Holocene sea level change in Northeastern Australia: a high-resolution record from Albatross Bay, Cape York Peninsula

There is a lack of data from the Gulf of Carpentaria (GoC), northeastern Australia, about the timing and elevation of sea level fluctuations during and following the culmination of the Holocene marine transgression, and how the coastal environment responded. A localised sea level reconstruction is needed because of the inherent variability in relative sea level change, both globally and within northeastern Australia. The study area of Albatross Bay also has an extensive archaeological record: Aboriginal shell mounds are present across the region that have a history of formation dating back to the mid-Holocene. A high-resolution palaeoenvironmental reconstruction and, in particular, a record of Holocene sea level change, are missing components that currently limit understanding of human-environment interactions in the past in the region. The overarching aim of the research reported in this thesis is to produce a Holocene sea level curve for Albatross Bay, on the northeastern coast of the GoC, which will address these gaps in current knowledge and improve our understanding of Holocene sea level change. The research reported in this thesis was conducted at three locations in Albatross Bay: Red Beach, Kwamter and Wathayn. At Red Beach, a sequence of thirteen beach ridges parallel to the shoreline contained a high-resolution record of sea level change and coastal progradation. At Kwamter and Wathayn, the presence of a single beach ridge at each location, formed during a period of higher relative sea level, along with extensive supratidal mudflats bordered with mangroves, provided ideal proxies for sea level reconstruction. All three field locations are also the focus of past and ongoing archaeological investigations. To establish the sedimentary stratigraphy and quantify the depositional history of each location, pits were hand-excavated into each beach ridge crest, most swales, and across the supratidal mudflats. A soil auger and a D-section corer were used to extract additional sediment samples from beneath the pits, and from the mangrove zone at Wathayn. A trench was also excavated through the modern beach at Red Beach to aid in understanding the formation of the beach ridges and relict beach ridge plain. A total of 38 excavations were conducted: 24 at Red Beach, 8 at Kwamter and 6 at Wathayn. Sediment samples were collected from each stratigraphic section, and organic carbon content, carbonate content, and particle size parameters were analysed in the laboratory. Samples were also collected for absolute age determination using radiocarbon and Optically Stimulated Luminescence techniques. Sandy beach and relict foredune sediments overlie intertidal or supratidal sediments in most of the beach ridges. At Red Beach, basal sediments of all but the three proximal ridges were cemented with carbonate, forming calcrete. Supratidal mudflat and intertidal mangrove deposits were uncovered at Kwamter and Wathayn. A transgressive shell lag was also discovered beneath the supratidal mudflat at Kwamter, formed during a period of higher relative sea level in the past. A sediment facies scheme for the Albatross Bay coastal environments, developed from the field and laboratory data, was combined with the geochronological data to create a high-resolution record of Holocene coastal evolution and sea level change in Albatross Bay. At approximately 6,500 years BP, relative sea level was approximately -1.7 m below present mean sea level (PMSL). Mangroves were present at Kwamter, corresponding to the “big swamp” phase of extensive mangrove habitats reported by other researchers from across the northern coastline of Australia. Relative sea level was rising, and reached PMSL prior to approximately 5,000 years BP. Relative sea level continued to rise, reaching a mid-Holocene highstand of approximately +1 m above PMSL approximately 4,000 years ago, leading to the formation of the beach ridges at Kwamter and at Wathayn. The transgressive shell-lag was deposited at Kwamter during the same period. Relative sea level then either remained at this elevation for a further 1,000 years before falling, or began to fall very gradually after approximately 4,000 years ago. Once relative sea level began to fall, the beach ridges at Kwamter and Wathayn were isolated from active coastal processes. Infilling of the Embley River estuary led to the formation of a mangrove zone on the proximal side of the beach ridge at Kwamter. As relative sea level continued to fall, shoreline progradation resulted in the formation of the supratidal mudflat at Kwamter and Wathayn. Relative sea level was between +0.5 to +1 m above PMSL approximately 2,200 years ago when beach ridge formation commenced at Red Beach, partially influenced by an increase in effective precipitation, and thus sediment supply into Albatross Bay, at this time. From 1,700 years ago to present, there was a change in the morphology, sedimentology and coastal progradation rates within the beach ridges at Red Beach, linked to a further increase in sediment supply into Albatross Bay, together with the re-commencement of tropical cyclone activity in the region. Relative sea level fell to approximately +0.7 m above PMSL around 500 years ago, and then to PMSL. This sea level curve for Albatross Bay provides a new, high-resolution sea level record for the GoC, and for northern Australia. It adds to our understanding of the magnitude, patterns and drivers of local and regional coastal evolution and relative sea level change during the mid- to late-Holocene. It will also have immediate utility for ongoing archaeological research within Albatross Bay, and elsewhere in northern Australia.