Late-quaternary paleoclimate of Mount Field, Tasmania

Abstract

The mid-latitudes of the Southern Hemisphere possess a complex history of paleoenvironmental change. I address two objectives using lake sediment records from Tasmania, one of the few landmasses situated within this critical region of the “Water Hemisphere”. First, cosmogenic dates from terminal moraines and sea surface temperatures (SST) suggest the climate of south-eastern Australia warmed rapidly after the Last Glacial Maximum, whereas pollen records indicate temperatures only began to increase at the onset of the Holocene, roughly 11.5 thousand calibrated years before present (cal ka BP). To explicitly address this discrepancy, I surveyed the modern environmental drivers of chironomid (Insecta: Diptera) distributions in Tasmania and developed an independent proxy of temperature. Second, a large body of literature has emerged invoking the Southern Westerly Winds (SWW) and El Niño/Southern Oscillation (ENSO) as major drivers of paleoenvironmental change. Some researchers have hypothesized that the SWW governed broad scale precipitation patterns from 14 to 5 cal ka BP, when ENSO became the main control of synoptic climate. Using a suite of lacustrine proxies, I investigated the nature of the transition from a SWW-to ENSO-dominated climate regime. In terms of the first objective, pH explains most of the variance in the modern chironomid distributions. This environmental variable delineates between two geological provinces in Tasmania: one province has infertile, acidic soils and the other has soils generally richer in clay, possessing a greater buffering capacity. However, temperature of the warmest quarter (TWARM) explains most of the variance in the chironomid fauna within either geological province. Consequently, I applied a chironomid-based transfer function for TWARM to a pair of fossil chironomid records from Mount Field National Park. Temperatures were at or above modern values early in the record, from about 15 to 13 cal ka BP, supporting the evidence from the cosmogenic dates and SST. After a prolonged cooling, temperatures again returned to modern, supportinga mid-Holocene thermal maximum between 6.7 and 4.6 cal ka BP. I used another lake sediment record from Mount Field to address the second objective. Based on lacustrine proxies, SSW controlled precipitation from 12 to 4.9 cal ka BP, fluctuating from wet to dry conditions around 8.4 cal ka BP. The abrupt shift to a generally dry though variable climate at 4.9 cal ka BP was characterized by an increased fire frequency. Considering all of the proxies, their transitions were nearly simultaneous and concurrent with the onset of ENSO activity in Ecuador, highlighting close links between the equatorial Pacific and south-eastern Australia.