New insights into the effect of the bipolar seesaw on the mid-latitudes of the Southern Hemisphere during the last glacial period

Throughout the last several million years, Earth's climate has undergone regular fluctuations between glacial and interglacial states. Previous studies show that the Antarctic climate experienced broad co-variance with the North Atlantic climate during the Last Glacial Period (LGP; 115ka to -12 ka). However, shorter-duration (millennial) climate events in the two hemispheres did not vary synchronously. This phenomenon, transmitted via the `the bipolar seesaw' of ocean circulation, has been interpreted as resulting from changes in the northward movement of heat by the Atlantic Ocean. Most palaeoclimate research covering this period is strongly biased towards Northern Hemisphere and sub-equatorial sites. Thus, little is known about the possible teleconnections during abrupt climate changes in the southern subtropics and middle latitudes. Evidence shows that timing of climate events from the North Atlantic to the southern equatorial tropics during the last glacial period principally coincide with that detailed in the Greenland ice-core record. There is, however, a lack of long, continuous, high-resolution palaeoclimate archives from mid-southern latitudes. Measurement of climate-sensitive geochemical properties (growth rates and stable isotope analysis; 813C, 8180) of a speleothem collected from Nettlebed Cave in NW Nelson, New Zealand provided a palaeoclimate time-series spanning the LGP. Sixty-two uranium-series ages were used to establish a chronology for all geochemical analyses performed, which aided in resolving the presence, nature and timing of millennial-scale climate fluctuations. To assist with interpretation of climate signals, the palaeoclimate series was compared to a previous Nettlebed Cave record (Hellstrom et al., 1998) and nearby Hollywood Cave (Whittaker et al. 2011). The 8180 composition of speleothem calcite was found to have a complicated relationship with changes in moisture source region, temperature and rainfall. 813C was proven to be a robust proxy for rainfall, exhibiting a negative relationship where positive excursions in carbon represented dry, cold periods in climate history, generally coupled with slow speleothem growth. Changes in these proxies were interpreted as reflecting changes in the strength of southern midlatitudinal westerly circulation. This is in response to extensive cooling events in the Northern Hemisphere, such as Heinrich events, causing a southward shift of major atmospheric pressure belts, including the Intertropical Convergence Zone (ITCZ). This would have led to a steepening of the temperature gradient across mid-southern latitudes and subsequent intensification of westerly circulation justifying an increase in precipitation on the West Coast of New Zealand. This occurrence demonstrates the extent to which the `northern response' penetrated the southern mid-latitudes supporting the hypothesis of interhemispheric teleconnections transported via the bipolar seesaw. i