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

Wellesley College, Wellesley Hills MA

Investigators

Abstract

Refining the Age of Neoproterozoic Glaciation in the Boston Basin, Massachusetts: New U-Pb Zircon Constraints on the Last Snowball Earth Margaret Thompson EAR-0001134 As 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.