High resolution record of the Last Glacial Maximum in eastern Australia

High resolution record of the Last Glacial Maximum in eastern Australia Lynda M. Petherick, Patrick T. Moss & Hamish A. McGowan School of Geography, Planning & Environmental Management, University of Queensland, St Lucia 4072, Queensland, Australia

1. Introduction: Several long, continuous palaeorecords have been reconstructed (fig. 1), but there are few comparable records geographically between these sites, with records from subtropical Australia (and the rest of the Southern Hemisphere) being rare. Previous work from the subtropics of Australia is limited to Fraser Island (Longmore 1997; Donders et al. 2006), although these records either have problematic dating results or a hiatus in lake sedimentation during the Last Glacial Maximum (LGM). With specific reference to eastern Australia, there is a lack of high resolution records of palaeoenvironmental & palaeoclimatic variability encompassing the LGM.

3. Methods:

Continuous, high resolution palaeorecords encompassing the LGM have been developed from multiple proxies (viz. aeolian sediment flux, moisture content, pollen and charcoal) in lake sediments from Tortoise Lagoon (TOR) and Native Companion Lagoon (NCL), North Stradbroke Island (NSI), Queensland, Australia.

Fig. 2: Location of North Stradbroke Island, showing TOR and NCL.

Fig. 3: Tortoise Lagoon Nov 2007

Fig. 1: Map showing the location of long palaeorecords for eastern Australia.

Tortoise Lagoon

NSI is a large sand island located in Moreton Bay, southeast Queensland, approximately 40 km off the coast of Brisbane (fig. 2). NSI has subtropical climate (Koeppen classification), experiencing warm, moist summers and relatively mild winters. The dominant wind direction is southeasterly. TOR and NCL are perched lakes, surrounded by vegetated quartz sand dunes. At the time of core extraction (TOR: November 2007; NCL: July 2004), both lagoons were dry and extensively colonized by reeds (figs. 3 & 4). However, it is believed that both lagoons had water in them, as recently as 2001 for TOR. Both lagoons are closed systems, with the majority of sediment input through aeolian processes.

Table 1: Specific details of analyses conducted on the NCL and TOR lake sediment cores

Table 1 shows the more specific details of the methods for each lake sediment core. The methods described apply to the full sediment cores, although the focus of this poster is the LGM. • Sediment cores were extracted using a Russian D-section corer from the dry lagoons • Selected samples underwent AMS 14C dating at either the University of Waikato, New Zealand or the Australian Institute of Nuclear Science & Engineering (AINSE) laboratory to provide age control • 14C ages were converted to calendar years using the Cariaco Basin chronology (Hughen et al. 2006). • The cores were sectioned and samples dried at 65oC for 60 hours, to remove moisture. • Samples were ashed in a high temperature furnace at 490oC for 12 hours to remove organic matter, leaving material believed to be predominantly deposited through aeolian processes. • Pollen and charcoal analyses were conducted on dried sediment samples at a 500 yr resolution, using the heavy liquid method of van der Kaars (1991).

4. Results:

Fig. 3: Native Companion Lagoon July 2004

2. Study site: North Stradbroke Island

NCL July 2004 4m

TOR November 2007 5m

age of full record Calibrate age of full record Sampling frequency Temporal resolution (average)

16 36.8 14C kyr. BP 42.6 cal. kyr. BP 5 mm 50 cal. yrs.

6 35.2 14C kyr. BP 40.7 cal. kyr. BP 2 mm 22 cal. yrs

Temporal resolution (pollen & charcoal) Depth encompassing LGM Calibrated age range encompassing LGM

500 cal. yrs. 2300 – 1500 mm 32 – 18 cal. kyr. BP

1000 cal. yrs. 3950 – 2800 mm 32 – 18 cal. kyr. BP

Date of core extraction Length of core Number of 14C dates 14C

(a)

(a)

(b)

(b)

LGM-2

LGM-1

Native Companion Lagoon

LGM-2

LGM-1

Fig.5: Moisture content and sediment flux for (a) TOR and (b) NCL

5. Palaeoenvironments of eastern Australia during the LGM The multiproxy records from TOR and NCL correlate well. Discrepancies in the timing of these periods between TOR and NCL are probably due to inaccuracies in the age chronology models developed for the records. I.e. the TOR record is only based on 6 14C dates (further samples are planned to undergo 14C dating to increase the robustness of the age chronology). Significantly cooler climate: • Indicated by the presence of ASTERACEAE, spineless ASTERACEAE and Nothofagus . Increased aridity: • Increased aeolian sediment flux in both TOR and NCL records. • Increased presence of CASUARINACEAE (replacing melaleuca wetland conditions at TOR, suggesting a lowering of the water table). Increased burning: • The NCL record in particular shows charcoal peaks coinciding with the peaks in sedimentation, indicating increased frequency of burning during the LGM. At TOR, charcoal begins to increase near the termination of the LGM, which may be a reflection of increased climatic variability leading into the deglaciation.

Fig.6: Pollen and charcoal record, for (a) TOR and (b) NCL

Double-peaked, extended LGM? Increased aeolian sedimentation in both palaeorecords suggest that the LGM in eastern Australia was an extended period of ca. 8 – 10 cal. kyr, characterized by two periods of increased aridity centered around 27.5 – 30.7 cal. kyr (LGM-1) and 21.7 – 20.9 cal. kyr (LGM-2) (Fig. 5). Peaks in CASUARINACEAE in both pollen records correlate well with the peaks in sedimentation, providing further evidence for a drier climate. Evidence for an extended, double-peaked LGM is also seen in South America (Denton et al. 1999), New Zealand (Suggate and Almond 2003; Alloway et al. 2007; Newnham et al. 2007) and Antarctica (Röthlisberger et al. 2002; EPICA 2006). Records from Australia also indicate that the LGM was not a period of uniform aridity and/or cooling (e.g. Nott & Price 1994; De Deckker 2001; Kemp & Spooner 2007). Mid-LGM amelioration? The period between the 2 apparent peaks in aridity is characterized by decreased aeolian sedimentation, indicating decreased aridity. The presence of RESTIONACEAE in the TOR pollen record indicates the re-establishment of rainforest locally, although this is not seen to the same extent at NCL. The similarities of the TOR and NCL records to other records from southeastern Australia (e.g. Caledonia Fen, Victoria (Kershaw et al. 2007), Redhead Lagoon, New South Wales (Williams et al. 2006) and Barrington Tops, New South Wales (Sweller & Martin 2001), suggests that the records represent regional climatic and environmental conditions during the LGM.

Acknowledgements I would like to thank AQUA/ARCNESS and the School of Geography, Planning & Environmental Management, University of Queensland, for travel support, without which I would not have been able to attend the PAGES YSM/OSM.

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