非表示:
キーワード:
Holocene sedimentology; pollen analysis; palaeovegetation; palaeoclimate; Amazon rainforest; Amazon water levels; sea-level; Brazil
要旨:
Holocene environments have been reconstructed by sedimentological, mineralogical geochemical and high resolution pollen analysis of a 1190 cm-long core from Lago Calado near Manaus in Central Amazonia. The sediment core, dated by eight AMS-radiocarbon dates, has a basal age of 8330 ±50¹⁴C yr BP and documents the landscape development from a river valley before 8280 ¹⁴C yr BP to a lake environment. This change is related to the rise of the Central Amazonian water level, controlled by the postglacial sea-level rise, Pollen analytical results show the formation of a local Mauritia palm-swamp along the river margin between 8280 and 7700 ¹⁴C yr BP. At that time, highly diverse terra firme (unflooded upland) Amazon rainforest and poorly developed várzea/igapó (inundated) forests along the river characterized the vegetation. The expansion of várzea/igapó forests and a strong presence of aquatic plants started at 7700 ¹⁴C yr BP. reflecting the rise of the Amazonian water level and the formation of the Lago Calado. The occurrence of abundant Poaceae and Cyperaceac pollen in the lake sediments since 7770 ¹⁴C yr BP, suggest both the local colonization of unflooded mud banks around the lake margin and the formation of flooding meadows. Open exposed unflooded mud areas, periodically colonized by herbs, are related to seasonal Amazonian high and low water stands. In respect to the abundance of Poaceae pollen during the early and mid Holocene, these open areas must have been large, suggesting brief annual high and extensive annual low Amazonian water stands. After 4070 ¹⁴C yr BP the proportion of várzea/igapó forest areas was larger than before. Herbs were less abundant, probably due to the smaller area of unflooded mud banks. The Amazonian water levels must have been higher and the period of the annual high water stands was probably longer than before. Since 2080 ¹⁴C yr BP, the largest proportion of várzea/igapó forests is recorded, reflecting the highest Amazonian water level. Herbs were rare, suggesting that open unflooded mud areas around the lake were relatively small. This may perhaps also indicate that the seasonal period of the Amazonian high water level was the longest since 2080 ¹⁴C yr BP. The increase of Cecropia, Poaceae and Alternanthera during the last 150 yr, suggests stronger human impact on the local vegetation. Changes to higher Amazonian water levels are related to the rise of the Atlantic sea-level. Relatively short annual high water levels during the early Holocene and relatively long annual high water levels since 4070 and especially since 2080 ¹⁴C yr BP, can be interpreted as a climate change from a drier early Holocene (with lower annual rainfall rates and longer dry periods) until 4070 ¹⁴C yr BP to a wetter late Holocene
