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

完善马萨诸塞州波士顿盆地新元古代冰川时代：最后一个雪球地球上的新铀铅 (U-Pb) 锆石约束

• 批准号: 0001134
• 负责人: Margaret Thompson
• 金额: $ 5.5万
• 依托单位: Wellesley College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2000
• 资助国家: 美国
• 起止时间: 2000-08-01 至 2003-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0001134&HistoricalAwards=false
• 关键词: Refining; Age; Neoproterozoic; Glaciation; Boston

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.

马萨诸塞州波士顿盆地新元古代冰川时代的精化：新的U-Pb锆石对最后一个雪球地球的约束气候学家在全球变暖的未来尺度上努力达成一致的同时，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非常极端，以至于世界海洋冻结，导致海洋微生物大规模灭绝，降水停止，陆地上的化学侵蚀也随之停止。大约1000万年后，由于火山喷发提供的大气中二氧化碳的积累，深度冻结让位于同样特殊的温室条件。这些戏剧性的雪球地球假说的主张将在这项研究中得到验证，该研究将改进目前可用的前寒武纪末期冰期的年龄限制，这些冰期记录在马萨诸塞州波士顿南部罗克斯伯里砾岩的量子“Tillite”成员中。改进的年龄将精确地确定Squantum冰川的持续时间，并澄清Squantum沉积物是否与纽芬兰东部阿瓦隆半岛的冰川沉积物年龄相同。年龄将通过测量从熔岩流、火山灰床和“泥质岩”中提取的矿物的U/Pb比率来获得，这些矿物可能与冰川床上下的岩石有关。最大沉积年龄(约。5.9亿年前)将使用马萨诸塞州兴厄姆海岸暴露的样本进行提炼，那里的熔岩流被含有石灰岩巨石的砾岩覆盖，将分析世界其他地方冰川后“碳酸盐岩帽”的C和Sr同位素特征。休伊茨湾剖面顶部的细粒砂岩也很重要，因为它最近在纽芬兰发现了比大约年轻的Aspidella化石。5.65亿年。由于量子矿床上方岩石的表面暴露程度较差，因此冰川作用的最小年龄（约为60岁）。5.7亿年的历史)将通过测定主排水隧道和岛际隧道的灰层和富含灰的泥质岩的年代来提炼。这项研究的数据不仅提供了前寒武纪末期冰川事件持续时间的第一个可靠估计，而且还提供了在寒武纪生命形式爆发之前的埃迪卡拉动物群的有价值的最大年龄限制。