PALAEOENVIRONMENTS OF THE GULF OF CARPENTARIA FROM THE LAST GLACIAL MAXIMUM TO THE PRESENT, AS DETERMINED BY FORAMINIFERAL ASSEMBLAGES A thesis submitted in fulfilment of the requirements for the award of the degree of Doctor of Philosophy (Science) from the University of Wollongong by Sabine Holt (BSc. Hons) School of Earth and Environmental Sciences, 2005 CERTIFICATION I, Sabine Holt, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the School of Earth and Environmental Sciences, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Sabine Holt 22nd February 2005 ABSTRACT This thesis presents a palaeoenvironmental study of the Gulf of Carpentaria, northern Australia, from around the Last Glacial Maximum (LGM) to the present. Foraminifers, microscopic unicellular aquatic organisms, occur throughout the sediment in the time frame studied. Data on the species composition and preservation of the microfossils found in the Gulf of Carpentaria cores are utilised to reconstruct past environments by comparison to the known assemblages of living foraminifers in various modern environments. The Gulf of Carpentaria is a shallow epicontinental sea, situated between Australia and Papua New Guinea, and is a maximum of 70m deep. It is separated from the Pacific Ocean to the east by Torres Strait, which is 12m deep at its shallowest, and from the Indian Ocean and Arafura Sea to the west by the Arafura Sill, which is 53m below sea-level (bpsl) at its shallowest. For at least ten thousand years in the lead up to the LGM (which reached its peak about twenty thousand years ago), and for about ten thousand years after, sea levels were lower than the 53m-deep Arafura Sill. The continental shelf in the Gulf of Carpentaria area between Australia and Papua New Guinea was exposed, creating a land bridge between the two islands, and a lake developed in the Carpentaria Basin. This palaeolake is termed Lake Carpentaria (named by Torgersen et al., 1983). Documentation of the timing in fluctuations in the extent and salinity of Lake Carpentaria provides information on local and regional climatic systems, such as the Australian summer monsoon. Constraining the nature and timing of the post- glacial rise in sea-level which flooded the lake provides evidence for global eustatic sea-level reconstructions. Analysis of sediment cores from the Gulf of Carpentaria, beginning around 40ka cal BP (forty thousand calendar years before present), shows the existence of Lake Carpentaria (a large, non-marine water body of fluctuating extent) until sea- level rose over the Arafura Sill and inundated the palaeolake around 10.5ka cal BP. i The earliest studied phase dates to around 40ka cal BP which is a marine- influenced brackish water lacustrine facies where Lake Carpentaria is briefly at its maximum extent: 12m deep in its deepest section. The existence of such a large body of water (around 150,000km2) supports the existence of a strong Walker Circulation in the region enhancing precipitation. Between 40ka and 18.8ka cal BP the non-marine, increasingly saline, Lake Carpentaria decreased to 7m maximum water depth, adding to the evidence of aridity around the LGM in northern Australia. At 18.8ka cal BP the lake freshened and monospecific bivalve, foraminiferal and ostracod populations dominated the still shallow (around 8m deep) lake. The lake was expanding, and from around 15±2ka cal BP, fluctuations are noted in the general trend of increasing precipitation. The recorded variations in precipitation intensity may result from stronger seasonality (i.e. monsoons) and/or interdecadal variability (e.g. El Niño Southern Oscillation). At 12.7ka cal BP Lake Carpentaria was at around 12m maximum water depth – the maximum documented extent in the studied period. At this stage there was some exchange of waters with the Arafura Sea via tidal outlet channels in the Arafura Sill (indicating sea-level around 60m below present), seen as a marine influence beginning in the western margins at 12.7ka cal BP. At 12.4ka cal BP the sea- level had risen to the same height as water levels within the lake (58m bpsl). By 12.2.ka cal BP sea-level was up to 2m higher than the previous lake level, and flowed into the lagoonal Lake Carpentaria via channels in the Arafura Sill. By 10.5ka cal BP the sea-level had overtopped the highest surface of the 53m-deep Arafura Sill and the transition to marine conditions began in the Gulf of Carpentaria, confirming the accepted models of sea-level rise. ii ACKNOWLEDMENTS The entire staff and associates of the School of Earth and Environmental Sciences at the University of Wollongong deserve many thanks for providing a stimulating and wonderfully friendly environment