Multi-millennia surface dynamics: Novel investigation approach unveils climate relation to mountain erosion (Serra da Estrela, Portugal)

Abstract

Multi-millennia data logs on surface denudation variation in alpine landscapes are scarce, yet they are needed to understand the impacts of environmental changes on denudation. On the example of the Serra da Estrela plateau in Portugal, we explored a new archive, vertical bedrock outcrops (tors), and the capability of the Tor Exhumation/Exposure Approach (TEA) to capture surface denudation variations even in formerly glaciated landscapes. Therefore, we used vertical in-situ 10Be to date surface exposure of tor slopes in formerly glaciated and non-glaciated parts of the plateau during the last glacial period.
Based on the surface exposure ages, surface denudation variations covering the last ∼200 ka could be derived that revealed glacial retreat dynamics in greater detail. Higher isotope contents and, thus, surface exposure ages were found in the non-glaciated area. At the formerly glaciated sites, the ice margin retreat is reflected in the isotope signature of the tors. The younger rock surfaces had a higher chemical weathering degree than older surfaces at the non-glaciated site as a result of a higher water availability. Highest-elevation tors have experienced (subglacial/hydrothermal) chemical weathering, mass wasting and stripping (∼6 ± 0.5 ka) during the transition from a cool moist, to an oceanic-Mediterranean climate.
Yet, tors in the non-glaciated area yielded lower surface denudation rates with a maximum of 0.53 [mm yr−1] compared to the glaciated area (reaching values of up to 18.29 [mm yr−1]). Since the LGM and the concomitant increase in air temperature, surface denudation also distinctly changed. Temperature trends and surface denudation developed analogously for the last ∼150 ka. Vegetation change or human activity's impact on surface denudation cannot be discerned due to the too low chronological resolution. Overall, we demonstrated that multi-millennia tor records of variation in surface denudation can be obtained using the TEA, even in formerly glaciated areas. Thus, this study contributes to revealing the sensitivity of mountain erosion rates to past environmental changes.