Development of Accurate Radiocarbon Chronologies for Antarctic Margin Sediments

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for development of improved methods for determination of the ages of sea-floor sediment layers. These methods are needed particularly for use with samples from Antarctic coastal environments. The results will lead to better reconstruction of the locations of the Antarctic ice margins during the past 20,000 years. In turn, that information will lead to a better understanding of relationships between the Antarctic ice sheets and global climate, including sea-level changes both in the past and in the future. Present estimates of the timing of Antarctic environmental changes during the past 20,000 years are based largely on radiocarbon ages of organic materials. On the Antarctic continent, most of these dates have come from recognizable remnants of biological materials for which the origin is clear. In contrast, when samples have come from coastal oceanic sediments, most dates have been based on the analysis of total ("acid-insoluble") organic carbon, which can derive from multiple sources. In particular, the mixing of old organic material, eroded from the continent, with fresh products from oceanic surface waters can lead to sediment layers with apparent ages significantly greater than the time that has elapsed since their deposition. The extent to which such effects explain discrepancies between continental and oceanic chronologies in Antarctica is not known. Resolution of these discrepancies is crucial to reduction of uncertainties regarding Antarctic effects on climate and sea-level change. Accurate dates for organic materials in sea-floor sediments can already be obtained by use of chemical techniques in which single organic compounds are isolated and submitted for radiocarbon analysis. The requirements of such techniques in terms of sample size, operator expertise, and time are, however, prohibitively high, given that large numbers of dates are needed. To circumvent that problem, this project will develop a streamlined procedure in which biologically related groups of compounds, securely identified as fresh, oceanic products, are isolated for dating. These will include both specific classes of compounds and, in a parallel project, materials trapped within an inorganic matrix of marine origin, namely the opal produced by diatoms. Very substantial increases in sample throughput should result. Compound-specific radiocarbon analyses will be used to guide the development of these techniques and to check the results. Accelerator mass spectrometry will be used for all determinations of the abundance of carbon-14.