A mountain landscape can react dramatically to surprisingly small changes in rainfall. In the southern Andes, erosion rose and fell in step with a slow rhythm in Earth’s orbit. Climate has long been thought capable of changing how much mountain rock is worn away, but direct geological evidence has remained unclear because long, detailed records from land are rare.

The record from the southern Central Andes shows long-term agreement between erosion rates and a four-hundred-thousand-year Milankovitch eccentricity cycle over the studied interval. The results also fit the idea that relatively modest rainfall changes can produce larger, uneven erosion responses as landscapes adjust to changing hydrologic conditions.

To trace that history, river sediment from a canyon in northwest Argentina was used to estimate past erosion rates across a span from six to one point eight million years ago. The basic idea is simple: particles made in rock by cosmic rays build up over time, and their concentration can be converted into an erosion rate.

Lower concentration means faster removal. A river sample works like a mixed bag collected from an entire neighborhood: if the river gathers sediment from the whole watershed, it carries an estimate of the watershed’s average erosion. When the sediment is quickly buried and preserved for millions of years, measuring those particles in a well-dated sequence reconstructs a time-resolved record of past erosion rates.

Around four million years ago, the record develops a clear repeating pattern in erosion that continues until about two point four million years ago. Across as many as three complete cycles, erosion changes by more than tenfold and is largely in step with a four-hundred-thousand-year cycle in Earth’s orbit.

After about two point four million years ago, the pattern changes. The four-hundred-thousand-year rhythm fades, while erosion rises sharply and fluctuates around a level consistent with the modern rate. The strongest result is the link between erosion and the changing strength of shorter, twenty-three-thousand-year summer sunlight cycles, whose amplitude rises and falls within the longer four-hundred-thousand-year rhythm.

The proposed explanation is a threshold response on steep slopes: landslides and river cutting may keep erosion modest until rainfall passes a tipping point, after which erosion rises disproportionately. The overall picture is that rainfall in the Andes varied with the strength of the South American monsoon, cycling every twenty-three thousand years while its intensity was shaped by the longer orbital rhythm.

Because the watershed had tectonically steepened, likely sparsely vegetated threshold slopes, small precipitation changes of about twenty percent could produce order-of-magnitude erosion changes through landslide dynamics, paced by the four-hundred-thousand-year cycle.

The study links modest precipitation changes to much larger swings in erosion, suggesting that climate can help set the pace of mountain change and the movement of carbon and sediment.