Increasing Tephra Deposition in Northeastern North America Points to Atmospheric Circulation Changes at the Early Mid Holocene Transition

Abstract

The number of cryptotephra (non-visible volcanic ash) records from northeastern North America is unique in the continent. The resulting tephrostratigraphic framework includes ash deposits sourced from volcanic arcs across the Northern Hemisphere and is an exceptional resource for correlating and dating paleoenvironmental records. It also provides an opportunity to explore more novel questions regarding the controls on ultra-distal tephra (volcanic ash >3,000 km from source) dispersal and deposition. Here, we examine temporal patterns in the tephrostratigraphy of northeastern North America to test the legitimacy of a previously noted change in ash deposition frequency at the Early Mid Holocene transition. We integrate five new cryptotephra records into the existing framework to improve its temporal and spatial extent and report further occurrences of widespread cryptotephra deposits including Mt. St. Helens We, Jala pumice, White River Ash east, Ruppert tephra, Mt. St. Helens Yn and Mazama Ash. Reexamination of the combined tephrostratigraphy using breakpoint analysis shows a significant increase in the frequency of ashfall after ca. 9,000 (7,860–9,650) cal yr BP (calendar years before C.E. 1950). We discuss this change in relation to volcanic and environmental controls of fine ash dispersal and preservation. We reject hypotheses relating to eruption frequency or depositional processes in favor of changing atmospheric transport patterns and tephra dispersal—possibly caused by the retreat of the Laurentide Ice Sheet. Our study is a novel example of how tephrostratigraphy can be used beyond traditional correlative and dating studies, in this case indicating large-scale changes in atmospheric circulation through time.