Strontium Isotope Ages of the Marine Merced Formation, near San Francisco, California
Strontium Isotope Ages of the Marine Merced Formation, near San Francisco, California
复制标题
加利福尼亚州旧金山附近海洋默塞德地层的锶同位素年龄
作者：
B. Ingram;J. Ingle
通过测定有孔虫（Elphidiellahannai）的锶同位素组成，研究了默塞德组海岸沉积物的年龄，并探讨了海进-海退旋回与更新世冰期-海平面升降的关系。在剖面的上半部分，年龄分辨率可能相对较好（± 0.1百万年）。锶同位素年龄提供了强有力的支持证据的裂变径迹年龄的火山灰层在顶部的形成（0.45百万年），以及最近的年龄估计为猛犸化石的0.45百万年。剖面上部的Sr同位素年龄也证实了默塞德组的海相/非海相旋回沉积受冰期-间冰期旋回的控制。由于全球海水中的87 Sr/86 Sr比值在2.4和4.3百万年之间变化不大，因此不可能对剖面的下部进行良好的年龄分辨率。地层基底的年龄仅限于2.4和4.8百万年之间。默塞德组上部（0 - 720 m）的沉积物累积速率明显高于计算的基底部分（720 - 1670 m）。堆积速率的变化可能是由于大陆风化速率的增加和/或大约80万年前开始的构造沉降的增加。
The strontium isotopic compositions of foraminifers ( Elphidiella hannaiwere determined to refine the age of coastal sediments of the Merced Formation and to evaluate the relation between transgressive–regressive cycles and Pleistocene glacio-eustatic sea-level fluctuations. Relatively good age resolution (± 0.1 myr) is possible in the upper part of the section. The Sr isotope ages provide strong supporting evidence for the fission track ages of an ash layer at the top of the formation (0.45 myr), as well as recent age estimates for the mammoth fossil of ∼0.45 myr. Sr isotope ages in the upper part of the section also confirm that marine/nonmarine cyclic sedimentation in the Merced Formation is controlled by glacial–interglacial cycles. Good age resolution for the lower part of the section is not possible, due to little variation of the87Sr/86Sr ratio in global sea water between 2.4 and 4.3 myr. The age of the base of the formation is constrained only between 2.4 and 4.8 myr. Sediment accumulation rates in the upper portion of the Merced Formation (between 0 and 720 m) are significantly higher than those calculated for the basal portion (between 720 and 1670 m). The change in accumulation rate may be due to increased continental weathering rates and/or increased tectonic subsidence beginning about 0.8 myr ago.