Jegen Marion

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  • Article
    Offshore freshened groundwater in continental margins
    (American Geophysical Union, 2020-11-20) Micallef, Aaron ; Person, Mark ; Berndt, Christian ; Bertoni, Claudia ; Cohen, Denis ; Dugan, Brandon ; Evans, Rob L. ; Haroon, Amir ; Hensen, Christian ; Jegen, Marion ; Key, Kerry ; Kooi, Henk ; Liebetrau, Volker ; Lofi, Johanna ; Mailloux, Brian J. ; Martin-Nagle, Renée ; Michael, Holly A. ; Müller, Thomas ; Schmidt, Mark ; Schwalenberg, Katrin ; Trembath-Reichert, Elizabeth ; Weymer, Bradley ; Zhang, Yipeng ; Thomas, Ariel T.
    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.
  • Article
    Structure of the mantle beneath the Alboran Basin from magnetotelluric soundings
    (John Wiley & Sons, 2015-12-19) Garcia, Xavier ; Seille, H. ; Elsenbeck, James R. ; Evans, Rob L. ; Jegen, Marion ; Holz, Sebastian ; Ledo, Juanjo ; Lovatini, Andrea ; Marti, Anna ; Marcuello, Alejandro ; Queralt, Pilar ; Ungarelli, Carlo ; Ranero, Cesar R.
    We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.
  • Article
    The coastal transition zone is an underexplored frontier in hydrology and geoscience
    (Nature Research, 2022-12-23) Weymer, Bradley A. ; Everett, Mark E. ; Haroon, Amir ; Jegen-Kulcsar, Marion ; Micallef, Aaron ; Berndt, Christian ; Michael, Holly A. ; Evans, Rob L. ; Post, Vincent
    We have better maps of the surfaces of Venus, Mars, and the Moon than of the Earth’s seafloor. There is even less information available about the geologic structure below the seafloor. In particular, the transition zone deep beneath and crossing the coastline is a very poorly studied frontier resulting from limitations of technology and logistical barriers. Here, we point out the significance of this region for understanding fundamental geologic processes, geohazards, and especially coastal aquifers. One prominent example is the increasing awareness of the importance of groundwater exchange between land and sea. This Perspective defines the region beneath the coastal transition zone, or coastal white ribbon as an underexplored frontier, and highlights the need for characterization of this critical region to depths of tens of km. We discuss available geophysical methods and their limitations with coastal groundwater used as the primary illustration. Advances in geophysical and drilling technology, coupled with numerical modeling, are needed to enable better accounting of this poorly understood component of the geosphere.