Isotope ratios of U, Nd, and Sr of ODP Hole 162-984A sediment samples (Table 1)

High precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition. The approach is demonstrated on fine-grained, late Pleistocene deep-sea sediments from Ocean Drilling Program Site 984A on the Bjorn Drift in the North Atlantic. The sediments are siliciclastic with up to 30% carbonate, and dated by delta18O of benthic foraminifera. Nd and Sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods - volcanic rocks from Iceland - and a distal continental source during glacial periods. An unexpected finding is that the 234U/238U ratios of the silicate portion of the sediment, isolated by leaching with hydrochloric acid, are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance. The 234U depletions are inferred to be due to alpha-recoil loss of 234Th, and are used to calculate "comminution ages" of the sediment - the time elapsed between the generation of the small (<=50 µm) sediment grains in the source areas by comminution of bedrock, and the time of deposition on the seafloor. Transport times, the difference between comminution ages and depositional ages, vary from less than 10 ky to about 300 to 400 ky for the Site 984A sediments. Long transport times may reflect prior storage in soils, on continental shelves, or elsewhere on the seafloor. Transport time may also be a measure of bottom current strength. During the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from Iceland that is rapidly transported to the site of deposition. The comminution age approach could be used to date Quaternary non-marine sediments, soils, and atmospheric dust, and may be enhanced by concomitant measurement of 226Ra/230Th, 230Th/234U, and cosmogenic nuclides.

本研究通过高精度铀同位素测量海洋碎屑沉积物，用以示踪沉积物从物源区至沉积点的搬运与储存时间。以北大西洋比约恩漂移体（Bjorn Drift）大洋钻探计划（Ocean Drilling Program）984A站位的细粒晚更新世深海沉积物为研究对象，验证了该方法的有效性。该沉积物属于硅质碎屑沉积，碳酸盐含量最高可达30%，通过底栖有孔虫（benthic foraminifera）的δ¹⁸O进行年代学定年。钕（Nd）与锶（Sr）同位素分析结果显示，沉积物物源在末次三次间冰期的近源端（冰岛火山岩）与冰期的远源大陆物源之间周期性振荡。一项意外发现为：经盐酸淋滤分离得到的沉积物硅酸盐组分，其²³⁴U/²³⁸U比值显著低于长期平衡值，且呈现出与冰期旋回及沉积物物源相关的大幅系统性变化。研究推断，²³⁴U的亏损源于²³⁴Th的α反冲损失，并据此计算了沉积物的“粉碎年龄”——即基岩经粉碎形成粒径≤50μm的细小颗粒（产生于物源区）至沉积物沉积于海底之间的历时。对于984A站位的沉积物，搬运时间（即粉碎年龄与沉积年龄的差值）介于不足10千年至约300~400千年之间。较长的搬运时间可能反映了沉积物曾在土壤、大陆架或海底其他区域预先储存。搬运时间亦可作为底流强度的衡量指标。在最近的间冰期，远源大陆物源的碎屑会被快速搬运至沉积点的冰岛来源沉积物所稀释。粉碎年龄法可用于第四纪陆相沉积物、土壤及大气尘埃的定年，若辅以²²⁶Ra/²³⁰Th、²³⁰Th/²³⁴U及宇宙成因核素的同步测量，该方法的应用潜力可得到进一步提升。