“Big Swamp” in Northern Australia and Colin Woodroffe
In early February I had the pleasure of attending a symposium at the University of Wollongong in honour of Colin Woodroffe. It was a great occasion to mark his retirement and allowed many of us to discuss a range of topics under the theme “Dynamic Coasts” while paying tribute to Colin’s remarkable academic career. His research covered many coastal themes and one of those has been his interest in the geomorphic evolution of estuaries. I first met Colin in 1976 on a visit to Cambridge at a time he was undertaking PhD research on mangroves in the Cayman Islands. This mangrove interest was revisited in the early 1980s when he joined the North Australia Research Unit (NARU) of ANU based in Darwin. Along with John Chappell and others I had the opportunity of spending some time in the field with Colin while he was at NARU. My role was to help drill lots of holes across the floodplain of the South Alligator River.
It was soon apparent after inspection of river banks and from drill cores that an organic layer existed under the blacksoil plains that were seasonally inundated by freshwater. I was reminded of what I had found some 12 years earlier in the Ord River estuary system where mangrove-rich sediments underlay the bare high tidal flats in this more arid climate area (Thom et al., 1975, Journal of Ecology, v 63, 203-232). We obtained 33 radiocarbon dates from the South Alligator River (SAR) which matched ages from the Ord (Woodroffe, Thom and Chappell, Nature, v 317, 711-713). The age range on the mangrove wood samples from the SAR area was c.5500 to 7000 years ago at depths of 2-7m beneath the floodplain surface. Pollen analysis and exposed stumps showed that the organic layer was from a mangrove environment. A similar organic layer rich in mangrove material occurred at King Sound and Fitzroy River in WA and Princess Charlotte Bay in Queensland.
We developed a model of estuarine sedimentation that expressed the effects of relative sea-level change in the early to late Holocene. Radiocarbon dates from areas under the blacksoil plains of the SAR (and bare high tidal flats of the Ord) indicated that mangrove swamps were flourishing at the time sea level was stabilizing. Before 7000 years ago the broad valley of the SAR was infilling with sediment in a shallow estuarine environment while sea level rose. Once sea level reached its present position sites for extensive mangrove colonization developed throughout the shoaling estuary. However, continued vertical sedimentation over these mangrove flats progressively eliminated intertidal environments effectively eliminating broad intertidal mangrove swamps. Mangroves continued to grow but were confined to intertidal habitats along the banks of the river and tidal creeks. After c.5500 years ago the SAR plains became vegetated by freshwater plants.
It was apparent that similar dramatic ecological responses were occurring across northern Australia due to sedimentary infill adjusting to the change from rising to stable sea level. There are some differences from estuary to estuary according to particular rates of sediment output and dimensions of the estuarine system. The conclusion is that vast tracts of mangrove vegetation was “lost” as the “Big Swamp”, as we called it, declined. For instance SAR mangrove alone during the Big Swamp phase was about 80,000 hectares, some 8-10% of the present total mangrove area of tropical Australia.
In concluding our Nature paper we stated that “it will be interesting to discover whether any changes in mangrove-based food chains can be detected in related fossil or even in archaeological deposits” (p 713). Well a paper appeared in 2023 which has made use of our model in reconstructing archaeological landscapes: J. Kowlessar, et al., 2023, “Reconstructing archaeological palaeolandscapes using geophysical and geomatic survey techniques: an example from Red Lily Lagoon, Arnhem Land, Australia”. PLoS One 18 (5): e0283006. (See also The Conversation, January 9, 2024).
Their work was on sites in the East Alligator area looking at different styles of rock art which changed over millennia. They found that different styles aligned with landscape changes driven by sea level rise. Interpretation of the art and material culture of Indigenous people in this area was linked to an understanding of past landscapes transitioning from a semi-arid open savanna in the Last Glacial Stage (MIS 3) through to estuarine conditions involving a mangrove swamp and then to the seasonally inundated floodplain in the Holocene. At their sites they found Big Swamp sediments and agreed with the sedimentation model we set out in our Nature paper. They mapped buried landscapes to help understand how the landscape has changed over time, and used this data to show how much of the landscape could be seen from each rock art site during the different periods of landscape evolution. During the Big Swamp phase rock art production was interpreted to be most “active, diverse in style and covered the most area of the plateau”. It is not clear whether this was because mangrove habitats provided abundant resources which sustained a large and stable population, or was a response to the contraction of available land caused by sea level rise.
It is very pleasing to see how our geomorphic research in the 1980s has contributed to this archaeological work. Kowlessar and colleagues have demonstrated that landscape models such as we developed “can yield profound insights into past human activities, even those as difficult to interpret as the incredible artwork of Arnhem land” (The Conversation, January 9, 2024). The NARU team that undertook the geomorphic and stratigraphic work is indebted to the commitment of Colin Woodroffe. His remarkable career has provided many other scientists, students and others with insights into coastal landscape change way beyond what we undertook on the SAR. It has been a great privilege to have worked with him on this and other projects.
Bruce Thom
Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2024. For correspondence about this blog post please email admin@australiancoastalsociety.org.au
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