Earth has undergone significant climate changes over the last ~2.6 million years (Quaternary period) through a series of glacial and interglacial cycles that have transformed our landscapes. This includes erosive action from glaciers moving directly across the area, as well as stream erosion from subsequent meltwater. In particular, mountain glaciers can devastate the local landscape by altering the topography through which water flows downstream, but it is unknown how quickly such activities will take effect.

A new study published in Geology focused on the Qilian Shan mountain range in China as an ideal experiment to determine the effect of glacial erosion on mountain topography due to the difference in glacier size on both sides of the mountain. Sunlight from the sun is more concentrated on the southern slope of the mountain, meaning that glaciers become smaller than the relatively colder and wetter (due to the Asian monsoons) northern slopes, a result known as asymmetrical glaciation. In fact, the point at which ice accumulation and ice melt are at equilibrium (known as the height of the equilibrium line) is 200 m lower on the northern slopes than on the southern slopes.

From the German Earth Sciences Research Center GFZ, Dr. Jingtao Lai and Dr. Kimberly Huppert explains that this phenomenon is not new, but rather that asymmetrical glaciation probably occurred from the late Oligocene (up to 27.8 million years ago), because the East Asian monsoon has been going on ever since.

“Many glaciers are shrinking due to climate change, but their potential impact on the landscape is still unclear,” says Dr Lai. “While studying the Qilian Shan, we found that glacial erosion affects downstream rivers and causes permanent changes that continue to shape the topography even after the glaciers have disappeared. This understanding helps us see how climate change is affecting Earth’s landscapes and gives us valuable insight into how glaciers, rivers and land surface have worked together in the past and possibly in the future.”

To test the terrain’s response to glacial-interglacial cycles, the researchers used topographic maps based on different heights of the equilibrium line and modeled glaciation in the study area. The greater the difference between the elevations of the equilibrium line at any given slope, the more pronounced the glacial asymmetry and therefore the more the drainage basin has moved.

Dr. Lai and Dr. Huppert found that the drainage divide, the boundary between two adjacent drainage basins into which all precipitation and runoff flow, shifts southward, and therefore the river beds flowing here become steeper, especially on their northern slopes. This increase in steepness is not only confined to glacial areas, but also leads to deeper erosion of the local landscape downstream.

The response time of land recovering after the ice age is the result of the difference between the rate of erosion and the rate of elevation of the land. Dr. Lai and Dr. Huppert’s modeling suggests that while erosion increased during interglacial periods, over time erosion and uplift reached a relative equilibrium that could take more than 6 million years. This is completely ahead of glacial and interglacial cycles that change every 40,000 and 100,000 years. Dr. “The long response time indicates that the effects of asymmetrical glaciation (through segregated migration) do not disappear easily once the glaciers have disappeared,” says Lai.

“In fact, stream erosion is still a dominant process in interglacial times, and the migration of the split influences the evolution of topography through changes in the rate of stream erosion,” adds Dr. “The long response time means that the effects of watershed migration and changes in erosion rates do not disappear easily and the land continues to adapt to glaciation. This has implications for the interpretation of some observational data. For example, we must be careful to interpret some observed erosion rates as long-term stable rates and to assume that these rates will not change in the future.

“In this study, we focused solely on the timescale of land rebalancing itself during interglacial periods in response to asymmetrical glaciation. In the future, I would like to fully understand the timescale of asymmetrical glaciation displacing the drainage basin. In this way, we can get a complete picture of the dynamic interaction between glaciers, rivers, and land during the glacial-interglacial cycle.”

At a global scale, this research is important as it relates to east-west oriented mountain ranges with significant differences in insolation on the northern and southern slopes and where winds and precipitation are more concentrated on one slope than the other. The rate at which glacial and fluvial erosion impacts local landscapes is important because climate change continues to shrink glaciers and large volumes of meltwater are carried downstream, leaving an imprint on the environment and communities along its path. Source