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Clocking an Ice Age Retreat: Dating Moraines in Alaska's Revelation Mountains
A boulder sitting on a glacial moraine carries a clock that started when the ice let go of it. Joseph P. Tulenko and Jason P. Briner of the University at Buffalo and Nicolas E. Young of Columbia University explain, in a 2021 article for Alaska Park Science, how glacial geologists read that clock, why Alaska makes it hard, and why the Revelation Mountains in the western Alaska Range are their best site for timing how glaciers retreated after the last ice age.
What a moraine records
During the cold stretch of the last ice age, roughly 26,000 to 19,000 years ago, glaciers in Alaska and elsewhere swelled with snow and spilled out of the mountains into the adjacent lowlands. They left moraines behind: ridges of debris, from fine sand and mud to boulders, that the ice gathered, carried and dropped. Moraines show up from the ground, on topographic maps and in aerial images, as narrow ridges or broad, lumpy ones. If a moraine's age is known, it marks where a glacier stood and when, and so how fast the ice changed. Because glaciers respond so closely to temperature and precipitation, such histories double as archives of past climate.
Two figures from the article frame the story. During the last ice age, Alaska's temperatures may have been 2-4 degrees Celsius colder than in modern times, and glaciers covered roughly 42% of the state, compared with roughly 3.5% in modern times. From 19,000 to 11,000 years ago the climate warmed worldwide and glaciers retreated substantially. This last deglaciation is a natural test of how glaciers behave when the climate warms significantly, and so a guide to how they will respond to present and future warming.
Reconstructing a glacier's history takes mapping, to show where ice has been, and dating, to show when. Mapping keeps improving with new satellite images and maps, such as the Arctic Digital Elevation Model from the Polar Geospatial Center, which the authors used for some of their maps. Dating is harder: serious attempts go back only about 50 years, and the dates must be precise enough to compare with other climate archives, such as ice cores from Antarctica and Greenland, ocean sediments and lake sediments, a comparison that new techniques made possible only in the few decades before the article.
A clock from cosmic rays
The technique is cosmogenic isotope exposure dating. High-energy particles from outside our solar system, known as cosmic rays, strike gas atoms in the atmosphere and burst them apart, setting off a chain reaction that showers the surface. Enough energy survives for particles to penetrate the crust and break apart some atoms in rock minerals. The leftover pieces are new isotopes, called cosmogenic isotopes, and they build up in exposed rock surfaces at a fairly steady rate: the longer a surface is exposed, the more isotopes it holds.
The method needs an event that exposes fresh rock from deep below, and glaciers supply one. They grind off previously exposed surfaces, leaving new ones with no cosmogenic isotopes, and they build moraines from boulders and sediment often never exposed before, plucked from beneath the ice or fallen onto it from steep valley walls. Once a glacier forms a moraine or retreats from a valley, fresh material is exposed and isotopes start to accumulate. Geologists later sample bedrock or boulders on a moraine, measure the isotopes, and use known production rates to calculate when the ice left them.
When the clock misleads
| Scenario | What happens | Effect on the age |
|---|---|---|
| Ideal | The glacier builds a moraine from freshly scraped boulders and sediment and leaves it exposed, and the moraine stays intact for millennia until sampled | A true age |
| Inheritance | The glacier does not erode away every earlier-exposed surface, which happens where ice moves very slowly, though not commonly in Alaska, so leftover isotopes remain | Ages look too old |
| Incomplete exposure | The moraine degrades, and boulders tumble over or rise out of it as mud washes away around them (boulder exhumation), altering or delaying the start of the clock | Ages look too young; the largest challenge in Alaska |
Alaska's landscape is exceptionally dynamic: faulting that causes earthquakes and landslides, the freeze-thaw cycle of the active layer in permafrost, and even volcanism all speed the erosion of fragile landforms like moraines, so moraine-dating results in the state vary. A moraine's makeup matters too. Moraines range from almost all boulders (clast-supported) to almost all clay and mud (matrix-supported). Fine sediments trap ice and water, and when the ice melts the moraine slumps; boulder-rich moraines slump less, so their boulders are more likely to have stayed put and their ages to date the moraine's formation.
Denali National Park and Preserve shows the problem. The moraines left there by the Muldrow Glacier have degraded significantly because they are mainly matrix-supported, with only a few large boulders; they also lie near the Denali Fault, and steady powerful earthquakes may have added to the damage. Earlier work found that exposure ages there probably record several phases of moraine stabilization after the glacier had already retreated, not the building of the moraine. Denali is a stunning showcase of how glaciers shape land, yet glacial geologists have found other Alaskan sites better suited to exposure dating. Since exposure dating was first applied to moraines in California, attempts in Alaska have brought a few successes and some failures, and both taught the importance of choosing moraines meticulously before sampling.
The Revelation Mountains
In the Revelation Mountains, glaciers existed during and after the last ice age and their moraines survive. In the early 2000s Briner and colleagues searched Alaska for the ideal place to build a precise glacier chronology, visiting several sites including the Swift River Valley in the Revelation Mountains. Their results were promising but incomplete: unlike the moraines in Denali and elsewhere, those here were less degraded, were not predominantly matrix-supported and held many large, stable boulders.
The team returned to the Swift River Valley beginning in 2016, after more than a decade of further research in Alaska and better sampling and lab techniques worldwide. The aim was a reliable chronology of the oldest moraines of the last ice age, both to show that exposure dating can succeed in Alaska and to compare Alaska's glacier change with past climate. One moraine was likely deposited sometime around 17,800 years ago, based on four reliable exposure ages; two much older outliers are suspected of carrying inherited isotopes. The remaining ages are in a 2018 paper by Tulenko and others, a beryllium-10 chronology. With that well-constrained dataset, the team found that although global temperature and atmospheric carbon dioxide stayed relatively low toward the end of the last ice age, glaciers in Alaska began retreating before some others around the world; the authors suggested that steadily increasing solar radiation in the Arctic caused the warming. What happened afterward, through the rest of the last deglaciation, and whether that early retreat was sustained, remains unknown.
In summer 2019 the team went back to study the detailed retreat of one glacier in the north Swift River Valley, which left multiple discrete moraines, and sampled the surfaces of 79 boulders. From moraine mapping and the 2018 chronology, the authors hypothesize that the moraines were deposited sometime between the last ice age and the article's writing in 2021, many during the last deglaciation. Dated as precisely as at the 2016 site, they would give the glacier's rate of retreat to set against other climate records and show why this glacier, and glaciers across Alaska, retreated.
Questions the work aims to answer
The authors hope to help answer three questions. Does one mechanism, such as greenhouse gases, control how glaciers behave worldwide, or do Arctic glaciers differ? How fast did Alaska's glaciers melt during the last deglaciation, one of the most recent times in the geologic record when the climate warmed quickly? And do those changes give context for how quickly glaciers are retreating, and will keep retreating, as the climate once again warms rapidly? Precise retreat records exist for many sites across the globe but not yet in Alaska; the project aims for comparable precision to learn whether Arctic alpine glaciers behaved like those elsewhere. Visitors to Alaska's parks can see for themselves how quickly glaciers are retreating, and the authors hope the geologic context will show why the current retreat is so alarming.
Sources
- Joseph P. Tulenko and Jason P. Briner (University at Buffalo) and Nicolas E. Young (Columbia University), Clues from glacier debris: Dating and mapping glacial deposits since the last ice age in the western Alaska Range, Alaska Park Science 20(1), 2021, on the National Park Service website; the authors' figures are not reproduced; rewritten in hubnx's own words.