Characterization Of Permafrost Development By Isotopic And Chemical Analysis Of Soil Cores Taken From The Copper River Basin And An Upland Loess Deposit In Interior Alaska

Abstract

Chemical and isotopic analyses of pore water from permafrost cores taken from the dry lake bed of ancient Lake Atna in the Copper River Basin and from an upland loess deposit northeast of Fairbanks, Alaska reveal information about the local past environments not available by other means. Thawed core samples from both sites were analyzed for delta18O and deltaD values using an isotope ratio mass spectrometer. Water content was determined as well, and subsamples of the cores were analyzed for nitrogen and carbon content. Water extracts of the core samples were analyzed for cations (Ca, Mg, K and Fe), as well as pH, electrical conductivity, and bicarbonate. Magnetic susceptibility was determined on samples from the Fairbanks site. Data from samples taken from the Lake Atna site revealed a sequence of events that occurred in the basin after the lake drained about 10,000 years ago. At one location, oxygen isotopes show that permafrost formed continuously down through the lakebed. At the other location, 400 m distant, oxygen isotopes show that after permafrost formed, a thaw lake was produced on top of the permafrost. After the thaw lake had drained at least one wildfire passed through the area, large enough to thaw the surface permafrost to a depth of over 2 m at both core sites. The surface permafrost refroze, and currently the permafrost at the research site extends from 1 m depth to between 45 and 55 m depth. At the Fairbanks site chemical analyses showed indications of pedogenesis at several depths in the loess profile of the permafrost core. Water isotope data from a site where the permafrost table is at less than 1 m depth show repeated episodes of thawing and refreezing. Charcoal and ash in several layers of soil in the area and from the sample core suggest that fire may be the cause for the thawing events. The primary core water isotope analyses also show several thawing and refreezing events, but the depths of the signatures in the core indicate that these episodes happened thousands of years ago when the ground surface was much lower than it is today.