Uranium-Series Dating and the History of Sea Level

Abstract

Abstract U-series dating of coral associated with former shorelines has provided the abscissa of time for diagrams of fluctuation in sea level during, especially, the last 250,000 years. Ordinates of amplitude have been provided by morphostratigraphic detail, by assumptions of rates of uplift, or by 18 O values in either littoral deposits or deep-sea sediments. The latter case provides mutual reinforcement of 18 O and sea level chronologies. The most complete reference sections come from the Huon peninsula of New Guinea. Twelve separate reef crests have been distinguished and dated: four in the interval 250 ka to 180 ka (core stage 7), seven in the interval 140 ka to 28 ka (core stages 5-4-3), and one in the in-terval 9 ka to 5 ka. Were 230 Th/ 234 U dating sufficiently accurate, sea level histories from other parts of the world might be compared to the New Guinea record, even with the identification of individual sea level events. Such, alas, is not the case. At least the following sources of error plague us. Statistical errors in 230 Th/ 234 U values are commonly quoted as ± 0.02, but an interlaboratory comparison suggests that ± 0.04 may be a fairer index of precision. This is ± 15 ka at 125 ka, nearly equal to the 17 ka interval between sea level maxima on New Guinea. Thus, indi-vidual samples from other areas cannot be assigned to a specific episode by “closed-system” ages alone. Close correlation must depend upon position in a morphostratigraphic sequence as well. Corals are not ideal closed systems. 230 Th ages may be either too old or too young. Concordancy of 230 Th and 231 Pa ages is the only useful check, but 231 Pa/ 235 U is seldom measured. Molluscan data are notoriously unreliable and would not be studied, were we not curious about the extra-tropical seas. Coexisting coral and mollusca can yield identical 230 Th ages. More commonly, molluscan ages are significantly younger. Concordant 230 Th and 231 Pa ages can be as little as 50% of those from coexisting coral. The molluscan systems have been “closed” to loss of daughter products, but “open” to addition of parent U. We are measuring the average age of the parent, which enters molluscan shell after death. Both coral and mollusca may also be “open systems” to which 234 U, 230 Th, and/or 231 Pa have been added in excess of that produced within the shell. In some cases, addition may have been systematic. The S-R model, which allows correction to “concordant” ages, is not demonstrably useful, but alternative models might be developed from sufficient data. There can be no doubt that “dating” the maximum sea level of the last interglacial at 125,000 ± 10,000 years, and confirmation of its wide-spread preservation are achievements of U-series dating. Detailed sea level records of the “last interglacial” (core stage 5), based on local morphostratigraphic evidence, still show conflicts (differences?) which cannot be resolved by U-series dating.