A 250,000-year record of sea level and climate from open-system coral ages (Greenland)

Earth's climate history contains clues about the global climate system and its natural range of variability that are crucial for assessing future climate change. Two frequencies of variability are of interest: orbital and sub-orbital. Although the idea that changes in the Earth's orbit affect climate in a predictable way has been a central tenet of climate change theory, this idea has been recently challenged by dating of the Penultimate Deglaciation, the timing of which appears to conflict with orbital predictions. Sub-orbital climate variability has been documented in many locations, particularly the Greenland ice cores and the North Atlantic, but it is not clear how significant these events are from a global perspective. Because sea level is the complement to global continental ice volume, sea-level changes are sensitive indicators of global climate. A detailed sea-level reconstruction is theoretically possible through uranium-thorium dating of corals. However, this potential has not been realized, primarily because of significant problems with coral dating. The source of excess 234U in fossil corals and its relationship to uranium-series age determinations has been an outstanding problem in geochronology for more than 20 years, and it has become increasingly apparent that a substantial fraction of coral isotope ratios cannot be explained by closed-system decay. Here, a set of decay equations is derived from first principles that accounts for this open-system behavior, permitting the calculation of open-system coral ages. This new approach dramatically increases the accuracy and resolution of sea-level reconstructions. For the first time, it is possible to create a high-resolution sea-level curve for the last 250,000 years from published isotope ratio data. Two fundamental conclusions can be drawn from this sea-level reconstruction: (1) Significant suborbital sea-level variability seems to be a persistent feature of the record. (2) In spite of this complexity, the signature of orbitally driven sea-level change is clear. For the last 250,000 years, except 135--150 ka for which sea level data are lacking, each peak in Northern hemisphere summer insolation is followed by a peak in sea level.