麻豆淫院

North America's deepest gorge, Hells Canyon, which slithers along the border of Idaho and Oregon, is a surprisingly new addition to the Earth's ancient landscape. A suggests that a monumental shift in Snake river drainage around 2.1 million years ago reshaped the topography, carving out Hells Canyon, which plunges an astonishing 2,400 m, significantly deeper than the Grand Canyon.

The research reported in PNAS collected ancient sediments transported by the Snake River and deposited in caves above the canyon to estimate their geological age using cosmogenic nuclide dating.

High-energy particles from space, known as cosmic rays, constantly bombard the Earth. When these particles strike rocks or sediments exposed at the surface, they can interact with certain elements, radioactive isotopes. These isotopes, called cosmogenic nuclides, accumulate over time in the uppermost layers of rock.

Cosmogenic nuclide dating involves measuring the concentration of these isotopes to determine how long a rock or sediment surface has been exposed to cosmic rays, giving us its age.

Canyons are important geographic features that are relics of changes caused by tectonic activity or shifts in climate. They also serve as natural corridors, enabling aquatic species to migrate through mountainous areas. However, scientists still don't fully understand how quickly or by what exact processes canyons form, leading to debates spanning several decades.

Previous studies have indicated that stream capture, a phenomenon where a river changes its course and begins draining in a new area, plays a key role in canyon formation, as seen in places like the Tennessee River in the Appalachians and the Rhine River in the Alps.

The process and timing of Hells Canyon's formation remain uncertain, largely due to the complex and debated history of its creator, the Snake River. Sediment analysis suggests that the canyon was carved between the late Pliocene (3.6 to 2.6 million years ago) and early Pleistocene periods (2.58 to 0.781 million years ago), but the absence of direct evidence leaves many questions regarding its age unanswered.

To piece together a detailed history of how Hells Canyon was carved over time, researchers sampled the sediments left behind in caves above the river when the land was being eroded during the formation of the canyon. These cave river deposits were dated by calculating the differential decay rates of two cosmogenic radionuclides, ¹⁰Be and ²⁶Al.

The team also analyzed the river profile, focusing on river features such as elevation changes and sharp drops (called knickpoints) in tributaries, as well as numerical modeling of stream capture events to understand how erosion occurred over time.

The results revealed that a major drainage capture event, where the Snake River rerouted into the Columbia River system around 2.1 卤 1.0 million years ago, played a central role in shaping Hells Canyon.

Cave deposit dating revealed an increase in erosion rates from ~0.01 mm/yr to ~0.16 mm/yr following drainage capture. The numerical models corroborated this, showing a sudden rise in discharge from stream capture, indicating accelerated canyon incision.

The researchers highlight that their findings demonstrate how abrupt changes in drainage areas can drive rapid landscape evolution, with implications for understanding canyon formation worldwide.

Detailed insights into the mechanics and timelines of this land-shifting geographical phenomenon can help inform better hydrological models and river management systems.

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