Des archives sédimentaires arctiques vieilles d’un million d’années éclairent un mystère climatique qui a rendu les scientifiques perplexes.

Le lac El'gygytgyn

Le lac El’gygytgyn est un lac de cratère d’impact situé dans le district autonome de Chukotka, au nord-est de la Sibérie. Crédit : UMass Amherst

Le plus ancien enregistrement continu de l’Arctique est un pas vers la compréhension du changement dramatique des cycles glaciaires.

Une nouvelle recherche est la première à fournir un aperçu continu d’un changement de climat, appelé la transition du milieu du Pléistocène, qui a laissé les scientifiques perplexes. L’étude a été menée par l’Université du Massachusetts Amherst et publiée récemment dans la revue .Climat du passé. Kurt Lindberg, le premier auteur de l’article et actuellement étudiant diplômé de l’Université de Buffalo, est le principal auteur de l’étude ; div class=glossaryItemBody>Fondée en 1846, l’Université d’État de New York à Buffalo est le plus grand campus du système de l’Université d’État de New York et le principal centre public de l’État de New York pour l’enseignement supérieur et professionnel. Il s’agit d’une université publique de recherche dont les campus se trouvent à Buffalo et Amherst, dans l’État de New York, aux États-Unis. Elle est communément appelée l’Université de Buffalo (UB) ou SUNY Buffalo, et était autrefois connue sous le nom d’Université de Buffalo.

;” data-gt-translate-attributes=”[{” attribute=””>University at Buffalo, was only an undergraduate when he completed the research as part of a team that included world-renowned climate scientists at UMass Amherst.

Somewhere around 1.2 million years ago, a profound shift in the Earth’s climate, known as the Mid-Pleistocene Transition, or MPT, happened. Previously, ice ages had occurred, with relative regularity, every 40,000 years or so. But then, in a comparatively short window of geological time, the time between ice ages more than doubled, to every 100,000 years. “It’s a real puzzle,” says Isla Castañeda, professor of geosciences at UMass Amherst and one of the paper’s co-authors. “No one really knows why this shift occurred.”

Lake El’gygytgyn Crater

False-color image of Lake El’gygytgyn, taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite on August 18, 2008 (red indicates vegetation, gray-brown indicates bare land, and deep blue indicates water). Credit: NASA

One of the big barriers to understanding the MPT is that very little data exists. The oldest Arctic ice cores only go back approximately 125,000 years. And older sedimentary cores are almost nonexistent, because as ice ages have come and gone, the advancing and retreating ice sheets have acted like enormous bulldozers, scraping much of the exposed land down to bedrock.

However, there is one place in the world, in far northeastern Russia, that is both above the Arctic Circle and which has never been covered by glaciers: Lake El’gygytgyn. This is where the world-renowned polar scientist, Julie Brigham-Grette, professor of geosciences at UMass Amherst and one of the paper’s co-authors, comes in.

Kurt Lindberg

Kurt Lindberg (class of 2020) analyzing ancient Lake El’gygytgyn sediments for vegetation and past temperature reconstruction. Credit: William Daniels

In 2009, Brigham-Grette led an international team of scientists to Lake El’gygytgyn, where they drilled a 685.5 meter (2,249 foot) sediment core, representing approximately the last 3.6 million years of Earth’s history. Lindberg and his co-authors used the portion of this sedimentary core that spanned the MPT and looked for specific biomarkers that could help them ascertain temperature and vegetation. With this information, they were able to reconstruct, for the first time, climactic conditions in the Arctic during the MPT.

While the team did not solve the mystery of the MPT, they did make a few surprising discoveries. For example, an interglacial period, or era when ice was in retreat, known as MIS 31 is widely recognized as having been abnormally warm—and yet the records at Lake El’gygytgyn show only moderate warmth. Instead, three other interglacial periods, MIS 21, 27, and 29 were as warm or warmer. Finally, the team’s research shows a long-term drying trend throughout the MPT.

“This couldn’t have been done without Lindberg’s enthusiasm,” says Castañeda. “I’ve always had lots of undergrads in my lab, and I love working with them. Kurt took off with this project, and did a wonderful job.”

Reference: “Biomarker Proxy Records of Arctic Climate Change During the Mid-Pleistocene Transition from Lake El’gygytgyn (Far East Russia)” by Kurt R. Lindberg, William C. Daniels, Isla S. Castañeda and Julie Brigham-Grette, 28 June 2021, Climate of the Past.
DOI: 0.5194/cp-2021-66

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