Cosmic-ray paleohydrology

Cosmogenic particles, such as neutrons and muons, have many practical applications in the earth and environmental sciences. One important application is measuring water content at and below the land surface using low-energy neutrons. The field of cosmic-ray hydrology provides the tools to measure soil water, snow water equivalent, or vegetation water, and water fluxes related to these reservoirs, such as precipitation and infiltration.

Cosmic-ray hydrology has its roots in cosmogenic geochronology, which uses cosmogenic isotopes to date depositional and erosional features on the land surface. In cosmic-ray paleohydrology geological proxy records are used to reconstruct past hydrologic conditions, such as precipitation rates or the distribution and size of water reservoirs. Such proxies should be clearly related to the environment, such as precipitation rates, lake levels, or river discharge in the geological past. This presentation will discuss several examples of geological records of ancient water systems, including mountain moraines, paleolake shorelines, river terraces, valley incisions, and ocean paleo-beaches. The combination of mountain moraine positions and their corresponding ages provides valuable information about the temperature and precipitation rates that allowed the glacier to reach the position marked by the moraines. Paleolake shorelines, dated with cosmogenic isotopes, offer insights into the size of the reservoir, which is useful in water mass balance reconstructions and paleoclimate studies. The height of river terraces indicates past river discharge rates at times determined through cosmogenic dating of the deposits. Dated valley incisions provide information on the last significant erosion episode, which is linked to high river discharge. Dated ocean paleobeaches provide a record of sea-level changes in the geological past.

Although measuring water reservoirs and fluxes today uses different tools than those which are used in paleohydrology, the two are linked by the cosmogenic particles that are at the heart of these tools.