of learning. I am grateful to my supervisor Prof. Allan Chivas for providing a place for me in such an exciting project, and for his dedication to see me complete my goal. I wouldn’t have chosen anyone else but the “GoC” crew to be locked up with in a “cold-room” for two years… Dr Adriana Garcia: mentor and friend; Dr Jessica Reeves: twin soul; and soon-to-be- Dr Martine Couapel (with new addition Korigan): no-nonsense support crew! I owe much to the kindness and inspiration of Prof. Brian Jones, who always looked out for ways to help. I thank Prof. Paul Carr for his caring guidance, even though I only play in the mud! David Carrie also made great efforts on my behalf. And the enthusiasm and concern of Dr Dioni Cendon was much appreciated. The technical staff always helped above and beyond their call of duty: Aivars Depers, John Marthick, Penny Williamson, and John Reid (who not only had to cope with computers, had to deal with people like me stressed about computers, and never seemed to hold it against me). Nick Mackie, of the Engineering Department, guided me through hours of SEM, and Dr Winston Ponder, of the Australian Museum kindly took the time to identify mollusc shells used in radiocarbon dating. Thanks to the special people in my life, Mum who gave me all the love in the Universe, and Sunirmalya who gave me immense spiritual support. I would like to dedicate this thesis to the memory of John Head, not only a very helpful colleague, but a good friend. iii TABLE OF CONTENTS page Abstract …..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..….. i Acknowledgements …..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..… iii List of Figures …..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..… viii List of Tables …..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…..…. xiii CHAPTER 1 – Introduction …..…..…..…..…..…..…..…..…..….. 1 1.1 Introduction to the topic …..…..…..…..…..…..…..…..…..….. 1 1.2 Introduction to the Gulf of Carpentaria …..…..…..…..……. 3 1.2.1 Tectonic stability …..…..…..…..…..…..…..…..…..….. 6 1.2.2 Water depth …..…..…..…..…..…..…..…..…..…..…..…… 10 1.2.3 Location and climate …..…..…..…..…..…..…..…..…..….. 12 1.2.4 The continuous presence of micro-organisms ……. 15 1.3 Previous related investigations …..…..…..…..…..…..…..….. 16 1.3.1 Comparing investigations …..…..…..…..…..…..… 16 1.3.2 Previous results …..…..…..…..…..…..…..…..… 17 1.3.3 Synthesis of previous investigations …..…..…. 22 1.4 The current project …..…..…..…..…..…..…..…..…..…..…..…..… 23 1.4.1 Thesis aims …..…..…..…..…..…..…..…..…..…..… 25 CHAPTER 2 – Palaeoclimate and Sea-level change …… 26 2.1 Introduction …..…..…..…..…..…..…..…..…..…..…..…..…..…..….. 26 2.2 Global overview …………………………………………………… 26 2.3 Changes in sea-level ….….….….….….….……...….….….….…… 29 2.4 Changes in oceanic circulation and salinity ….…...…….… 33 2.5 Changes in temperature ….….….…….….….….….….….….….. 36 2.6 Atmospheric circulation (including rainfall) …………..… 40 2.3.1 Walker Circulation, ENSO and WPWP ………….….. 42 2.3.2 Australian summer monsoon, ITCZ and Hadley Cell... 43 2.3.3 Combined effects ….….……………………………….….. 44 2.1 Summary ……....…..…..…..…..…..…..…..…..…..…..………..…..….. 48 iv CHAPTER 3 – Foraminifers ….…….…….…….…….…….……. 49 3.1 Introduction …………………………………………………………. 49 3.2 General systematics and biology ……………………………. 49 3.2.1 Systematic nomenclature ……………………………. 49 3.2.2 Features of foraminiferal tests ………………….. 49 3.2.3 The habitat of living foraminifera ………………….. 52 3.3 Introduction to living assemblages and indicator species 55 3.4 Isolated water bodies ….…….…….…….…….…….……. 57 3.4.1 Isolated brackish waters ……………………………. 58 3.4.2 Isolated water bodies of marine salinity and higher 64 3.5 Transitional environments ….…….…….…….…….…….……. 65 3.5.1 Tidally influenced environments …………………… 66 3.5.2 Lagoonal environments ……………………………. 76 3.6 Marine environments ……………………………………………….. 82 3.6.1 Continental shelf sea environments …………. 82 3.6.2 Open marine environments ……………………………. 88 3.7 Thanatocoenoses ……………………………………………….. 92 3.8 Summary …………………………………………………………. 95 CHAPTER 4 – Methods ……………………………………………….. 96 4.1 Introduction …………………………………………………………. 96 4.2 Material selection and collection ……………………………. 96 4.2.1 Seismic data ……………………………………………….. 96 4.2.2 Piston Core collection ….…….…….…….…….… 96 4.3 Core preparation and sub-sampling …………………… 97 4.4 Foraminiferal analysis ……………………………………… 100 4.5 Sedimentary analysis ……………………………………… 102 4.6 Radiocarbon dating ……………………………………… 102 4.6.1 Selection of material ……………………………………… 102 4.6.2 Dating method ……………………………………… 103 4.6.3 Calibration of dates ……………………………………… 103 4.7 Summary …………………………………………………………. 104 v CHAPTER 5 – Results