Isotopic composition of strontium in planktonic foraminiferas from sediments of the DSDP Holes 90-588 and 94-607

Measurements of 87Sr/86Sr on samples of planktonic foraminifers were used to reconstruct changes in the Sr isotopic composition of seawater for the past 8 Ma. The late Neogene was marked by a general, but not regular, increase in 87S/86Sr with two breaks in slope at 5.5 and 2.5 Ma. These times mark the beginning of two periods of steep increase in 87Sr/86Sr values, relative to preceding periods characterized by essentially constant values. During the last 2.5 Ma, 87Sr/86Sr values increased at an average rate of 0.000054/Ma. This steep increase suggests that the modem ocean is not in Sr isotopic equilibrium relative to its major input fluxes. A non-equilibrium model for the modern Sr budget suggests that the residence time of Sr is ~2.5 Ma, which is significantly less than previously accepted estimates of 4-5 Ma.
Modelling results suggest that the increase in 87Sr/86Sr over the past 8 Ma could have resulted from a 25% increase in the riverine flux of Sr or an increase in the average 87Sr/86Sr of this flux by 0.0006. The dominant cause of increasing 87Sr/86Sr values of seawater during the late Neogene is believed to be increased rates of uplift and chemical weathering of mountainous regions. Calculations suggest that uplift and weathering of the Himalayan-Tibetan region alone can account for the majority of the observed 87Sr/86Sr increase since the early Late Miocene. Exhumation of Precambrian shield areas by continental ice-sheets may have contributed secondarily to accelerated mechanical and chemical weathering of old crustal silicates with high 87Sr/86Sr values. In fact, the upturn in 87Sr/86Sr at 2.5 Ma coincides with increased glacial activity in the Northern Hemisphere.
A variety of geochemical (87Sr/86Sr, Ge/Si, d13C, CCD, etc.) and sedimentologic data (accumulation rates) from the marine sedimentary record are compatible with a progressive increase in the chemical weathering rate of continents and dissolved riverine fluxes during the late Cenozoic. We hypothesize that chemical weathering of the continents and dissolved riverine fluxes to the oceans reached a maximum during the late Pleistocene because of repeated glaciations, increased continental exposure by lowered sea level, and increased continental relief resulting from high rates of tectonism.

本研究通过对浮游有孔虫（planktonic foraminifers）样品的87Sr/86Sr比值进行测定，重建了过去8 Ma以来海水的锶同位素组成演化历史。晚新近纪（late Neogene）整体呈现87Sr/86Sr比值逐步升高的趋势，但并非匀速变化，在5.5 Ma和2.5 Ma处存在两次斜率转折；这两个时间节点标志着两段快速升高阶段的起始，此前的时期内该比值基本保持恒定。
过去2.5 Ma以来，87Sr/86Sr比值的平均增长速率为0.000054/Ma。这一快速升高现象表明，现代海洋相对于其主要锶输入通量并未达到同位素平衡。针对现代海洋锶收支的非平衡模型显示，锶的停留时间约为2.5 Ma，这一数值远低于此前公认的4~5 Ma估算结果。
模拟结果显示，过去8 Ma间海水87Sr/86Sr比值的升高，可能源于河流锶输入通量增加25%，或是该输入通量的平均87Sr/86Sr比值升高0.0006。学界普遍认为，晚新近纪海水87Sr/86Sr比值升高的主导成因是山地隆升速率与化学风化强度的增强。计算表明，仅喜马拉雅-青藏高原区域（Himalayan-Tibetan region）的隆升与化学风化，即可解释晚中新世以来观测到的绝大多数87Sr/86Sr比值升高现象。大陆冰盖对前寒武纪地盾区（Precambrian shield areas）的剥蚀作用，可能次要促进了高87Sr/86Sr比值的古老地壳硅酸盐矿物的机械与化学风化加速。事实上，2.5 Ma处87Sr/86Sr比值的上升拐点，与北半球冰川活动增强的时间相吻合。
海洋沉积记录中的多种地球化学指标（包括87Sr/86Sr比值、Ge/Si比值、δ13C、碳酸盐补偿深度（Carbonate Compensation Depth，CCD）等）以及沉积学数据（如沉积堆积速率），均与晚新生代以来大陆化学风化速率与河流溶解态输入通量的逐步升高相契合。我们提出如下假说：受多期冰川作用、海平面下降导致的大陆暴露面积增加，以及构造运动速率提升引发的大陆地形起伏加剧共同影响，晚更新世时期大陆化学风化与河流向海洋的溶解态输入通量达到峰值。