This is a review from of an interesting paper looking at Oxygen 18 records in water driven cave formations (stalactite, stalagmite or flowstone) than span a 4000 year period. Here is a basic description from the NZ Climate Centre:

"These deposits occur within karst terranes in subterranean caverns mainly as calcite (CaCO3) precipitated from groundwater that percolated through overlying limestone or marble rock. Interior cave climates and environments are generally stable; temperatures have little annual variation and are usually close to the external local mean annual air temperature. Oxygen and carbon stable isotope values (18O/16O and 13C/12C) obtained from speleothem calcite have been employed at many locations in the world to determine past climate conditions and can be used to interpret environmental changes."

What was done
Two master speleothem (stalactite, stalagmite or flowstone cave deposit) δ18O records were developed for New Zealand's eastern North Island (ENI) and western South Island (WSI) for the period 2000 BC to about AD 1660 and 1825, respectively. The WSI record is a composite chronology composed of data derived from four speleothems from Aurora, Calcite, Doubtful Xanadu and Waiau caves, while the ENI record is a composite history derived from three speleothems from Disbelief and Te Reinga caves.

What was learned
For both the ENI and WSI δ18O records master speleothem histories, their warmest periods fall within the AD 900-1100 time interval, which is also where the peak warmth of a large portion of the temperature records found in our Medieval Warm Period Project fall (see our Interactive Map and Time Domain Plot).

What it means
Not wanting to acknowledge that the earth was likely as warm as or even warmer than it is currently a thousand or so years ago (when the atmosphere's CO2 concentration was much lower than it is today), the world's climate alarmists have been loath to admit there was an MWP or Medieval Warm Period anywhere other than in countries surrounding the North Atlantic Ocean. Consequently, the seven independent speleothem records that produced the results reported by Lorrey et al. are of great importance to the ongoing global warming debate, as they greatly advance the thesis that the MWP was indeed a global phenomenon, and that there is thus nothing unusual, unnatural or unprecedented about earth's current warmth, and that it therefore need not be attributed to the historical increase in the air's CO2 content.

Lorrey, A., Williams, P., Salinger, J., Martin, T., Palmer, J., Fowler, A., Zhao, J.-X. and Neil, H. 2008. Speleothem stable isotope records interpreted within a multi-proxy framework and implications for New Zealand palaeoclimate reconstruction. Quaternary International 187: 52-75.