The potential of groundwater as a geochemical archive of past environments

Abstract

Archives of past climates and environments are essential for understanding Earth history and provide a baseline for studying the impacts of anthropogenic climate change. Because groundwater in large basins typically has residence times of thousands to tens-of-thousands of years, it potentially records changes to the hydrosphere over the transition from the last glacial maximum. However, the resolution of the palaeoenvironment information preserved in groundwater is limited by groundwater not having a distinct age. Even with simple aquifer geometries, uniform hydraulic properties, and no cross-formational flow, variations in flow path lengths and dispersion results in groundwater containing individual water aliquots that have residence time distributions of hundreds to thousands of years. Groundwater in regional aquifers will potentially record longer-term (tens of millennia) environmental changes, such as the transition between the last glacial maximum and the current climate that occurred between approximately 17 and 11 ka. Evidence for shorter timescale (decades to centuries) environmental changes may also be preserved in younger (<1 ka) groundwater. However, because the width of the residence time distribution scales to mean residence time, evidence for these shorter timescale changes are unlikely to be preserved in groundwater with mean residence times of more than a few thousand years. The attenuation of the initial geochemical variations also results in the magnitude of environmental change recorded being potentially underestimated, especially in older groundwater.