Uranium and thorium isotopes and sedimentation rates in metalliferous sediments from the western flank of the East Pacific Rise at 21°-22°S@en

Isotopic compositions of uranium (234U and 238U) and thorium (230Th and 232Th) were measured in metalliferous sediments from the western flank of the East Pacific Rise at 21°-22°S, in the area of hydrothermal activity and massive sulfide accumulation at the axis of the EPR. Concentration of 232Th (on the carbonate-free base) is consistent with composition of mafic extrusive rocks; isotope ratios 232Th/238U and 234U/238U indicate that about 70% of uranium passes into sediments from sea water with hydrothermal iron hydroxide. Mean sedimentation rates are calculated for seven cores by the nonequilibrium 230Th method with use of the constant concentration model. Flux of 230Th to bottom sediments is calculated and its mean value is used to determine sedimentation rate in four other cores. The constant flux model is used to calculate change of sedimentation rate with depth for seven cores over time interval of 100-300 ky. Sedimentation rates varied not much (0.3-0.6 cm/ky). The greatest changes occurred in two cores: one located near massive sulfide structures, and another near the spreading axis. Determinations of mean rates by the radiocarbon method and the nonequilibrium thorium method are in good agreement.本数据集针对东太平洋海隆（East Pacific Rise, EPR）21°~22°S西翼的热液活动区及轴部块状硫化物堆积带的含金属沉积物，测定了铀（234U、238U）与钍（230Th、232Th）的同位素组成。232Th的浓度（以无碳酸盐基体计）与镁铁质喷出岩的元素组成一致；同位素比值232Th/238U与234U/238U显示，约70%的铀以热液氢氧化铁结合形式从海水进入沉积物中。采用恒定浓度模型，通过不平衡230Th法计算了7个岩芯的平均沉积速率；同时对230Th向海底沉积物的沉积通量进行了计算，并以其平均值估算了剩余4个岩芯的沉积速率。采用恒定通量模型，计算了7个岩芯在100~300千年的时间跨度内沉积速率随深度的变化。研究区的沉积速率波动幅度较小，介于0.3~0.6 cm/ky之间；其中2个岩芯的沉积速率变化最为显著，分别位于块状硫化物构造附近与扩张轴周边。放射性碳法与不平衡钍法测得的平均沉积速率结果具有良好的一致性。