The Marine 234U/238U Record and the Timing of the Marine 18 O/ 16 O Record

9402693爱德华兹对海洋S18 O记录的直接测年在过去40年里一直是一个经典的问题，而且在很大程度上仍然没有解决。随着测量234U/238U(Chen等人，1986)和230Th/238U(Edwards等人，1987年a，b)的分析能力的改进以及进一步的分析改进(Edwards等人，1993)，人们对这个问题的兴趣增加了。然而，试图使用230 Th技术测定深海海相碳酸盐的年代将面临至少一个主要的、也许是不可逾越的障碍：将上覆水柱中的铀产生的污染物230 Th与碳酸盐中的铀产生的230 Th进行分离(Delaney和Boyle，1983)。因此，将研究两种替代方法来限制海洋S18O记录的时间：富碳酸盐沉积物的234U/238U测年(Ku，1965；Ludwig等人，1991年)和热带岛屿洞穴的230Th测年。234U/238U比值的测量精度为+1/mil(Edwards等人，1993)，这对应于+4Ka的234U/238U年龄的误差。因此，高分辨率的234U/238U年龄在原则上是可能的。将从原生铀含量较高且接近100%碳酸盐的岩心中分析大量碳酸盐。此外，热带岛屿的洞穴化石测年将使用TIMS 230 Th技术(Li等人，1989；Dorale等人，1992；Baker等人，1993；Richards等人，在出版中)，并可能具有模拟海洋价值的S18O值，从而提供了一种测定海洋记录的方法。这两种方法将直接制约海洋S18O记录的时间(过去350,000年)，并将有助于理解第四纪气候变化的根本原因。

9402693 EDWARDS Direct dating of the marine S18 O record has been a classic problem for the last four decades, and is still largely unsolved. Interest in this problem has increased in the wake of improvements in analytical capabilities for measurement of 234 U/ 238 U (Chen et al., 1986) and 230 Th/238 U (Edwards et al., 1987 a,b) and further analytical refinements (Edwards et al., 1993). However, attempts to use 230 Th techniques to date deep-sea marine carbonates will be faced with at least one major, perhaps insurmountable, obstacle: separation of contaminant 230 Th produced by uranium in the overlying water column from 230 Th produced by uranium in the carbonate (Delaney and Boyle, 1983). Consequently, two alternate approaches will be investigated to constrain the timing of the marine S18 O record: 234 U/238 U dating of carbonate-rich sediments (Ku, 1965; Ludwig et al., 1991) and 230 Th dating of tropical-island speleothems. The 234U/238U ratio is measured with a precision of +1 per mil (Edwards et al., 1993), which corresponds to an error in 234U/238U age of +4Ka. Thus high-resolution 234U/238U ages are in principle, possible. Bulk carbonate will be analyzed from cores that have high primary uranium contents and are close to 100% carbonate. Also, dating of speleothems from tropical islands will be dated with TIMS 230 Th techniques (Li et al., 1989; Dorale et al., 1992; Baker et al., 1993; Richards et al., in press) and may have S18 O values that mimic marine values, thereby providing a means of dating the marine record. These two approaches will place direct constraints on the timing of the marine S18 O record (over the past 350,000 years) and will help to understand the fundamental causes of Quaternary climate change.