Southern Hemisphere mid-latitude hydroclimate during Termination I: links to Antarctic iceberg discharge

The last deglaciation (Termination I, T-I) in the terrestrial mid latitudes of the Southern Hemisphere is well represented by records from lakes and alpine glaciers, which together provide information on regional vegetation, fire and temperature change. However, much less is known about variations in hydroclimate across this interval. We present a high-resolution, multi-proxy time series from a Tasmanian stalagmite spanning T-I. Coherent patterns in stable oxygen and carbon isotopes, growth rates, initial uranium isotope ratios and trace elements (Mg and Sr) reveal multiple, millennial-scale episodes of regional hydrological change during the deglaciation. Periods with a more positive moisture balance, inferred from low carbon isotope ratios and low Mg/Ca and Sr/Ca, occur when the climate was cooler, and coincide with lower growth rates and oxygen isotope ratios. Converse excursions reflect warmer periods with a reduced water balance. These swings in hydroclimate occur at a higher frequency than the classical millennial-scale deglaciation model of early warming - Antarctic Cold Reversal – late warming.

The stalagmite hydroclimate record shows a strong similarity to iceberg-rafted debris fluxes off the Antarctic Peninsula, with periods of increased iceberg flux coinciding with increased effective moisture over northern Tasmania. This suggests that enhanced iceberg and meltwater release altered regional ocean and atmospheric circulation, bringing cooler (and potentially wetter) conditions to Tasmania. Results from climate-model freshwater hosing experiments show that during such episodes, sea-ice cover increases and negative surface-temperature anomalies are present over the Indian, Atlantic and SW Pacific sectors of the Southern Ocean. Although rainfall itself does not increase, water balance over Tasmania is substantially enhanced during these events, consistent with the speleothem data. There is also some evidence that the patterns of retreat (re-advance) of some NZ glaciers may coincide with reduced (enhanced) moisture balance over Tasmania, implying that NZ glacier history through T-I may be more nuanced than previously proposed.