Early–Mid Holocene climatic variations in Tasmania, Australia: multi-proxy records in a stalagmite from Lynds Cave

Abstract

Mass spectrometric uranium-series dating and C–O isotopic analysis of a stalagmite from Lynds Cave, northern Tasmania, Australia provide a high-resolution record of regional climate change between 5100 and 9200 yr before present (BP). Combined δ18O, δ13C, growth rate, initial 234U/238U and physical property (color, transparency and porosity) records allow recognition of seven climatic stages: Stage I (>9080 yr BP) – a relatively dry period at the beginning of stalagmite growth evidenced by elevated 234U/238U; Stage II (9080–8600 yr BP) – a period of unstable climate characterized by high-frequency variability in temperature and bio-productivity; Stage III (8600–8000 yr BP) – a period of stable and moderate precipitation and stable and high bio-productivity, with a continuously rising temperature; Stage IV (8000–7400 yr BP) – the warmest period with high evaporation and low effective precipitation (rainfall less evaporation); Stage V (7400–7000 yr BP) – the wettest period with highest stalagmite growth and enhanced but unstable bio-productivity; Stage VI (7000–6600 yr BP) – a period with a significantly reduced precipitation and bio-productivity without noticeable change in temperature; Stage VII (6600–5100 yr BP) – a period of lowest temperature and precipitation marking a significant climatic deterioration. Overall, the records suggest that the warmest climate occurred between 8000 and 7400 yr BP, followed by a wettest period between 7400 and 7000 yr BP. These are broadly correlated with the so-called ‘Mid Holocene optimum’ previously proposed using pollen and lake level records. However, the timing and resolution of the speleothem record from Lynds Cave are significantly higher than in both the pollen and lake level records. This allows us to correlate the abrupt change in physical property, δ18O, δ13C, growth rate, and initial 234U/238U of the stalagmite at ∼8000 yr BP with a global climatic event at Early–Mid Holocene transition.