Geochemical Evidence for Quaternary Sea-Level Changes

Reconstruction of sea-level history is a major goal of climate research, as changes in sea level are a measure of the volume of ice stored on the continents. Much of our knowledge of the basic characteristics of the Late Quaternary ice-age cycles comes from sea-level records. Sea-level history can be estimated directly by dating features that constrain past sea levels. Dating of fossil coral skeletons is the main method of direct sea-level reconstruction; however, dating of cave deposits is also important. Typically, ages are determined by ²³⁰Th dating, although ¹⁴C dating can be applied to the youngest part of the record (<40ky). Since the development of mass spectrometric methods for the measurement of the nuclides pertinent to ²³⁰Th dating, it has been possible to obtain extremely precise ²³⁰Th dates for materials that formed in the last ~500ky. However, many corals behave as open systems, potentially leading to inaccurate ²³⁰Th dates. Strategies for testing for diagenesis include checking whether the coral has retained its marine uranium isotopic composition and testing for age concordance between ²³⁰Th and ²³¹Pa dates. Alternatively, one may apply methods that aim to correct for shifts in age due to diagenesis. In addition to direct sea-level reconstruction methods, past sea levels can be estimated from the oxygen isotope record derived from foraminifera recovered from marine sediments. Such records have the advantage of being long and continuous, but generally have lower resolution, larger dating error, and require some strategy for isolation of the sea-level component of the oxygen isotope signal. From both methods of sea-level reconstruction, we now have a good idea of the general features of the sea-level curve over the past ~600ky. Particularly for the younger portion of the record, we know important details of the curve. However, many details have yet to be worked out. We know that orbital forcing is important in driving sea-level variations and have begun to understand some of the mechanisms whereby orbital changes affect sea level, including some of the factors that cause nonlinear responses of sea level to orbital changes. We also have clearly resolved millennial-scale changes in sea level for the last glacial period and the last deglaciation. We are also beginning to reach some kind of understanding of the interplay among ice sheets, sea-level change, atmospheric circulation, ocean circulation, and the carbon cycle.