A millennium-long perspective of flood-related seasonal sediment yield in Mediterranean watersheds

摘要

Mediterranean mountains have been extremely vulnerable to land degradation and soil erosion due to climate factors (summer hydric stress, high storminess) and the long history of human pressure on these terrestrial ecosystems. The short-time span of instrumental monitoring datasets limits our ability to obtain a full depiction of the long-term drivers controlling flood intensity and frequency and soil erosion in Mediterranean watersheds. Here we have applied a novel methodology based on detailed microfacies analyses on annually-laminated (varved) lacustrine sediments to reconstruct floods and seasonal sediment yield and denudation rates in a mountainous Mediterranean watershed during the last 2775 years. The sediment yield reconstruction in this study agrees reasonably well with soil erosion rates from Pyrenean experimental watersheds supporting the validity of this methodology to assess the soil erosion and sediment production from a long-term perspective. The comparison with instrumental precipitation datasets demonstrates the different seasonal sensitivity of sediment yield to heavy rainfall magnitudes mostly depending on soil moisture conditions, soil and regolith erodibility and vegetation cover. During periods of reduced human impact in the watershed, the seasonal maxima in sediment production occurred in autumn, which corresponds to the season with more frequent and intense heavy rainfall in the region. The highest soil erosion rates occurred during periods with higher human impact in the watershed due to sustained burning, overgrazing and cereals cultivation that modified the seasonal sediment yield distribution, with the highest sediment production happening in winter. The most significant periods of increased sediment yield occurred during the Middle Ages (1168-1239 CE) and the 19th century (1844-1866 CE) due to an interplay between increased frequencies and magnitudes of heavy rainfall and intensive agropastoral activities in the lake's watershed. This study highlights the potential of seasonally-resolved archives to adequately evaluate the environmental drivers and mechanisms controlling flood dynamics and soil erosion at decadal to centennial time-scales in areas with strong seasonality.