Late quaternary mangrove distribution in northern Australia

Abstract

Although mangroves have long graced the north Australian coastline, stable sea levels required for the formation of extensive mangrove swamp forests have occurred only intermittently over the late Quaternary. Most ancestral mangrove swamps are likely to have been formed below present sea level. The only well-preserved deposits that have been described, developed on the present continental surface as sea level reached its present position in the early Holocene. Gradual upstream shifting of mangrove communities from about 8400 BP is recorded in sediments from the wet tropics, followed by the establishment of extensive Rhizophora forests over the newly drowned estuaries. More extensive Rhizophora swamps developed in the monsoon tropics where an earlier transitional phase has not been preserved. These 'big swamps' infilled over the next 1500–4500 years as sediments accumulated above the now stable sea level. The present mangrove estate, though more restricted, is fairly stable, with maintenance of mangrove forests in protected prograding bays and in estuaries kept open by adequate river flow. In the short term, mangroves may be threatened by human influences, but any change in climate leading to a gradual change in sea level should again provide conditions for expansion of mangrove habitats across northern Australia.