Holocene palynology of five wetland basins in the Becher Point area, southwestern Australia

The wetlands on the Becher Cuspate Foreland provide an opportunity to undertake palynological studies in a series of discrete small basins all belonging to the same wetland suite - the Becher Suite - from middle to late Holocene. The wetlands have formed progressively as the beachridge plain prograded westwards, and provide a landscape within a temporal framework wherein wetland initiation ranges from circa 4500 years in eastern locations to circa 900 years in western locations. Using patterns of surface pollen composition in relation to extant vegetation as a baseline the middle to late Holocene record was investigated in five wetlands, selected to incorporate different ages and plant/ vegetation assemblages. The five wetland basins, in relatively close proximity and in the same climatic setting, have markedly different pollen history reflecting intra-basinal evolution. Pollen content of the sediment to shallow depth suggests that the following vegetation assemblages will be useful to interpret past vegetation patterns: Centella asiatica (L.) Urb. herb assemblage, Baumea articulata (R. Br.) S. T. Blake sedge assemblage, Typha (L.) sp. sedge assemblage, mixed B. articulata and Typha sp. sedge assemblage, Melaleuca teretifolia Endl. scrub assemblage, M. rhaphiophylia Schauer forest/shrub assemblage, with understorey of C. asiatica, M viminea Lindley heath assemblage, M. cuticularis Labill. assemblage, and wetland margin of Xanthorrhoea preissii Endl., Isolepis nodosa (Rottb.) R. Br. and Sporobolus virginicus (L.) Kunth assemblages. Radiometric dating of the sediment cores was used to provide an age structure. Pollen diversity and abundance against this age structure indicated that, except for X. preissii and I. nodosa, most wetland species were present over the interval of 4500 years BP to the present in the middle to late Holocene. However, there were a number of other important patterns in the down profile abundance of wetland pollen taxa and their inter-basin variation at isochronous levels: a lack of continuity down profile for some species; fluctuations in numbers of pollen taxa that were continuous; lack of correlation in timing of the peak pollen numbers between separate basins; variable total composition at the same isochron level from wetland to wetland; variable total composition of wetland pollen at different ages within the same wetland; the association of pollen species with sediment types; increases and decreases of wetland margin pollen taxa in the down profile composition; and the recent appearance of I, nodosa and X. preissii within the last circa 1500 years. The patterns of pollen derived from wetland vegetation in individual wetlands suggest that the ancestral distribution and abundance of plant assemblages in the Becher wetlands was a function of intra-basin environmental changes caused by wetland evolution. In contrast, pollen derived from upland vegetation exhibited continuity down profile, suggesting that delivery of upland pollen has been largely consistent, though variable in abundance and composition from basin to basin, and being exogenic, it has not reflected (hydrochemical, edaphic or hydroperiod) environmental conditions within the wetlands. The combination of slow rates of sedimentation and bioturbation have obliterated any potential fine scale sequencing of pollen, resulting in a pollen record which may contain a composite of up to several hundred years of mixed wet and dry climate assemblages, making it difficult to interpret detailed climate history. However, the appearance of X. preissii and I. nodosa circa 1500 years ago, coupled with a corresponding change in stratigraphy, probably signals a recent increase in rainfall.