Offshore freshened groundwater in continental margins

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Micallef, A., Person, M., Berndt, C., Bertoni, C., Cohen, D., Dugan, B., Evans, R., Haroon, A., Hensen, C., Jegen, M., Key, K., Kooi, H., Liebetrau, V., Lofi, J., Mailloux, B. J., Martin-Nagle, R., Michael, H. A., Mueller, T., Schmidt, M., Schwalenberg, K., Trembath-Reichert, E., Weymer, B., Zhang, Y., & Thomas, A. T. Offshore freshened groundwater in continental margins. Reviews of Geophysics, 59(1), (2021): e2020RG000706, https://doi.org/10.1029/2020RG000706.
First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography-driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three-dimensional hydrological models. We show that such advances, combined with site-specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub-seafloor geomicrobiology.
This study has received funding from the European Research Council (ERC), under the European Union's Horizon 2020 research and innovation program (grant agreement No. 677898 (MARCAN) to A. M.) and the U.S. National Science Foundation (NSF FRES 1925974 to M. P.; NSF OCE 0824368 to B. D.; and NSF EAR 1151733 to H. A. M.). T. M., B. W. and Y. Z. were funded by the SMART project through the Helmholtz European Partnering Initiative (Project ID Number PIE-0004) involving GEOMAR and the University of Malta.