Carbon-14 calibration and sea-level history from high-precision thorium-230 and protactinium-231 dating of corals.

摘要

In this work I employ high-precision 230Th and 231Pa dating methods to improve the chronology of late-Quaternary climate change. 230Th-age determinations were made on fossil corals obtained from drill cores and surface exposures in Papua New Guinea, Vanuatu and Barbados. To eliminate diagenetically altered samples, I filtered the analyses according to the proximity of the coral δ234U _i value to the modern marine value. In addition, I applied the newly developed 231Pa dating methods as an independent check on the accuracy of 230Th ages older than ∼20,000 years. This second check adds significant confidence to records established from corals with concordant 230Th-231Pa ages and represents an important advance in efforts to constrain the timing of climate change. This is the second coral study to make use of such methods.; Using the filtered 230Th-age results, I address two key areas of late-Quaternary climate studies that require improved age control: the 14C time scale and the sea-level record. In the 14C calibration study, the new data map 14C variations ranging from the current limit of the tree-ring calibration, 11,900 cal yr BP, to the 14C-dating limit of 50,000 cal yr BP, and provide detailed structure during the List glacial maximum period. The results reveal two major 14C-age plateaus between 14–15 cal ky BP and 22–25 cal ky BP that are likely linked to changes in global climate. In the sea-level study, the new data reveal four intervals of dramatic sea-level fall since the last interglacial period, the most rapid of which occurred between marine oxygen-isotope stages 5a and 4. During this transition sea level fell 50% of the glacial-interglacial amplitude in less than 6000 years. In addition, comparison of the results with deep-sea δ18O records reveals a large deep-ocean temperature shift following 5e and during deglaciation, and smaller changes between 5c and 2. With these studies, I hope to bring the paleoclimate community one step closer to achieving an accurate chronology of late-Quaternary climate change.