| New developments in mass spectrometry and a better understanding of open-system
processes are ushering in a new era of precision and accuracy in coral geochronology. An
effort is underway to develop a uniform set of reference materials and reporting standards to
assure age comparability between laboratories and eliminate inter-laboratory and age
interpretation biases.
PALSEA is a PAGES/IMAGES working group that aims to extract information about ice
sheet response to temperature change by examining the history of sea during past
interglacials. As reef-building corals are one of the primary archives of past sea
levels, the U-series coral dating community is well represented in this group. During
workshop discussions, it became clear that further progress on the sea level problem
requires engaging the coral dating community in a cooperative standardization
effort.
Improvements in analytical precision continue to extend the potential precision and range
of the U-Th chronometer. As a result, assuring comparability of ages reported by different
labs becomes a crucial issue. Ideally, all measurements should be traceable to the same set of
reference standards. Unfortunately, internationally recognized standards are not currently
available. A widely used U/Th uraninite standard, HU-1, is no longer be suitable, as different
aliquots have different isotope ratios and the assumption of radioactive equilibrium no longer
appears valid when measured at current levels of precision. The time is ripe for
the development of new reference standards. A strategy for their production and
distribution has been initiated in collaboration with the NERC Isotope Geosciences
Laboratory, UK, and drawing on the experiences of the EARTHTIME initiative
(http://www.earth-time.org).
Quaternary sea level data is presently scattered across the scientific literature
with widely varying reporting formats, screening and correction criteria, and decay
constants. Stratigraphic information is often incomplete, and elevations are not tied
to consistent benchmarks. It would be highly desirable to compile existing data
in a uniform format that can be made available to the wider community, and to
adopt a uniform set of standards for future data reporting. While best practices for
sample screening and/or age correction are still keenly debated, reported ages depend
heavily on assumptions about the 234U/238U history of seawater over the last 800
thousand years. A standard history of ocean 234U/238U for quality and correction
criteria, with associated error estimates, would make ages reported by different
labs more directly comparable. Data reduction and archiving software has been
developed as part of the EARTHTIME project, and discussions are underway to
adapt this software for the U-Th chronometer. Standardized reporting through data
reduction and databasing software has great potential to make U-series dating of coral
sea-level indicators more useful and accessible to the wider paleoclimate community. |