The gravitas of gravitational isotope fractionation revealed in an isolated aquifer

Despite the ubiquitous effects of gravitation on Earth, its potential influence on relative distribution of isotopic substances has remained elusive - and so far only identified in confined gaseous systems (Craig et al., 1988; Severinghaus et al., 1996, 1998). Yet, in a motionless and chemically homogeneous water column, dissolved isotopic substances must be distributed according to their masses. Here we report the first resolvable isotopic variations resulting from gravitational effects on solutes, identified on dissolved chloride (Cl-) and bromide (Br-) in a sedimentary aquifer from the Illinois Basin (USA). We show that the correlations between depth and both ³⁷Cl/³⁵Cl and ⁸¹Br/⁷⁹Br - varying by 1.1 ‰ and 1.6 ‰ respectively - reflect the evolution toward a gravity-diffusion equilibrium of porewater in the sediment column. This observation reveals that these deep groundwaters have been mostly stagnant for at least 20 Myr, possibly up to 300 Myr. As chloride and bromide are often conservative in groundwater systems, we highlight their essential role in unravelling the hydrodynamics and residence times of isolated aquifers. Furthermore, this study reveals gravitational fractionation as a viable process, potentially affecting other isotopic systems in various geological settings.