87Sr/86Sr chemostratigraphy of Neoproterozoic Dalradian carbonates below the Port Askaig Glaciogenic Formation, Scotland
87Sr/86Sr chemostratigraphy of Neoproterozoic Dalradian carbonates below the Port Askaig Glaciogenic Formation, Scotland
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DOI：
10.1016/j.precamres.2010.02.021
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发表时间：
2010-05
影响因子：
3.8
通讯作者：
Y. Sawaki;T. Kawai;T. Shibuya;Miyuki Tahata;S. Omori;T. Komiya;N. Yoshida;T. Hirata;T. Ohno;B. Windley;S. Maruyama
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作者：
Y. Sawaki;T. Kawai;T. Shibuya;Miyuki Tahata;S. Omori;T. Komiya;N. Yoshida;T. Hirata;T. Ohno;B. Windley;S. Maruyama
苏格兰新元古代-寒武纪Dalradian超群包含三个冰川沉积单元，按升序排列为Port Askaig组、Stralinchy-Reelan组和洛赫na Cille Boulder层。虽然这个想法，相对寒冷的气候发生在最后的元古代，被广泛接受，冰川作用的发生的原因仍然是有争议的，我们使用新的carbonate 87 Sr/86 Sr数据，以更好地约束这种环境变化。一些达尔拉德期灰岩具有高锶（> 1000 ppm）、低锰和低Mn/Sr（<0.2）的特征，根据其硅、铝、钾含量和Rb/Sr比值，它们未受到硅质矿物的污染，因此适合于估算新元古代古海水的87 Sr/86 Sr值。Islay组灰岩的87 Sr/86 Sr值（约1000 Sr/1000 Sr）为1000 Sr/1000 Sr。0.7067），采样岛的艾莱，是类似的值以前的研究。然而，87 Sr/86 Sr值的石灰岩下港Askaig组（约。0.7064），收集在加尔布艾里奇岛，有最低的值记录在达尔拉德超群。将这些87 Sr/86 Sr值与世界上其他地区的87 Sr/86 Sr值进行比较，如此低的87 Sr/86 Sr值（<0.7065）仅见于早于更古老的Cryogenian（Sturtian）冰川作用的层序中，这支持了Port Askaig组是在更古老的Cryogenian冰川作用期间沉积的观点。我们重建了从Tonian到Cryogenian和Ediacaran到最早寒武纪的187 Sr/86 Sr比值的变化。结果表明，海水中87 Sr/86 Sr比值在早寒武纪冰期之前下降，在埃迪卡拉纪早、中期上升了两个台阶，在埃迪卡拉纪中期和前寒武纪-寒武纪界线至少出现了两次突然的正偏移。就在更古老的低温期斯特期冰川作用之前，海水中87 Sr/86 Sr比值的下降表明，与罗迪尼亚超大陆的分裂和大陆溢流玄武岩火山作用有关的风化作用的增强可能是雪球地球条件下产生全球冷却的驱动因素（Goddéris等人，2003; Donnadieu等人，2004年）。
The Neoproterozoic–Cambrian Dalradian Supergroup of Scotland contains three glaciogenic sedimentary units, the Port Askaig Formation, the Stralinchy-Reelan Formation, and the Loch na Cille Boulder Bed, in ascending order. Although the idea, that relatively cold climates occurred during the latest Proterozoic, is widely accepted, the cause of the onset of glaciation is still controversial; we use new carbonate87Sr/86Sr data to better constrain this environmental change. Some Dalradian limestones have high strontium contents (>1000ppm), low manganese contents and Mn/Sr ratios (<0.2) and, according to their silica, aluminum and potassium contents and Rb/Sr ratios, they suffered insignificant contamination from siliciclastic minerals, thus they are suitable for estimating the87Sr/86Sr values of paleoseawater in the Neoproterozoic. The87Sr/86Sr values of limestones of the Islay Formation (ca. 0.7067), sampled on the island of Islay, are similar to the values of previous studies. However, the87Sr/86Sr values of limestones just below the Port Askaig Formation (ca. 0.7064), collected on the island of Garbh Eileach, have the lowest values recorded in the Dalradian Supergroup. Comparing these87Sr/86Sr values with coeval87Sr/86Sr values of other regions in the world, such low87Sr/86Sr (<0.7065) has only been reported from successions pre-dating the older Cryogenian (Sturtian) glaciation, which supports the idea that the Port Askaig Formation was deposited during the older Cryogenian glaciation. We have reconstructed the variations in thei87Sr/86Sr ratio from the Tonian through the Cryogenian and Ediacaran to the earliest Cambrian. The results show that the87Sr/86Sr ratio of seawater decreased just before the older Cryogenian glaciation, that it increased by two steps in the early and middle Ediacaran, and that there were at least two abrupt positive excursions in the middle Ediacaran and at the Precambrian–Cambrian boundary. The decline of the87Sr/86Sr ratio of seawater just before the older Cryogenian Sturtian glaciation suggests that enhancement of weathering, related to break-up of the Rodinia supercontinent and continental flood basalt volcanism, was a likely driver for generating global cooling for the Snowball Earth conditions (Goddéris et al., 2003; Donnadieu et al., 2004).