Complex conductivity hysteresis of diatomites and chalks with varying pore water salinity

ABSTRACT Diffusion-driven changes in pore water salinity are prevalent in natural environments. We show that the electrical conductivity of saturated porous rocks at a given salinity depends on the history of salinity change, that is, whether salinity was reached by increasing or decreasing concentration. Understanding this salinity-induced conductivity hysteresis is crucial for accurate geophysical interpretation of subsurface processes. We measured complex conductivity spectra across 16 diatomite and chalk samples subjected to increasing and decreasing pore water salinity. For a given salinity, in-phase conductivity is up to approximately 120% higher on the decreasing salinity path. The hysteresis is minor above 3 S/m bulk water conductivity but increases monotonically below that threshold, becoming most pronounced at the lowest salinities, where bound water increasingly contributes to conduction. Our data indicate that the hysteresis arises from distinct equilibrium states of the electrical double layer for each salinity path, not from kinetics. The magnitude of the consequent surface conductivity hysteresis scales with Archie’s m-exponent and the fraction of pore space occupied by bound water. By contrast, the induced polarization (IP) response, characterized by the Cole–Cole parameters of relaxation time, distribution exponent, and normalized chargeability, shows limited dependence on salinity history. This relative stability indicates a decoupling between conduction and polarization, where IP dispersion reflects pore geometry, whereas polarization magnitude reflects the equilibrium state of the interface. Using the equilibrium surface conductivity at high- and low-frequency limits, a new method was introduced to estimate the bound water fraction involved in polarization. The analysis shows that approximately 9%–26% of the bound water contributes, with greater spatial extent along the increasing salinity path. This trend may reflect that adsorbed ions are more hydrated during the increasing salinity path and remain more dehydrated during the decreasing salinity path. These findings provide a new framework for interpreting geophysical data in variable-salinity environments.