Testing peat humification analysis in an Australian context: identifying wet shifts in regional climate over the past 4000 years

SUMMARY Peat humification analysis is presented as a robust palaeoclimatic proxy, suitable for use on mid–late Holocene peat sequences situated in the Southern Hemisphere. The proxy is shown to permit the identification of wet and dry shifts in a peat sequence from the humid tropics of north-eastern Australia. A significant correlation is found between the humification record and other proxies indicative of past climate conditions such as pollen, δ 13 C, C/N and macrocharcoal. Sixteen wet shifts detected in the humification record for Bromfield Swamp occur at the following dates (with 2σ range): 3830 (3920–3740), 3560 (3640– 3480), 3490 (3560–3420), 3380 (3450–3300), 3120 (3250–2970), 2950 (3100–2790), 2560 (2710–2450), 2430 (2600–2260), 2120 (2330–1910), 1750 (1980–1520), 1430 (1660–1200), 1170 (1390–960), 1010 (1220–820), 620 (770–500), 300 (400–200) and 100 (200–10) cal. yr BP. Eleven dry shifts are also identified in the record at 4220 (4330–4110), 3670 (3750–3590), 3330 (3420–3220), 3020 (3170–2870), 2350 (2530– 2160), 2020 (2230–1800), 1730 (1980–1510), 1290 (1510–1070), 700 (870–560), 400 (470–300) and 260 (360–150) cal yr BP. Blechnum and Poaceae are identified by pollen analysis to be the dominant plants of the swamp surface over the past 4000 years. The ratio of these two plant taxa in the pollen record correlates well with identified wet and dry shifts. It is suggested that a ratio ≤1 possibly indicates dry conditions, a ratio of >1–3 indicates wet or dry conditions, and a ratio >3 implies wet conditions. Large macrocharcoal peaks are recorded during the initiation phase of the peat sequence at approximately 4090 cal. yr BP, and at 3700–3620 cal. yr BP, both of these time periods being coincident with dry phases. Isolated minor macrocharcoal peaks at ca. 2860, 2820, 2620, 2560, 2130, 1930, 1740 and 200 cal. yr BP are found to coincide with periods of average effective precipitation (based on the humification proxy) and so may reflect fire on the swamp surface, transport and re-deposition down-slope of old charcoal after a high rainfall event, or burning in the landscape by indigenous people.