Glaciers hold layers of history preserved in ice, offering unique insights into Earth's past that can also help us interpret the future. Trapped amidst the frozen water are microscopic deposits of dust, pollen, and even pollutants that scientists can use to examine environmental changes through time. DRI's Ice Core Lab has used this technique to highlight atmospheric lead pollution and economic turbulence in Ancient Rome. Now, their latest study found that a glacier in the French Alps dates back to the last Ice Age – the oldest known glacier ice in the region. Serving as a record that spans through the development of agriculture in Western Europe and the advent of industrialization, the glacier holds insights into an era of rapid change.
The new study, published in the June issue of PNAS Nexus , examines a 40-meter long ice core from Mont Blanc's Dôme du Goûter. Using radiocarbon dating techniques, the research team found that the glacier provides an intact record of aerosols and climate dating back at least 12,000 years. Aerosols are small droplets and particles in the air such as desert dust, sea salts, sulfur from volcanic eruptions, soot from forest fires, as well as pollutants and other emissions from human activities. Glacier ice offers the most detailed record of past atmospheric aerosols, and this is the first ice core record from the European region that extends back to the last climatic transition. Aerosols play an important role in regional climate through their interactions with clouds and solar radiation, and the insights offered by the ice record can help inform accurate climate modeling for both the past and future.
"For the first time, we have a fairly complete Alpine record of atmospheric and precipitation chemistry going all the way back to the Mesolithic Period," said Joe McConnell , Director of DRI's Ice Core lab who co-authored the study. "And that's a big deal, because you have two major climate states – glacial and interglacial – and to get a record of atmospheric precipitation chemistry across that huge climate change tells you the most extreme natural aerosol concentrations that you'd expect. On top of that, you have humans going from hunter-gatherers with a very low population through the development of agriculture, domestication of animals, mining, etc, and then a vast population increase and the clearing of land. All of that is happening around this ice core site. It spans the full range of natural and anthropogenic change, and it's right in the center of Europe – where much of Western civilization evolved."
The glacier's location in the Alps is important because it serves as a more intact record of Europe's local climate than those found in distant Arctic ice. Many aerosols play important roles in driving Earth's climate, so scientists would like to know how sources and concentrations in the air have varied in the past.
"Ice cores collected from glaciers and ice sheets can provide such information, but since these droplets and particles stay in the air only for a few days to maybe a week, records developed from glaciers close to the sources often are the most informative," said lead author, Michel Legrand.
The ice core analyzed in this study was first collected in 1999 by some of the study's French authors. It was stored in a freezer in France for more than 20 years before McConnell and his team brought it to DRI's Ice Core Lab in Reno, Nevada, where specialized equipment and methods known as continuous flow analysis allowed it to be melted down and the chemistry measured, layer by icy layer.
"Determining what year or period of time a layer in the ice represents can be challenging, so here we used a unique combination of radiometric methods to establish the chronology in the ice," said coauthor Werner Aeschbach.
"We were relieved to find that even under the unusually warm climate of the 20th century, the cold temperatures at over 14,000 feet near Mont Blanc's peak had preserved the glacier so that the ice record hadn't yet been impacted by melting," said co-author Nathan Chellman.
The historic age of the ice at the base of the core, around 40 meters deep into the glacier, surprised the researchers. Another core collected from a glacier located less than 100 meters away at Col du Dome was found to contain ice only about a century old, despite being much deeper. The scientists attribute this to the strong wind patterns found on Mont Blanc.
"It's exciting to find the first ice core from the European Alps containing an intact record of climate that extends back through the current ten-thousand-year warm period and into the very different climate of the last ice age," said coauthor Susanne Preunkert, who was a member of the field team that collected the ice core in 1999.
Insights into Europe's Past Climate
The uniquely detailed ice record revealed a temperature difference of about 3 degrees Celsius between the last Ice Age and the current Holocene Epoch. Using pollen records embedded in the ice, reconstructions of summer temperatures during the last Ice Age were about 2 degrees Celsius cooler throughout western Europe, and about 3.5 degrees Celsius cooler in the Alps.
The phosphorous record also told researchers the story of vegetation changes in the region over the last 12,000 years. Phosphorous concentrations in the ice were low during the last Ice Age, increased dramatically during the early to mid-Holocene, and then decreased steadily into the late Holocene. This is consistent with the spread of forests under the warmer climate, and their decline following the proliferation of modern society and the land-clearing that resulted from agriculture and the spread of industry.
Records of sea salt also helped the researchers examine changes in historical wind patterns. The ice core revealed higher rates of sea salt deposition during the last Ice Age that may have resulted from stronger westerly winds offshore of western Europe. Sea salt aerosols can scatter solar radiation back to space and affect climate via their impacts on cloud droplet, size, and albedo, making them important drivers of the regional climate.
The ice record tells a more dramatic story for the changes in dust aerosols during the climatic shift. Dust serves as an important driver of climate by both absorbing and scattering incoming solar radiation and outgoing planetary radiation, and impacts cloud formation and precipitation by acting as cloud condensation nuclei. During the last Ice Age, dust was found to be about 8-fold higher compared to the Holocene. This contradicts the mere doubling of dust aerosols between warm and cold climate stages in Europe simulated by prior climate models. The difference may be explained by increased plumes of Saharan dust depositing in Europe, which remains the main source of dust in the region. The ice core record is consistent with other paleoclimate records that suggest more arid conditions over the Mediterranean during colder climates.
The 1999 expedition team collecting the ice core from Dome du Goûter on the shoulder of Mont Blanc. Credit: LGGE/OSUG, Bruno Jourdain
Uncovering More Stories Entombed in the Ice
This study is only the beginning of the Mont Blanc ice record's story, as the researchers plan to continue analyzing it for indicators of human history. The first step in uncovering every ice core's record is to use isotopes and radiocarbon dating to establish how old each layer of ice is. Now, with that information, the scientists can take an even deeper look at what it can tell us about past human civilizations and their impact on the environment.
"Now we can start to interpret all these other records that we have of lead and arsenic and other things like that, in terms of human history," said McConnell.
The information can also be used to help interpret how changes in aerosols impact the climate and improve modeling to help us understand current and future climatic shifts.
"If you're really going to go back and examine all possible climate states, past and future, you need a model that captures true climate variability," McConnell said. "It's a laudable goal, but to evaluate how good the models are, you've got to be able to compare them to observations, right? And that's where the ice cores come in."
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More information: The full study, Alpine ice core record of large changes in dust, sea-salt, and biogenic aerosol over Europe during deglaciation, is available from PNAS Nexus at https://doi.org/10.1093/pnasnexus/pgaf186
Study authors include: Michel Legrand, Joe McConnell, Susanne Preunkert, David Wachs, Nathan Chellman, Kira Rehfeld, Gilles Bergametti, Sophia Wensman, Werner Aeschbach, Markus Oberthaler, and Ronny Friedrich
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