LAKE-SWITCH · Using lake sediments to reconstruct soil weathering trajectories over the Holocene

Chemical weathering is a central biogeochemical process that shapes the Earth’s Critical Zone (CZ), regulates the global carbon cycle and sets the pace for nutrient delivery to soils and ecosystems. Most knowledge on the rates and controls on chemical weathering are from laboratory experiments and from the short-term observation of modern soil and river systems. In contrast, little is known about past changes of chemical weathering over hundreds to thousands of years, which limits our understanding of how long-lasting human-climate-ecosystem interactions have impacted the CZ trajectories. Because of this knowledge gap, it is not possible to fully understand the response and feedbacks of the CZ to the climatic and environmental perturbations of the Holocene period, nor to predict their future evolution during the Anthropocene. To fill this gap, LAKE-SWITCH aims to produce new quantitative weathering records over 10^2-10^4 year timescales, with a temporal focus on the Holocene period. There are 3 main challenges: 1) developing quantitative proxies of chemical weathering, 2) calibrating these proxies for paleo-reconstructions, 3) measuring these proxies in paleo-archives of 10^2-10^4 year timescale integration. To provide these records, we will measure lithium and strontium isotopic proxies in lake detrital and authigenic – carbonates and biogenic silica – sediment archives. To calibrate these proxies and archives, we will use a source-to-sink approach and track weathering product pathways from soils, through rivers, to lake deposits. Then we will apply these proxies back in time in Holocene lake cores. As rapidly-eroding mountain dominate the global chemical erosion budget, we will focus on the study of the European Alps. New data from Alpine watersheds and lake records spanning gradients in erosion, runoff and land use will serve to quantify and model the impact of climate and human drivers on soil trajectories from the onset of the Holocene to the Anthropocene.