Six thousand years of peat accumulation in a Dendritic, Intermontane Drainage basin of Tropical tasek Bera, Malaysia : evidence of a complex evolution history based on stratigraphic records

Tasik Bera is a dendritic, intermontane basin and the largest natural freshwater lake system in Malaysia. The basin is 30 m above present sea level, with an average precipitation exceeding, 2,000 mm/a. Organic-rich sediment accumulation in the basin began some 6,000 years BP, during the time of maximum mid-Holocene transgression. Since then, climatic and vegetational changes sea level led to terrestrialization of channels and subbasins, within the wetland system and to paludification of the deeply weathered, riparian lowland dipterocarp forest, peat swamp forest (PSF) with various floral composition, Cyperaceae and Pandanaceae swamp and open water areas. The organic-rich sediments of the Tasik Bera basin are unique to Southeast-Asia in composition and stratigraphic diversity mainly as a result of the natural shifting of the flora due to climatic and hydrological changes. Previous descriptive models of peat deposits from the tropics assumed a layer-cake structure of peats and are clearly not applicable to the Tasik Bera peat deposits. Stratigraphic analyses and radiocarbon-dates reveal that peat accumulation started first in the southern tributaries and progressively expanded into the northen part of the lowland basin, mainly by paludification of the lowland riparian areas. Core sections from the southern Tasik Bera basin record a variety of depositional settings including high discharge rivers, lakes with anoxic bottom waters, acidic peat water channels and freshwater tributaries. Around 5,500 years BP, the climate changed to wetter and warmer conditions in Penisular Malaysia, which combined with the lowland, low-gradient and deeply weathered landscape setting, favoured the evolution of lake and pond system in the Tasik Bera area. Subsequent paludification during the following one thousand years resulted in organic-rich and peat deposition. Vegetational changes to swap forest with dense, interwoven root system, may have low ash, low sulfur, woody peat accumulated during the past 5,000 years with a depositional rate of 1 to 1.4 mm/a. Two thousand years after the onset of peat accumulation in the southern part, organic material accumulation began in the most northen Tanjung Kuin area. Accumulation rate in the northern open area was slow during the forested period ( 1 mm/a), but rapid after the progressive retreat of the swamp forest in favor of Pandanaceae (2.5 mm/a) and a peat thickness of more than 500 cm was deposited. Peat accumulation slowed to 1.3 mm/a when Cyperaceae, such as Lepironia aritculata, began to expand and replace Pandanaceae (Pandanus helicopus) habitats in the littoral open area, probably as a result of decreasing precipitation and a drop in water level in the swamp basin. At present, most of the peat deposits have an epipedon enriched in clay minerals formed in response to the distrubance of the wetland environment during the last four decades.