The Earth has experienced major changes in its climate over the past ~2.6 million years (the Quaternary) with successive glacial and interglacial cycles that have changed our landscapes. This involves erosive action directly from glaciers moving across the area, as well as fluvial erosion from subsequent meltwater. In particular, mountain glaciers can destroy the local landscape, changing the topography where the water flows downstream, but it is uncertain how quickly such activity will affect.
New research, published in Geology, focused on the Qilian Shan mountains in China as an ideal experiment to determine the effects of glacier erosion on mountain topography due to the difference in glacier size on either side of the mountain. Solar insolation from the sun is more concentrated on the southern side of the mountain, meaning that the glaciers grow to a smaller size than the relatively cooler and wetter (from the Asian monsoon) northern slopes, a result which is known as asymmetric glaciation. In fact, the point where ice accumulation and ice melt are in equilibrium (known as the equilibrium line altitude) on the northern slopes is 200m lower than on the southern slopes.
Dr. Jingtao Lai and Dr. Kimberly Huppert, from the GFZ German Research Center for Geosciences, explained that this phenomenon is not a recent phenomenon, rather asymmetric glaciation probably occurred since the late Oligocene (up to 27.8 million years ago) as the East Asian monsoon continues since then.
“Many glaciers are shrinking due to climate change, but their potential impact on landscapes is still unclear,” said Dr. Different. “By studying the Qilian Shan, we found that glacial erosion influenced the rivers below, causing long-term changes that continue to shape the land even after the glaciers have disappeared. how the glaciers, rivers and land have worked together in the past and possibly in the future.”
To test the topographic response to glacial-interglacial cycles, the researchers used topographic maps and simulated glaciation across the study area based on different altitudes of the balance line. The greater the difference between the equilibrium line altitudes on either slope, the more pronounced the glacial asymmetry and therefore the drainage divide moved further.
Dr. found. Lai and Dr. Huppert that the drainage divide, the border between two adjacent drainage basins where all the rain and runoff flows, has moved south and because of this the rivers that flow in it have become steeper, noticeable on the northern slopes. Such subsidence results in deeper erosive action on the local landscape, not only in areas that experienced glaciation, but also further downstream.
The response time of self-equilibrating topography after glaciation results from the difference between the rate of erosion and the rate of land uplift. The modeling of Dr. Lai and Dr. Huppert suggests that while erosion worsened during the interglacial period, over time erosion and uplift reached a relative equilibrium, possibly lasting more than 6 million years. It completely overlaps with the changing glacial and interglacial cycles every 40,000 and 100,000 years. Dr. Lai says that “the long response time suggests that the effect of asymmetric glaciation (through divide migration) does not disappear quickly after the glaciers disappear.”
“In fact, fluvial erosion is still the dominant process in interglacial times, and the distribution of migration affects the evolution of topography by changing the rate of fluvial erosion,” added Dr. Different. “The long response time means that the effects of drainage divide migration and erosion rate changes do not dissipate quickly and the topography continues to adjust itself to deglaciation. This has implications for the interpretation of some observational data. For example, we should be careful about interpreting some observed rates of erosion as long-term stable rates and assuming that these rates will not change in the future.
“In this work, we only focused on the timescale of the topography that rebalanced itself during the interglacial periods in response to the asymmetric glaciation. In the future I would like to have a full understanding of the timescale of the asymmetric glaciation that moves the drainage divide location. can have a complete picture of the dynamic interaction between glaciers, rivers, and topography during the glacial-interglacial cycle.”
Globally, this research is important to link east-west trending mountain ranges that experience large differences in solar insolation on their north and south slopes, as well as those with wind and precipitation patterns that are more focused on one slope rather than the other. The rate of glacial and fluvial erosion affecting local landscapes is important because climate change continues to shrink glaciers and more volumes of meltwater are brought down, making their mark on the environment and community on their way.
Jingtao Lai et al, Asymmetric glaciation, divide migration, and postglacial fluvial response times in the Qilian Shan, Geology (2023). DOI: 10.1130/G51086.1
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Citation: Asymmetry in China’s mountain glaciers irreversibly changes landscape (2023, July 19) retrieved on July 19, 2023 from https://phys.org/news/2023-07- asymmetry-china-mountain-glaciers-irreversibly.html
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