SHORT-LIVED UPWELLING AND VIGOROUS OCEAN CIRCULATION CAUSED BY A BRIEF GLACIAL PERIOD DURING THE LATE ORDOVICIAN

A dramatic shift in climate occurred during the Hirnantian when the first major glaciation of the Phanerozoic was initiated on Gondwana and cooled global temperatures. This glaciation led to the End-Ordovician mass extinction and a large positive carbon isotopic excursion known as the Hirnantian Isotopic Carbon Excursion (HICE). Hirnantian glaciation also invigorated ocean circulation, leading to increased upwelling and productivity along the southern margin of Laurentia and is hypothesized to have also increased upwelling along the western margin. The Upper Ordovician – Lower Silurian Ely Springs Dolomite of eastern California and western Nevada provides a means to study the effects of shifts in ocean circulation on Earth’s tropical biota during a glacial period and constrain the duration of Late Ordovician cooling by determining how long changes in ocean circulation persisted. The Ely Springs Dolomite transitions from thinner, outer shelf and slope deposits at Willow Springs Canyon, CA to thicker, middle to inner shelf deposits to the southeast at White Swan Mine, CA. At both localities, the Ely Springs Dolomite comprises a lower member consisting of a chert-rich dolostone, a massive dolostone middle member, and an upper member consisting of chert-rich dolostone that contains thick (m-scale) chert beds at Willow Springs Canyon. This sequence records relatively high sea levels that drop in the uppermost lower member due to the establishment of glaciers on Gondwana during the Hirnantian, followed by a relative rise in sea level due to the deglaciation of Gondwana. A positive δ¹³C excursion of 1.6‰ to values of +1.3‰±0.03‰ in the middle member at Willow Springs Canyon is interpreted to be the HICE. Enrichment Factors for trace elements (Ba, Cu, Ni, and Zn), a proxy for primary productivity, increase before the HICE and remain elevated until the HICE returns to preexisting δ¹³C levels, indicating that upwelling occurred from the upper portion of the lower member and throughout the middle member at Willow Springs Canyon. Overall, the Ely Springs Dolomite indicates that upwelling and high primary productivity is a short-lived phenomenon that occurs over deposition of the middle member and suggests that vigorous ocean circulation was persistent and likely driven by short-lived glaciation during the Hirnantian.