Our Rising from the Depths project in northern Mozambique has been concerned with understanding the natural environment as a context for marine and underwater cultural heritage. A large dataset of geophysical survey results captured around Mozambique Island (Ilha de Mozambique) allows for new insights and features to emerge demonstrating the long history of sea-level change and its relevance for today’s communities in the context of climate change. In this blog we provide some of the first imagery derived from the survey work, which was conducted with colleagues from Centro de Arquelogia Investigção e Recursos da Ilha de Moçambique (CAIRIM) and the local community in Mozambique Island and marine heritage practitioners from the region.
Global sea level was -120m to -130m lower than present at the height of the last Ice Age (roughly 20,000 years ago) because vast quantities of ocean water were locked up in ice sheets. The coastal landscape inhabited by prehistoric people was therefore very different to today. Away from the large continental ice sheets the fall in sea level exposed large tracts of land, allowing rivers to cut across what is now the continental shelf and pushing coastlines out towards the shelf edge. Over time, global climate warmed, the ice melted and sea level rose. These landscapes, and any archaeological evidence they contained, were flooded and now lie on, or buried under the seabed.
Previous studies in Southeast Africa have identified remnants of these submerged landscapes. These types of evidence have been found off the KwaZulu Natal coast of South Africa and as far north as Maputo in southern Mozambique. The evidence includes former shoreline complexes, incised valleys and their sedimentary fills and shallow water/lagoonal sediments found at depth on the continental shelf. The resulting evidence has also been used to provide insights into the timing, pattern and rate of the post-Ice Age sea-level rise. However, elsewhere on the East African coast, investigations of submerged landscapes and sea-level change are few and far between. The new evidence from northern Mozambique therefore represents a step towards filling this gap.
Consequently, the main aim of the December 2019 marine geophysical survey of Mozambique Island was to see if we could find any evidence of past sea level in this area. We chose to focus on two main areas. Firstly, the outer edge of the shelf fronting the Baie de Mozambique. Secondly, the channels which form the bay’s deeper entrances and allow access to the shallow waters behind the Island . By using both multibeam echosounder (MBES) and sub-bottom profiler (SBP), we hoped to capture the geomorphic expression of relict landforms exposed on the seabed as well as features and stratigraphy which are currently buried under the seabed. The acquired data are still being analysed, but even so, a preliminary examination has been able to identify a number of features of geological and archaeological interest.
Our main area of MBES survey covered the outer shelf fronting the Baie, an area of ~13km2. The resulting Digital Elevation Model (DEM) has a spatial resolution of up to 1m and shows a steep shelf which descends from ~-20m at the Baie entrance to almost -200m within a kilometre offshore. Several features of interest are visible on the MBES, the clearest being a narrow channel cut into the seabed between the Ile de Goa and Ile de Sena. At face value, this seems to provide a great example of a former river valley which was incised when sea level was lower . Also apparent on the MBES are several submerged breaks in slope. The clearest one forms a distinct cliff line both north and south of the incised channel (but is absent in front of the channel). Where the base of the cliff is clearly visible, its depth is at ~-65m. At least two other low ridges/breaks in slope occur landward of the cliff line at depths of ~-35m to -40m. It is presently unclear whether these features represent former palaeo-shorelines. The depth of the submerged cliff line superficially matches palaeo-shoreline complexes (-60m) from KwaZulu Natal, but further analysis is needed to conform this. Fortunately, there was also enough time (and budget!) to acquire smaller patches of MBES around the northern end of the Ilha. These give glimpses of the inner parts of the Baie showing in particular the deeply incised nature of the channel between the Ilha and the mainland . These data also captured some of the historic shipwrecks which are known to lie here and which will be the subject of future blog posts.
Meanwhile, SBP acquisition was arranged to give a series of profiles running offshore from the coast to the shelf edge and across and along the channels. These were sited to establish the wider stratigraphic sequence and provide targeted data over the channels which could demonstrate how they responded to sea-level change. Starting with the outer shelf, the SBP profiles clearly show that the channel visible on the MBES is actually incised to a considerable depth below the seabed and was later infilled . In fact, the SBP data also show that it extends seaward of its surface expression as a completely infilled valley. The SBP data also confirmed the existence of the distinct cliff line on the outer shelf, and also suggests that its base is buried, and in some cases there may also be a deeper buried break in slope at ~-92m . In the outer part of the Baie, channels are also clearly visible on the SBP data, incised below the seabed and subsequently infilled . In all cases, the nature of these infills requires further analysis. Other potential features of interest include terraces cut into the flanks of the channels and secondary channels paralleling the main channel . Finally, the stratigraphy becomes more complex in the inner part of the Baie behind the Ilha. Whilst the seabed surface appears to be relatively undulating, the SBP results show a more rugged, buried topography. These include high points with an acoustic character suggestive of reefs and basins which have been infilled by horizontal or gently-dipping layers sediment, possibly suggestive of deposition in lagoonal or sheltered water conditions.
All the above is still work in progress, with interpretations to be confirmed by additional processing and analysis. However, even this preliminary glimpse has shown us the potential of these data to contribute to our understanding of sea-level change and palaeo-landscape evolution on the coast of East Africa. The survey work will form a component of the training on the tools and techniques used being delivered to the community on the Ilha and will inform new exhibition materials for the CAIRM facility on the island.