Troublesome Tephra and Ambiguous Age Models: The Challenges of Volcanic Records From North Pacific Ice Cores

Ice cores drilled in the North Pacific provide important records of ocean-atmosphere interactions. However, their potential to document volcanic activity remains underexplored, an oversight that is somewhat surprising given proximity to many active volcanoes in the Pacific Ring of Fire. The few studies that have investigated volcanism in North Pacific ice cores have largely been limited to electrical conductivity and sulfate measurements. As such, tephra data from ice cores in the region are limited. This is particularly significant because traditional ice core dating methods (e.g., seasonal variations in stable water isotopes) have proven unreliable in some North Pacific records, thus tephra are an important alternative and underexploited chronological tool.A 325 m ice core drilled from the summit plateau of Mt. Logan, southwest Yukon, in 2022 (60.604°N, 140.493°W; 5,334 m asl) presents an opportunity to revisit the characteristics and sources of tephra in a North Pacific ice core. An annual chronology has been developed using seasonal variations in H₂O₂, NH₄⁺, Na, and insoluble particle concentrations. Annual layers have been counted to 1911 CE, which corresponds to a depth of ~ 257.4 m, or ~ 80% of the core length. Although the basal age of the core remains unconstrained, preliminary age-depth modelling suggests the record spans one to two millennia at most. However, an abrupt and distinctive change in the seasonal cycle of the proxies used for annual layer counts, together with an absence of independent tie-points between 257 – 325 m, has complicated an accurate chronology. Here, we present our efforts to complete the age-depth scale for the 2022 Mt. Logan ice core. In doing so, we outline some challenges and successes encountered thus far in working toward the first high-resolution tephrochronology for a North Pacific ice core.One approach is to compare the 2022 Mt. Logan record with the limited but important volcanic records available for ice cores from elsewhere in the North Pacific region. Sites include an earlier record drilled on Mt. Logan, the 2002 Prospector Russell Col ice core, which is one of the few sites in the North Pacific reported to contain pre-Holocene ice. Additional locations include the Begguya plateau (Mt. Hunter, Alaska), which similarly preserves pre-Holocene ice, and the Eclipse Icefield located close in proximity to Mt. Logan in the St. Elias Mountains. Volcanic records, if correlated across the sites and/or to well-dated eruptions, may provide a means to constrain the age of the 2022 Mt. Logan ice core.