Integrating lacustrine and coastal sediment records of environmental change in Northern Spain during the Anthropocene

Reconstructing past environmental changes and identifying their main drivers is essential to predict the future response of natural systems to climate change under ever increasing anthropogenic pressure. To achieve this goal and understand the natural variability (prior to human disturbance) of the main processes involved, it is necessary to extend our temporal records back in time to pre-industrial conditions through the analysis of natural archives. The Cantabrian region (Northern Spain) constitutes an excellent natural laboratory to analyze and evaluate the magnitude of recent environmental change because of: i) its particular location, near to the boundary between Eurosiberian and Mediterranean biogeographic regions; ii) its strong elevation (from sea level to >2600 m asl) and climate (oceanic to continental mediterranean) gradients; and iii) the strong human impact to which this region has been subjected during the past few centuries. This research aims at understanding how recent (19th to 21st centuries CE) warming and increasing human land use have affected the geomorphological and geochemical dynamics of Northern Spanish watersheds, in the context of the environmental changes occurred during the last millennia. We use a multi-site approach, integrating high-resolution lake sediment records (Valle, Ausente, Isoba, Pozo Curavacas, Pozo Tremeo and Antuzanos) located along a West to East transect with a strong altitudinal gradient (17—1800 m asl), covering a wide range of climatic conditions and land management. To quantify the contributions of human and climate drivers to the recorded environmental changes, we use a multidisciplinary approach , involving geomorphological and paleolimnological proxies. We particularly focus on three main components of watershed dynamics: i) sediment delivery and depositional dynamics, ii) heavy metal loads, and iii) carbon fluxes. The multiproxy analysis of lake sediment cores (sedimentology, geochemistry, environmental magnetism, pollen and diatoms) dated by radiometric techniques (210Pb, 137Cs and 14C) reveals a dominant climate forcing at millennial to centennial timescales on depositional processes, in agreement with speleothem records. This signal has been modulated locally by changing anthropogenic landscape transformations driven by arable and pastoral farming as revealed by biological and geochemical proxies. In contrast, human-driven, abrupt increases in watershed erosion, heavy metal concentrations and nutrient loads occurred since the early to mid-20th century CE, coinciding with the Great Acceleration, in agreement with estuarine records along the Central and Eastern Cantabrian Sea coast analyzed by our research team and collaborators. According to available erosion models, this increase in sediment production has been influenced by a warmer and drier climate, with increasing flood frequency. This environmental change has been particularly intense at low-elevation sites subject to higher anthropogenic pressure, but it has been attenuated during the last two decades in high-elevation areas as a consequence of changing land use and environmental management. This research demonstrates the importance of combining different natural archives and methodologies to achieve a comprehensive understanding of the nature, timing, spatial variability, and consequences of the synergistic effects of human activities and climate change on watershed and regional scales. This is a contribution to CALACLIMP project (PID2021-122854OB-I00).