Paper: Gulick, S. P. S., et al., 2015. Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska. Proceedings of the National Academy of Sciences, v. 112: p. 49.
When you go to a beach, you probably already know that the sand you sink your toes into has been eroded from rocks over thousands or millions of years. You also probably know that much of this eroded sand and sediment comes from wave action, or perhaps rivers that transport sediments thousands of miles until it can reach the sea to be deposited along shorelines or in submarine delta fans.
But, did you know that climate may tear down mountains to create such sandy features as well? A new study shows that mountain ranges can actively evolve with Earth’s climate, a process that can be recorded in marine sediments near shorelines.
Typically, an orogeny, or a mountain-building event due to plate tectonics, builds mountains much faster than they can be weathered away – the Himalayas, Andes, and Rockies are examples of such growing mountain ranges. However, in some cases, glaciers may erode mountains faster than they can be built. Evidence of this erosion can be found in off-shore delta fans, like one found off the southern coast of Alaska. The St. Elias Mountain Range, near the south coast of Alaska, is one such area where climate-triggered erosion has occurred in the past.
Led by scientists from the University of Texas at Austin, the University of Florida, and Oregon State University, researchers with the Integrated Ocean Drilling Program (IODP) attempted to measure the net material leaving and entering this mountain range over the course of more than a million years. It was the first time a study like this has been attempted.
The study, “Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska,” sheds light on an ongoing debate over the balance of climate and tectonic forces that influence how mountains are formed. The researchers found that erosion from this mountain range accelerated sharply about 1 million years ago, correlating with a time when global climate cooling triggered stronger, more persistent ice ages than had previously occurred.
A race between tectonics and glacial erosion
To investigate the net material building and leaving the St. Elias range, the researchers studied an offshore fan in the Gulf of Alaska that has recorded sedimentary runoff and tectonic convergence in the area. Variations in the sedimentary fan allowed them to construct a budget of tectonic material influx, erosion, and sediment output from the coastal mountain range. In addition to seismically-imaged sediment volumes, sediment cores from IODP boreholes allowed them to create timelines to show when erosion may have accelerated in the past.
Their results showed that that erosion first accelerated about 2.7 million years ago in response to Northern Hemisphere glacial intensification. At the time, however, tectonics were still building the mountain range, exceeding the outflow rate of eroded sediments from the range. This changed when erosion further accelerated about 1.2 – 0.7 million years ago during the mid-Pleistoncene climate transition, when shorter, more frequent glacial cycles changed to longer, approximately 100,000 year cycles. Since that transition, erosion and transport of sediment out of the coastal mountain range has exceeded tectonic building rates by 50% to 80%, effectively tearing down mountains, though still very slowly.
Climate has become an ever-present topic of interest in the Earth sciences these days. Usually, it is spoken about when focused on global temperatures, or sea level rise, for instance. But climate has played a key role in many other Earth processes, and can at times be even more powerful of a force than plate tectonics. This study supports that, and demonstrates just how interconnected Earth’s systems can be.