Year

2005

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

The degree of aspartic acid racemisation measured in radiocarbon dated specimens of fossil molluscs collected from Holocene barrier estuaries on the southeast coast of Australia is evaluated in the context of results of laboratory-induced racemisation established in heating (simulated ageing) experiments. The general kinetic trend of aspartic acid racemisation, in both heating experiments, and at ambient temperatures during diagenesis in the fossil molluscs Anadara trapezia and Notospisula trigonella conforms to a model of apparent parabolic kinetics. Using the apparent parabolic kinetic model, numeric ages based on the degree of aspartic acid racemisation in fossil specimens of A. trapezia and N. trigonella have been determined. Aspartic acid D/L ratios in Holocene specimens of A. trapezia and N. trigonella range from 0.049±0.005 to 0.510±0.009, representing an age range from 8,000 yr. Accordingly, the Holocene amino- and chronostratigraphies of the wavedominated barrier estuaries Lake Illawarra, St Georges Basin, Swan Lake and Burrill Lake have been established based on the extent of aspartic acid racemisation measured in 290 specimens of fossil molluscs. For fossil material beyond the time span of the radiocarbon dating method (ca >50 ka) and the calibration range of aspartic acid, relative ages have been determined based on the slower racemising acids alanine, valine, leucine and proline. The relative age determinations on older fossils indicate that Last Interglacial successions have been preserved at depth within the incised valleys on the southeast coast of Australia. Aminostratigraphy, in conjunction with thermoluminescence dating, additional radiocarbon ages, and the litho- and biostratigraphic analysis of 141 vibracores, allowed the detailed assessment of the geomorphological evolution of wave-dominated barrier estuaries that formed in both broad, and narrow, incised valleys on the southeast coast of Australia. Results from the stratigraphic and geochronological analysis show that the estuaries investigated in this thesis have an evolutionary pathway that is different to previous models of barrier estuary evolution. In contrast to the earlier models of barrier estuary evolution, this thesis places a greater emphasis on the influence of the antecedent Late Pleistocene landsurface on hegeomorphological evolution of barrier estuaries on the southeast coast of Australia. The Late Pleistocene sediments are represented by dense, mottled estuarine clays and/or by remnant barrier systems comprising medium-grained quartz sand with a clay matrix. The Late Pleistocene sediments preserved within the incised bedrock valleys represent sedimentary successions deposited during previous high-stands in sea-level that have undergone minor diagenetic alteration during subsequent sea-level lowstands. Therefore, the Late Pleistocene sediments now represent low-stand weathering profiles and provided the substrate over which Holocene sedimentary successions have been deposited. In particular, the preservation of remnant Last Interglacial barrier systems are found underlying the Holocene barrier systems and provide a core over which Holocene barrier sediments have accumulated. Results from this study have also shown that Holocene transgressive sands are more extensive than previously anticipated and form a basal, near basin-wide shell-rich deposit that extends almost up to present sea-level. The transgressive sandsheet facies lies unconformably over the Late Pleistocene antecedent landsurface and was deposited during the most recent post-glacial marine transgression (PMT) when rising sea-levels breached Last Interglacial remnant barriers and inundated shallow incised valleys causing more open marine conditions to persist between 8,000 and ca 5,500 years ago. This contrasts with established models for barrier estuary evolution on the southeast coast of Australia where Holocene PMT sandsheets were restricted to the mouths of incised valleys and back-barrier central basin muds lie directly over the antecedent Pleistocene landsurface. This study also presents a database of 121 previously published radiocarbon ages obtained from fossil molluscs, organic material and fixed biological indicators from back-barrier sedimentary successions and the marginal marine environment on the southeast coast of Australia that have been used to constrain Holocene sea-level fluctuations. The database has been limited to fossil materials that have an accurate description of facies association and stratigraphic relationship to present mean sea-level (PMSL). An assessment of the uncertainty terms associated with the various proxy sea-level indicators is made and radiocarbon ages are calibrated to sidereal years. Additional radiocarbon ages and aspartic acid racemisationderived ages obtained on fossils preserved in the Holocene PMT sandsheets deposited inshallow incised valleys are presented in this thesis. A synthesis of the previously published radiocarbon ages and new data presented in this thesis has permitted a revised Holocene sealevel curve for the southeast coast of Australia to be delineated. Results show that rising sea levels during the most recent PMT attained an elevation of around -10 m by 10,000 cal yr BP and continued to rise to ca -5 m by 8,500 cal yr BP. Between 8,300 and 8,000 cal yr BP sealevel had risen to at least 3 m below PMSL and inundated shallow incised valleys, resulting in the deposition of shell-rich transgressive sandsheets. The most recent PMT sea-levels attained present levels around 7,700 cal yr BP, slightly earlier than a previously proposed culmination of ca 7,000 cal yr BP. Results indicate that sea-level continued to rise to between 1 and 1.5 m above PMSL by 7,400 cal yr BP during the culmination of the most recent PMT and was followed by a sea-level highstand that lasted to some time between 3,000 and 2,000 years ago. This was followed by a relatively slow and smooth regression of sea-level from ca +1.5 m to present levels. A series of minor negative and positive oscillations in relative sealevel associated with variations in ocean topography and/or climate change during the mid to late Holocene appear to be superimposed over the Holocene sea-level highstand and subsequent smooth sea-level regression. This study highlights the potential of aspartic acid racemisation as an important supplement for dating of fossils preserved in Holocene marginal marine deposits. This thesis also represents the most rigorous application of the aspartic acid racemisation dating method to the investigation of Holocene sedimentary infill of incised valley systems, and sea-level change during the most recent PMT. The evolutionary models of barrier estuary evolution developed in this thesis show that both the antecedent Late Pleistocene substrate and fluctuating Holocene sea levels have had a greater influence on the geomorphological evolution of barrier estuaries than previously anticipated. These revised models of estuary evolution can be applied to other shallow incised valley systems on tectonically stable, wavedominated coastlines, and provide a template for future studies in wave-dominated barrier estuaries on the southeast coast of Australia.

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