Development of Accurate Radiocarbon Chronologies for Antarctic Margin Sediments

该奖项由极地项目办公室的南极地质和地球物理项目提供，为开发测定海底沉积层年龄的改进方法提供资金。这些方法特别需要用于南极沿海环境的样本。这些结果将有助于更好地重建过去2万年南极冰缘的位置。反过来，这些信息将有助于更好地了解南极冰盖与全球气候之间的关系，包括过去和未来的海平面变化。目前对过去2万年南极环境变化时间的估计主要基于有机物质的放射性碳年龄。在南极大陆上，这些日期大多来自可识别的生物材料残余物，其起源是明确的。相比之下，当样本来自沿海海洋沉积物时，大多数日期都是基于对总有机碳（“酸不溶性”）的分析，而总有机碳可以来自多个来源。特别是，从大陆侵蚀而来的古老有机物质与来自海洋表层水的新鲜产物的混合，可能导致表面年龄明显大于其沉积时间的沉积层。这种影响在多大程度上解释了南极洲大陆和海洋年代学之间的差异尚不清楚。解决这些差异对于减少南极对气候和海平面变化影响的不确定性至关重要。海底沉积物中有机物质的准确年代已经可以通过使用化学技术获得，其中分离出单一有机化合物并提交放射性碳分析。然而，考虑到需要大量的数据，这种技术在样本量、操作人员专业知识和时间方面的要求非常高。为了避免这个问题，这个项目将开发一种简化的程序，在这个程序中，生物相关的化合物组，安全地识别为新鲜的海洋产品，被分离出来进行年代测定。这些将包括特定类别的化合物，以及在一个平行项目中，被困在海洋来源的无机基质中的材料，即由硅藻产生的蛋白石。结果将导致样品吞吐量的大幅增加。特定化合物的放射性碳分析将用于指导这些技术的发展并检查结果。加速器质谱法将用于所有碳-14丰度的测定。

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.