About: Roda-Boluda et al., (2023), JGR-Earth Surface (link)
Where rocks are uplifted, they get eroded by wind, water, ice, and gravity. Erosion creates large volumes of sediment that are transported from the mountains to sedimentary basins. The rates of erosion are fundamentally driven by mountain uplift. However, the climate can also impact the breakdown and movement of rock. For example, heavy and sustained precipitation can trigger landslides, glaciers grind their bases to a fine powder, and cycles of freezing and thawing can efficiently break down solid bedrock. Geologists currently debate how climate affects erosion on the scale of an entire mountain range.
The Southern Alps of New Zealand are a fantastic place to dig deeper into the link between climate and erosion. Along a narrow range, metamorphosed sandstones are lifted up at multiple millimeters per year – making the Southern Alps one of the fastest deforming mountain ranges on the planet. A relief of over 3000 m captures the westerly winds and leads to heavy rains on the Western Southern Alps with yearly precipitation of 2 – 10 meters. Moreover, the Southern Alps are subject to so-called “paraglacial” (conditioned by recently retreated glaciers) and “periglacial” (in a zone where temperatures fluctuate around 0ºC) erosion processes: Where glaciers recently retreated, hillslopes have become unstable and temperatures around freezing cause efficient freeze-thaw cycles.
During one month in the field, we sampled sand from a number of rivers that drain the Western Southern Alps. Measuring the concentration of cosmogenic beryllium-10, we estimated the average erosion rate upstream of each sample point. Then, we studied how erosion rates vary with different topographic and climatic parameters.
We found that erosion rates were highest in those rivers that had a substantial portion of their catchment at an elevation of 1500 – 2000m. At these elevations para- and periglacial processes are particularly strong in the Southern Alps. In contrast, rainfall and erosion rates did not correlate well.
Overall, the pattern of erosion is set by the uplift of the rocks. However, our data suggest that these erosion rates can be modulated substantially by processes related to freeze-thaw and glacier retreat.
