KEYNOTE ADDRESS: THORIUM-230 DATING OF LATE QUATERNARY CAVE CALCITE: ESTABLISHMENT OF THE TIMING OF CLIMATE CHANGE AND WORLD-WIDE CORRELATION

The precision with which carbonates can be dated by the Th-230 method improved with the development of mass spectrometric methods for measuring Th-230 and U-234, almost twenty years ago. Along with this improvement came a reduction in sample-size requirement. Additional improvements in mass spectrometric measurement of both Th-230 and U-234 have been made, including some in recent months. The technical advancements, of course, do not change natural processes, and cannot change the extent to which processes that produced a sample satisfy the assumptions used in calculating an age from isotope ratios. However, the advancements do allow us to test, at high precision, the degree to which a particular sample satisfies those assumptions, thereby allowing us to choose the most suitable samples for dating. In this regard, experience has shown that low porosity cave calcite with minimal detrital material is ideal. It has also been demonstrated that, in many cases, changes in the oxygen isotopic composition of cave calcite reflect changes in the isotopic composition of meteoric precipitation above the cave. In such cases, it is possible to obtain long (hundreds of thousands of years), absolute-dated records of the isotopic composition of meteoric precipitation with sub-decadal to multi-decadal resolution. Records of this sort have been produced from a number of localities world-wide, including China and Brazil. Because of the dating, the cave records can be correlated precisely to each other. Furthermore, the cave records can be correlated, given certain assumptions, with Greenland and Antarctic ice, marine oxygen isotope records, and the wealth of data contained in each of these archives. Thus, we can put together geographic and temporal patterns of climate change over multiple glacial cycles. Notable observations pertain to the timing and geographic pattern of millennial-scale events and the worldwide sequence of events during glacial terminations, both of which will be discussed in my presentation.