The middle and late Holocene environment of Sydney Harbour (NSW, Australia): a millenial story preserved in live and dead molluscan assemblages

Understanding how biological communities have responded to past climate change is key to evaluating, modelling and predicting the future changes to biological communities due to anthropogenic climate change. This thesis investigates ecological and environmental variability in Sydney Harbour during the middle and late Holocene using the palaeoecological and sclerochronological information preserved in living and subfossil molluscan assemblages. This thesis is divided in three chapters linked sequentially in a logical order. First, I used radiocarbon-calibrated amino acid racemization ages from 173 Fulvia tenuicostata shells collected from Sydney Harbour (NSW, Australia) to quantify time-averaging in surficial and excavated death assemblages (Chapter 1). I determined that the upper 1.6 m of the sedimentary column preserved molluscan assemblages stratigraphically ordered from ~ 100years old in the most surficial layer to ~ 5000 years old in the deepest layers. This first chapter provides the geochronological framework for the historical comparisons in Chapter 3 and was the first quantitative study of time-averaging from temperate Australian waters. Secondly, I evaluated the potential of Callista disrupta shells, the most common living subtidal bivalve in the area currently, for sclerochronological and palaeoecological studies (Chapter 2). I found that C. disrupta form annual growth bands which I used to determine age, growth and population structure, I also quantified seasonality patterns in shell trace elements (Mg/Ca, Ba/Ca and Sr/Ca). I derived a linear calibration equation that explained the relationship between Mg/Ca and water temperature in C. disrupta which permitted me to use Mg/Ca ratios as a reliable proxy for maximum annual temperature. I also documented a strong relationship between Ba/Ca peaks and influx of freshwater into the system. This second chapter demonstrates that C. disrupta shells are viable proxies for maximum summer temperature and freshwater flux. Thirdly, using C. disrupta shells collected from the excavation layers dated in Chapter 1, I compared age, growth and shell composition of the living C. disrupta population with the historical population of the middle, late and recent Holocene (Chapter 3). I found notable differences in growth rates and lifespan between modern and middle Holocene population, with the modern populations being significantly larger and living longer periods; I also found that C. disrupta became the dominant species in the area in the last decades, while it was rare in the same area during the most part of the middle and late Holocene. Using Mg/Ca ratio in the shells as environmental proxies, I suggest a middle Holocene significantly colder with summer ~ 2 ˚C lower than today which may have affected the growth of C. callista ~ 5000 years ago. Also, based on Ba/Ca ratio profiles, I found evidence of a strong change in the hydrological dynamics of Sydney harbour during European colonization ~ 150 years ago. Together these three chapter build a compelling picture of the changes in Sydney Harbour over the past ~ 5,000 years. I suggest a link between the anthropogenic impact during this period with strong changes in the molluscan community structure. I provide new insights into how ecological communities have varied since the mid-Holocene and the relationship between these variations and historical environmental changes, naturally and anthropogenically driven.