School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada;
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Proc Natl Acad Sci U S A. 2019 Sep 17;116(38):18874-18879. doi: 10.1073/pnas.1909570116. Epub 2019 Sep 4.
Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δC excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δC compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δC excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.
年龄为非冰川的Cryogenian（659 到 649 百万年前）和早埃迪卡拉纪（635 到 590 百万年前）的碳酸盐沉积物在纳米比亚北部的浅水海台地上显示出大的正的和负的 δC 偏移。同样的偏移也记录在边缘深海扇和其他古大陆的碳酸盐台地上。然而，纳米比亚台地上部前斜坡的同期碳酸盐，以及在最外部台地的程度较小的范围内，具有与现代海洋中溶解无机碳（DIC）相容的相对均匀的 δC 组成。我们将这些均匀的值归因于流体缓冲的成岩作用，这种作用发生在海水侵入沉积物中以响应地热孔隙水对流的地方。这种归因可以通过对钙和镁同位素进行配对测试，这意味着在新元古代台地上观察到的大的 δC 偏移虽然是沉积成因的，但并不反映古代开阔洋 DIC 的组成。
