Environmental and sea-level variations on the southeastern Brazilian coast during the Late Holocene with comments on prehistoric human occupation

摘要

Analysis of organic sediments, including their diatom and pollen contents, was carried out in a swamp forest on the coastal plain of Sao Paulo State (southeastern Brazil). A palaeoenvironmental reconstruction was made regarding this region where communities of fisher-gatherers lived in prehistory, producing numerous shell mounds. From at least 4900 until 3470 cal yr BP the site was a lagoon surrounded by a relatively open forest. The occurrence of shell mounds 50 km from the coast and the 'peak' of human occupation between 5700 and 3300 cal yr BP can be linked to the presence of this great lagoon which formed an important food source. The lagoon desiccated ca. 3470 cal yr BP, i.e. during the Late Holocene sea-level fall, and was replaced by a swamp forest that still exists today. This forest was altered during a short period between 1300 and 675 cal yr BP, when a freshwater lake or marsh was formed as a result of a rise in groundwater level. The climate in the area throughout the Middle-Late Holocene (ca. 5000 cal yr BP to the present) has not changed significantly except for three slightly more humid episodes at 3470-2750, ca. 1940, and 1300-675 cal yr BP. No indications of human impact on the vegetation during this period could be identified. Our results show that from 4900 to 3470 cal yr BP sea-level was higher than at present, contradicting a widely accepted relative sea-level trend that suggests that at least two 'negative' sea-level oscillations occurred during the Holocene, i.e. at 4560-4150 and 3200-2800 cal yr BP. Our data confirm recent studies instead proposing a continuously declining relative sea-level during the Late Holocene.