increasing human settlement, and ensuing use of coastal resources. A comprehensive multi-proxy analysis, including pollen, phytolith, charcoal, stratigraphy, particle size, and geochemical data from sedimentary cores extracted from mangrove ecosystems combined with soils from archaeological contexts, provided valuable insights into vegetation dynamics, environmental changes, and human interactions within the mangrove ecosystem of Songo Mnara Island, Tanzania over the last 2590 BCE (4540 cal yr BP). The bottommost layers indicate a lack of vegetation, as deduced from the presence of coral rags and high calcium and carbonate content, possibly due to high mid-Holocene sea-level. Evidence of mangrove taxa suggests a decrease in sea level, enabling the establishment of mangroves from around 2590 BCE. A brief period of sea-level rise occurred between 90 BCE and 320 CE before sea-level fell until 1570 CE. Significant evidence of human activity is recorded from around 1400 CE indicated by increased charcoal, crop phytoliths, and evidence of marine resource utilisation. The timing of this human-environment interaction is also linked to the time of lower sea level. However, there was evidence suggesting human abandonment of the island from around 1500 CE. This coincided with a subsequent rise in sea levels and potentially prolonged drought conditions spanning from 1570 to 1700 CE. These factors likely contributed to a shortage of food resources in the area, impacting both agricultural practices due to the scarcity of natural freshwater and the accessibility of marine food resources. From 1700 CE to the present, fluctuations in sea level have been observed, with a signal of recent sea-level rise in tandem with shifts in mangrove, terrestrial herbaceous taxa and fire activity.

The low sedimentation rates within mangrove areas suggest that the mangroves on Songo Mnara Island may not keep pace with the current rate of sea-level rise.