A pollen analytical investigation of Holocene palaeoenvironments at Lake Tyrrell, semi-arid Northwestern Victoria, Australia

Abstract

This thesis presents a pollen analytical reconstruction of mallee vegetation history in the vicinity of Lake Tyrrell, a large active salt lake in semi-arid northwestern Victoria. The project combined studies of the modern pollen rain, pollen depositional processes and sedimentological characteristics of lake deposits to provide an analytical framework appropriate to the interpretation of fossil pollen spectra from the novel salt lake setting. Pollen trapping in northwestern Victoria and western New South Wales indicates that the characteristic plant communities of semi-arid southeastern Australia can be identified from the pollen spectra they produce. Mallee heath communities produce spectra containing a diverse array of heathland taxa with limited pollen dispersal capacities, including Banksia, Baeckea behrii, Cryptandra and Calytrix tetragona. Mallee heaths also produce large amounts of Calli tris pollen but can be distinguished from Callitris woodland by the regular presence of pollen from restricted heathland taxa. Pollen spectra from chenopod shrublands are characterised by overwhelming dominance by Chenopodiaceae pollen. Other halophytic taxa often represented include Selenothamnus and Disphyma. Riverine forests produce pollen spectra dominated by Eucalyptus Muehlenbeckia cunninghamii and Amyema pollen occur commonly. It may be possible to identify pollen of Eucalyptus camaldulensis in fossil assemblages allowing this community to be more clearly delineated in the fossil record. Mallee communities can be distinguished from eucalypt dominated communities in moister areas by producing pollen spectra containing relatively high percentages of chenopod pollen and low percentages of grass pollen. Eucalypt woodlands in areas receiving more than 400 mm mean annual rainfall produce pollen spectra containing appreciable quantities of Callitris pollen. No relationship could be discerned between pollen production and rainfall in this study. Pollen trapping at Lake Tyrrell suggests that the majority of pollen arriving at the lake surface is wind borne. Few are washed from the lake margin or imported down Tyrrell Creek. This contrasts strongly with the situation in humid areas where pollen washed from the catchment or carried in creeks are a significant part of a lakes pollen budget. Pollen reaching the surface of Lake Tyrrell are rapidly redistributed and are preferentially deposited in areas marginal to the persistent salt crust. Maximum pollen concentrations occur on relatively high parts of the lake bed, again contrasting strongly with models derived from permanently wet lakes where maximum deposition of pollen occurs in the deepest parts of the basin. The sediments of Lake Tyrrell record a history of hydrological change extending to approximately 10,000 BP. Between 10,000 BP and 6600 BP water in Lake Tyrrell was shallow, saline and probably ephemeral. Water depths and the frequency I duration of flooding were most likely similar to those experienced today but there was no persistent salt crust. Between 6600 BP and 2200 BP the lake was a permanent though fluctuating waterbody. The lake waters were saline throughout this period. Water balance calculations suggest average rainfall in the lake catchment would have been approximately 2.6 times modern levels between 6600 BP and 2200 BP. The lake was dry between 2200 BP and 800 BP. The local groundwater table fell below the lake bed. There was no salt crust until about 800 BP when rainfall increased slightly allowing local watertables to rise and modem salt lake conditions to develop. Changes in vegetation around Lake Tyrrell occur in association with changes in rainfall. Between 10,000 BP and 6600 BP Lake Tyrrell was surrounded by open woodland dominated by Allocasuarina Eucalyptus and Callitris were probably present in limited areas. At 6600 BP mallee communities began to dominate the landscape. It is likely the appearance of mallee reflects the arrival of mallee eucalypts spreading from refugial areas occupied during the last glacial maximum. Callitris patches were a prominent element of the regional vegetation during this the wettest interval in the Holocene record. They appear little affected by the active fire regime of the times. Between 2200 BP and 800 BP mallee persisted and Allocasuarina experienced a modest expansion. Callitris declined drastically. The dense mallee vegetation which surrounded the lake at the time of European settlment was established after 800 BP. The history of Holocene environmental change identified from Lake Tyrrell provides a possible explanation for the patterns of archaeological site distribution observed in the Mallee Districts of northwestern Victoria. Show more