Refining the Age of Neoproterozoic Glaciation in the Boston Basin, Massachusetts: New Uranium-Lead (U-Pb) Zircon Constraints on the Last Snowball Earth

马萨诸塞州波士顿盆地新元古代冰川时代的精确化：新的U-Pb锆石对最后一个雪球地球的约束Margaret Pampsonnel-0001134当气候学家努力就全球变暖的未来规模达成一致时，研究前寒武纪末期冰河时期形成的沉积岩的地球科学家之间的争论也在升温。 根据被称为“雪球地球”的假设，这个时代的气候条件是如此极端，以至于世界上的海洋冻结，导致海洋微生物大规模灭绝，降水停止和相关的陆地化学侵蚀。 深度冻结让位于同样非凡的温室条件后，估计一千万年的积累大气二氧化碳提供的火山爆发。 这些戏剧性的说法雪球地球假说将在这项研究中进行测试，完善目前可用的年龄限制终端前寒武纪冰川记录在斯昆恩“Tillite”成员的罗克斯伯里砾岩南部的波士顿，马萨诸塞州。 改进后的年龄将确定斯昆恩冰川作用的持续时间，并澄清斯昆恩矿床是否与纽芬兰东部阿瓦隆半岛的冰川矿床年龄相同，年龄将通过测量从熔岩流、火山灰床和“泥质岩”中提取的矿物中的U/Pb比值来获得，这些矿物与冰川床上下的先前定年岩石有关。 最大沉积年龄（大约）将使用来自马萨诸塞州欣厄姆沿海暴露的样品进行提炼，在那里，熔岩流被含有石灰石巨砾的砾岩覆盖，将分析世界其他地方冰后期“帽碳酸盐”的C和Sr同位素特征。 休伊特湾剖面顶部的细粒砂岩也很重要，因为它最近发现了阿斯皮德拉化石，纽芬兰的阿斯皮德拉化石年龄约为10年。5.65亿年 由于Squantum矿床上方的岩石表面暴露很差，冰川作用的最小年龄（约为20岁）。5.7亿年）将通过确定主排水隧道和岛际隧道中的Scouron层上覆的灰层和富含灰的粘土岩的年代来进行改进。 从这项研究的日期不仅提供了第一个强大的估计持续时间的终端前寒武纪冰川事件，但也将提供一个宝贵的最大年龄限制软体“埃迪卡拉”动物群之前的生命形式在寒武纪的时间爆炸。

Refining the Age of Neoproterozoic Glaciation in the Boston Basin, Massachusetts: New U-Pb Zircon Constraints on the Last Snowball EarthMargaret ThompsonEAR-0001134As climatologists struggle to agree upon future dimensions of global warming, debate is also heating up among earth scientists who study sedimentary rocks formed during ice ages at the close of Precambrian time. According to the hypothesis dubbed "snowball earth", climatic conditions in this era were so extreme that the world's oceans froze over resulting in mass extinction of marine micro-organisms and shut-down of precipitation and related chemical erosion on land. The deep freeze gave way to equally extraordinary greenhouse conditions after an estimated ten million years with the accumulation of atmospheric carbon dioxide supplied by erupting volcanoes. These dramatic claims of the snowball earth hypothesis will be tested in this study by refining currently available age constraints for terminal Precambrian glaciation recorded in the Squantum "Tillite" Member of the Roxbury Conglomerate south of Boston, Massachusetts. Improved ages will pinpoint the duration of the Squantum glaciation, and also clarify whether the Squantum deposits are the same age as glacial deposits in the Avalon Peninsula of eastern Newfoundland.Ages will be obtained by measuring U/Pb ratios in minerals extracted from lava flows, volcanic ash beds and "argillite" that can be related to previously dated rocks above and below the glacial beds. The maximum depositional age (approx. 590 million years) will be refined using samples from coastal exposures in Hingham, Massachusetts where a lava flow is overlain by conglomerate containing limestone boulders that will be analyzed for C and Sr isotope signatures characteristic of post-glacial "cap carbonate" elsewhere in the world. Fine grained sandstone at the top of the Hewitts Cove section is also significant because it has recently yielded Aspidella fossils which in Newfoundland are younger than approx. 565 million years. Because surface exposure of rocks above the Squantum deposits is poor, the minimum age of glaciation (approx. 570 million years) will be refined by dating ash beds and ash-rich argillite overlying the Squantum beds in the Main Drainage Tunnel and the Inter-Island Tunnel. Dates from this study will not only provide the first robust estimate of the duration of a terminal Precambrian glacial event, but will also furnish a valuable maximum age constraint on soft bodied "Ediacaran" fauna preceding the explosion of life forms in Cambrian time.