How Old Is a Moraine? Dating the Debris at Denali's Muldrow Glacier

A moraine, the ridge of debris a glacier leaves behind, has two ages: when the debris was dropped and when the ridge stopped moving. At Muldrow Glacier in Denali National Park and Preserve, a team measured the gap between them at one moraine and found it was 1,260 years. Jason M. Dortch, Lewis A. Owen, Marc W. Caffee and Phil Brease report the work in Alaska Park Science, and argue that the gap matters to anyone comparing moraines dated by different methods.

Why moraines are read

Glaciers respond to temperature and precipitation. Cooler temperatures with more snowfall make a glacier advance, while warmer temperatures or less snow make it thin and retreat. Denali's glaciers were considerably larger in the past, and the moraines they left mark where their edges once were. The moraines that Muldrow Glacier left in the McKinley River area serve as a standard for comparing the timing of glaciation in other parts of Alaska. The authors say that learning how geologic systems answered past climate changes helps predict what comes next, and they cite climate models (IPCC 2007) showing that Alaska is likely to feel climate change faster than mid-latitude areas such as the lower 48 states.

Two kinds of clock

Cosmic rays striking the atmosphere set off a cascade of neutrons that bombards the top of exposed rock and triggers nuclear reactions in its atoms. Beryllium-10 is produced from oxygen and silica in the rock at known rates, so the amount in a sample shows how long that rock has sat at the surface.

The catch

On moraines younger than 10,000 years, beryllium-10 can give younger ages than radiocarbon or lichenometry from the same landform, because the methods date different stages of the moraine's development. Many Alaskan moraines have an ice core that melts slowly, and the boulders on top roll as it goes. A rolled boulder shows a fresh surface with no beryllium-10, so sampling it gives an age that is too young. The cosmogenic age therefore marks the moment the ice core melted and the landform stabilized, while the other methods mark the arrival of the debris. Studies have dealt with many problems of beryllium-10 dating, the authors say, but this lag has received little attention, and measuring it is essential for comparing recent glacial fluctuations driven by climate change.

The test at Muldrow Glacier

On the northern side of the Alaska Range the team studied two moraines, called X and Y here, with two questions. How long is the lag at moraine X, whose deposition age lichenometry had already set? And do unstable landforms really give beryllium-10 ages near zero? Moraine Y had been judged unstable by an earlier study (Werner 1982), which the authors share. They remapped moraines first mapped by Werner in the McKinley River area using field methods, aerial photographs and IKONOS satellite imagery provided by the park. Samples were chiseled from the top 1 to 2 inches (2.5-5 cm) of large granitic boulders, several to each landform. Chemical steps isolated the beryllium, and a mass spectrometer at Purdue University's Rare Isotope Measurement Laboratory counted its atoms. Dividing by the rate at which they accumulate gives an exposure age.

What they found

Only those two ages, one from moraine Y and one from the active ice, showed prior accumulation. The authors say that is likely because boulders keep toppling, being exhumed and breaking up during glacier transport and moraine stabilization, which resets their ages. The beryllium-10 ages stay reset to zero until a moraine stabilizes, they say, which supports reading them as stabilization ages.

Why the lag matters

The authors argue that moraines with an ice core can take a long time to stabilize after the debris is deposited, though the time might differ in other climates, and they call for measuring stabilization times in different climatic settings to build a model linking landform stability to beryllium-10 ages. A lag of this size is small for deposits older than 100,000 years. It can matter for moraines younger than 10,000 years, and for comparisons that mix methods such as radiocarbon, lichenometry and tree-ring dating. Two moraines given the same age by different methods may not have formed at the same time, which would skew estimates of how far glaciers retreated. The authors say that correlations of glacial deposits under 10,000 years old dated by different methods will need to be reevaluated, because climate models compare glacier positions and sizes at the same time over large areas, and wrong correlations would make their results inaccurate. They describe the study as a preliminary framework for determining the lag.

Sources

• Jason M. Dortch, Lewis A. Owen, Marc W. Caffee and Phil Brease, Understanding moraine formation around Denali's Muldrow Glacier, Alaska Park Science, Volume 10, Issue 1, National Park Service; the project was funded by the Murie Learning and Science Center; rewritten in hubnx's own words.