1. Environmental changes indicated by grain-size and trace metal analysis over the past 700yrs at Annaburroo Billabong, NT, Australia Xinrong Zhang 1,2,3, Henk Heijnis 1, John Dodson 1, Atun Zawadzki 1, Gary Buchanan 4 1.Institute for Environment Research, Australian Nuclear and Science Technology Organization, Menai, 2234, Australia; 2.College of Earth Sciences, Jilin University, Changchun, 130061, China 3.Key-Lab for Evolution of Past Life and Environment in Northeast Asia, Ministry of Education, China, Jilin University, Changchun, 130026, China 4.Department of Environmental Science, University of Technology, Sydney, 2001, Sydney, Australia Main References: [1] Fett D.E.R. & Hall I.R. 1983. Report on the land units of the western section of Annaburroo Station, Northern Territory[C]. Technical Memorandum Number 85/1. Land Conservation Unit, Conservation Commission of the NT. [2] Glew JR., Smol JP., Last WM.2001.Sediment core collection and extrusion. In: William M. Last & John P. Smol, Tracking environmental change using lake sediments (Vol.1), Kluwer Academic Publishers, Netherland: 73-105. 1. Introduction The Annaburroo billabong ( 12°55′S,131° 40′E) is a localized depression on the flood plains of the Mary River, west of Kakadu National Park, NT, in the tropics of Australia (Fig.1&Fig.2). Its water-levels are maintained by groundwater and filled by floods from both the Mary River and Mckinlay River. Around it, mining, farming and natural climatic influence and supply sediment to it. Little bio-turbation happens in the deep and anoxic water body. Core AC3-2002 & AC5-2001 collected in the 2001yr & 2002yr were analyzed and results are herein described to establish the deposition rates, the variability of grainsize distribution and trace element concentration in the past centuries of this lagoon, and then their relations with the natural climate changes and local human activities were examined, which aims to investigate and determine what proportions of changes in natural archives are due to human activity and climate variability in the past 700yrs of the Annaburroo area. Fig.1 Photographs from Annaburroo Billabong (1)Overlook of the lagoon; (2) Lotus and Restionaceae in the lagoon; (3)Grasses and Pandana’s; (4) Grass lands affected by the stocks; (5) Eucalypts show high water line with ground affected by cattle; (6) Succession from left to right, grasses, sedges, Lilly, Myriophyllum. (1)(2)(3) (4)(5)(6) Fig.2 Location map of the study area and major land units (Modified from Fett & Hall, 1983) U1: floodplains, deep gray earths, Perennial grassland, U2: Colluvial foot- slopes and slopes, moderately deep to deep gravelly yellow, deep red and yellow earth, open woodland; U3: Erosional slopes, shallow stony lithosols and gravelly yellow and brown earth, woodland; U4-1: Low hills and strike ridges, shallow stony lithosols and shallow yellow earth, woodland; U4-2: High hills and strike ridges, very shallow stony lithosols, woodland; U5: Head of alluvial plains, deep weakly developed solodic soil, low shrubland. 3. Discussion Grain-size and trace-metal analysis in the sediment profile (Fig.5) of Annaburro billabong suggest that the climate in Annaburroo Billabong has become drier since about 700yrsBP and sediments has been accumulated steadily since 1355AD. Climate changes from 1314AD to 1836AD at Annaburroo Billabong can be divided into three stages: 1314AD - 1355AD, a wet and warm climate with highest precipitation. 1355AD -1630AD, drier than the earlier period. 1630AD -1836AD, warmer than the former stage, with similar rainfall. Large grain size distributions suggest that at least six floods deposited coarser materials in Annaburroo Billabong from 1314AD to 1882AD: 1328AD, 1355AD, 1422AD, 1537AD, 1629AD, and 1744AD. The largest was in 1355AD, and the smallest occurred in 1744AD. The overall chemical fingerprints shows that variable levels of metals, but no gradual increase due to possible “European” mining activities in the catchments. Despite the well documented mining activities in the area the lagoon seems to be a natural sink for fine sediments and associated metals are related to the geology of the catchment, which hints that natural processes were still important parts in the formation of the sediment records of this lagoon. Most of the sediment in it autogenic, and only inundated in the most severe monsoonal floods and most of the time has no connection to Mary River. The high lead values in 1318AD and 1790AD might be caused by two forest fires and was perhaps associated with lower precipitation and drier climate conditions, but more evidence, such as charcoal analysis is necessary to verify this. 2. Methods and Results 2-1.Cores were taken by using a messanger operated gravity corer in Fig.3 (Glew, et al., 2001). 2-2.Chronosequence time frame (Fig.4) was built using CIS 210 Pb ages and AMS 14 C dating (calibrated through Quickcal 2007 ver.1.5.) Fig.3 Coring demonstrated by Prof. Henk Heijins (1) Loading the corer; (2) Pipe emplacement; (3) Ready for coring with weight attached and a vacuum seal; (4) core sediment ma- terials; (5) Capped the core for transportation (1)(2) (3)(4)(5) 2-1. Coring Fig.4 Chronology performed by 210 Pb and AMS 14 C dating 2-2. Chronology Fig.5 Trace-metal and grain-size curves of the sediments in the AC3-2002 core and AC5-2001 as compensation 2-3. Trace elements and grain-size analysis