University of Tasmania
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Mid to late Holocene sea level history and coastal evolution in Tasmania

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posted on 2023-05-28, 09:11 authored by Brigid MorrisonBrigid Morrison
An understanding of past sea level change in Tasmania is essential to understanding the contribution of Antarctic ice melt to the global oceans. Research into Holocene sea level history in Tasmania has been sporadic and conflicting. The primary goal of this research was to develop precise Holocene sea level index points (SLIPs) for Tasmania, south eastern Australia, achieved by: establishing the relationship between Tasmanian coastal salt marsh biogeomorphology and mean sea level; and locating and documenting palaeo-environments that contain a Holocene sea level signal of substantial duration. The northern hemisphere quantitative, base of basal salt marsh approach was combined with the Australian coastal geomorphic landform evolution approach to reconstruct Holocene sea level history. Pleistocene transgressive surfaces in micro tidal drowned river valleys in south eastern Tasmania provided a set of depth sequenced base of basal sea level index points. A full intertidal foraminifera based transfer function was calibrated with a sediment core to fill gaps in this sea level record. A contrasting coastal evolution sedimentary record in a meso tidal, open coast intertidal setting in north western Tasmania was examined by targeting an organically enriched sedimentary deposit underlying the modern salt marsh, that was inferred to be a drowned Holocene salt marsh facies. Tasmanian salt marshes are constrained to elevations relative to MSL within sites, but inundation period is a better predictor of marsh zonation than elevation. The greatest between site variation in inundation period occurs at marsh seaward edges that are limited to no more than 33% tidal inundation period per year. This regional variation in inundation period of the seaward edge limits the suitability of regional modern analogues for transfer function based sea level reconstructions. Sea levels were below present throughout the Holocene, with a mid-Holocene highstand absent. The slow rates of sea level rise after 6000 yr BP indicate continued melt water contribution from Antarctica after 6000 yr BP to the present. The open coast site in north western Tasmania did not yield a longer Holocene sea level history. The age of the putative marsh was from ~ 37 000 yr BP to ~ 26 000 yr BP. It was a preserved, semi indurated Pleistocene swamp deposit that occupied the swales between sand dune ridges through the latter stages of glacial advance, and just prior to the last glacial maximum, when aeolian processes dominated the landscape. Integrating the sea level reconstruction for Tasmania with the accepted geochronological framework for estuarine evolution added further detail on the geomorphic evolution of coastal landforms in south eastern Australia, particularly the prevalence of the transgressive sand sheet, now shown to be present in shallow marine environments outside of estuaries. Further research to better populate the early and late Holocene sea level curve for Tasmania should focus on locating the transgressive surface in shallow subtidal, and upper intertidal locations.


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