Microradiocarbon age constraints in the Mt. Hunter, Alaska ice core: implications for Central Alaska Holocene ice extent and climate

Abstract:

Investigating North Pacific climate variability during warm intervals prior to the Common Era is essential to understand the behavior of ocean-atmosphere teleconnections between low latitudes and the Arctic under future warming scenarios. However, most of the existing ice core records in the Alaska/Yukon region only allow access to climate information covering the last few centuries. Here we present a complete depth-age scale for a 208 meter surface-to-bedrock ice core recovered in 2013 from the summit plateau of Mt. Hunter (Denali National Park, central Alaska). The annual layer signal is unambiguous to a depth of 190 meters (152.8 m w.eq.), where the age of the ice is 1203 ± 41 years old (i.e. representing the year 810 CE). Below that depth, annual layering is difficult to identify due to glacier flow-induced thinning. By applying radiocarbon dating on the carbonaceous aerosols in the ice core and a two-parameter glacier flow model, a continuous depth-age relationship to the bottom of the core has been established. This is the first time micro radiocarbon dating has been applied to a high latitude Northern Hemisphere ice core, where concentrations of ice impurities (including carbon) are particularly low. To achieve this, the amount of ice sampled to obtain the required carbon mass was increased (>1 kg) and ¹⁴C analysis of 16 samples was done using established (water insoluble organic carbon [WIOC]) and new (dissolved organic carbon [DOC]) techniques. Calibrated ¹⁴C ages from the two deepest samples (207.9 m, 7,946-10,226 years cal BP; 208.2 m, 7,018-7,975 years cal BP) indicate that basal ice on Mt. Hunter has an early Holocene (>8 kyr) origin. Samples from depths of 198.7 to 205.6 m have nearly uniform ¹⁴C ages (3,200 to 3,500 years cal BP), which may reflect a significant increase in snow accumulation at Mt. Hunter in the mid-Holocene coeval with regional Neoglaciation. We will discuss initial stable isotope data from the early Holocene portion of the Hunter ice core and place it into a regional climate context.