Measuring Quaternary time: A 50-year perspective

Over the last 50 years, there have been significant developments in the range and sophistication of the chronological tools now available to Quaternary scientists. Notable milestones include the introduction of new methods (e.g. optically stimulated luminescence, cosmogenic radionuclide dating, ice-layer counting, molecular clocks') and the refinement of established techniques (e.g. accelerator mass spectrometry C-14 dating, C-14 calibration, high-precision uranium-series dating, argon-argon dating, cryptotephra analysis). Developing new techniques and methodologies for measuring Quaternary time is not an end in itself, however: it is stimulated by a parallel need, which is a deeper understanding of the mode, pattern and rates of environmental processes, and how different processes inter-connect. Here we review some of the important limitations that continue to constrain our ability to provide coherent chronologies for Quaternary environmental reconstructions at both millennial and sub-millennial timescales. We focus our discussion on two unique stratigraphic templates: first, the marine oxygen isotope sequence that spans the full Quaternary period; and, second, the Greenland ice-core record, which provides a basis for dating environmental changes through the last glacial cycle.