Environmental changes indicated by grain-size and trace metal analysis over the past 700yrs at Annaburroo Billabong, NT, Australia

Xinrong Zhang1,2,3, Henk Heijnis1, John Dodson1 , Atun Zawadzki1, Gary Buchanan4

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, China4.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 floodplains of the Mary River, west of Kakadu National Park, NT, in the tropics of Australia(Fig.1&Fig.2). Its water-levels are maintainedby groundwater and filled by floods from boththe Mary River and Mckinlay River. Around it,mining, farming and natural climatic influenceand supply sediment to it. Little bio-turbationhappens in the deep and anoxic water body. Core AC3-2002 & AC5-2001 collected in the 2001yr & 2002yr were analyzed and resultsare herein described to establish the deposition rates, the variability of grainsize distributionand trace element concentration in the past centuries of this lagoon, and then their relationswith the natural climate changes and local human activities were examined, which aims toinvestigate and determine what proportions of changes in natural archives are due to humanactivity 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 210Pb ages and AMS14C 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 210Pb andAMS14C dating

Calendar Ages (calAD.yrs)

Dep

th (

cm)

1900 1800 1700 1600 1500 1400 1300 1200calAD

0

5

10

15

20

1950 1850 1750 1650 1550 1450 1350 12502000

1314 46AD±

1333 42AD±

1882 22AD±1910 18AD±

1930 15AD±1946 12AD±

1964 10AD±1981 8AD±

1996 6AD±

210Pb CIC ages

210Pb CRS ages

AMS C ages14

2-2. Chronology

Fig.5 Trace-metal and grain-size curves of the sediments in the AC3-2002 core and AC5-2001 as compensation

Fig.3 Trace-metal and grain-size curves of the sediments in the AC3-2002 core and AC5-2001 as comp ensation

0 10 20

Comg/kg

1333 42±

0 20 40

Cumg/kg

Nimg/kg

0 20 40 60 0 20 40 60

Pbmg/kg

0 50 100 150

Znmg/kg

0 1 2 3 4

Snmg/kg

0

Umg/kg

1 2 3 4 5 6 7

0.0

24.0

1.02.03.04.05.067.089.0

10.011.012.013.014.015.01617.018.019.020.021.022.023.0

.0

.0

.0

Depth(cm)

1318

1882

19641930

1314 46±

132213261330

13761422146815141560160616521698174417901836

Cal . (AD)

Mdd(0.5)

Sand>63um

Clay<2um

Silt2-63um

5 15 25 35 45 55 65 75 85 95 0 10 20 3040 50

AD1922

AD1978AD2000

2-3. Trace elements and grain-size analysis