@article{Chesley.2021,
title = {Fluid-rich subducting topography generates anomalous forearc porosity},
author = {Christine Chesley and Samer Naif and Kerry Key and Dan Bassett},
url = {https://www.nature.com/articles/s41586-021-03619-8.epdf?sharing_token=059dEpg3Ed0LSl8d02FaDdRgN0jAjWel9jnR3ZoTv0M_rJgybuzy9QY1Pp-7852_sl1dUqsZYNhDbIkAbAwN8O00dlhHX0kHWAqHdpqZsFD8QgiZoZHW8WJbEVUSVahCUzKCDjMjf_MTwwA70y77ZzabPEx99qB99b1SDa08GkI%3D},
doi = {10.1038/s41586-021-03619-8},
issn = {0028-0836},
year = {2021},
date = {2021-01-01},
journal = {Nature},
volume = {595},
number = {7866},
pages = {255--260},
abstract = {The role of subducting topography on the mode of fault slip—particularly whether it hinders or facilitates large megathrust earthquakes—remains a controversial topic in subduction dynamics1–5. Models have illustrated the potential for subducting topography to severely alter the structure, stress state and mechanics of subduction zones4,6; however, direct geophysical imaging of the complex fracture networks proposed and the hydrology of both the subducting topography and the associated upper plate damage zones remains elusive. Here we use passive and controlled-source seafloor electromagnetic data collected at the northern Hikurangi Margin, New Zealand, to constrain electrical resistivity in a region of active seamount subduction. We show that a seamount on the incoming plate contains a thin, low-porosity basaltic cap that traps a conductive matrix of porous volcaniclastics and altered material over a resistive core, which allows 3.2 to 4.7 times more water to subduct, compared with normal, unfaulted oceanic lithosphere. In the forearc, we image a sediment-starved plate interface above a subducting seamount with similar electrical structure to the incoming plate seamount. A sharp resistive peak within the subducting seamount lies directly beneath a prominent upper plate conductive anomaly. The coincidence of this upper plate anomaly with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event7 directly links subducting topography to the creation of fluid-rich damage zones in the forearc that alter the effective normal stress at the plate interface by modulating the fluid overpressure. In addition to severely modifying the structure and physical conditions of the upper plate, subducting seamounts represent an underappreciated mechanism for transporting a considerable flux of water to the forearc and deeper mantle. Electromagnetic data collected at the northern Hikurangi Margin, New Zealand show that a seamount on the incoming plate allows more water to subduct, compared with normal, unfaulted oceanic lithosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Key.2021,
title = {Inverted long-baseline acoustic navigation of deep-towed CSEM transmitters and receivers},
author = {Kerry Key and Steven Constable},
doi = {10.1007/s11001-021-09427-z},
issn = {0025-3235},
year = {2021},
date = {2021-01-01},
journal = {Marine Geophysical Research},
volume = {42},
number = {1},
pages = {6},
abstract = {We develop an inverted long-baseline (ILBL) acoustic navigation system for determining the position of deep-towed instruments, such as controlled-source electromagnetic transmitters and receivers. The ILBL system uses a deep-tow mounted acoustic transceiver system to measure travel times to a pair of surface transponders towed on paravanes behind the survey vessel. The travel times, transponder positions and pressure depth data are inverted for the lateral position of the deep-tow vehicle, as well as the positions of any relay transponders on the antenna and receiver array that are towed horizontal behind the deep-tow vehicle. Three example applications demonstrate position accuracies of about 5 and 37 m in the inline and crossline directions for 3 km water depths and around 6 m for 1 km depth. The portability and generality of the system make it suitable for deep-tow applications for geophysical, geochemical and geological surveying purposes. We have shown that the accuracy of the ILBL system is similar to that of commercial USBL systems, but is considerably more cost effective than even portable USBL systems, and can be used on vessels lacking permanently installed USBL transceiver heads. Further, it can be used in water depths of 5,000 m or more. It could also be readily modified for further improving its position accuracy if desired.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{sgur,
title = {Offshore Freshened Groundwater in Continental Margins},
author = {Aaron Micallef and Mark Person and Christian Berndt and Claudia Bertoni and Denis Cohen and Brandon Dugan and Rob Evans and Amir Haroon and Christian Hensen and Marion Jegen and Kerry Key and Henk Kooi and Volker Liebetrau and Johanna Lofi and Brian J Mailloux and Renée Martin‐Nagle and Holly A Michael and Thomas Müller and Mark Schmidt and Katrin Schwalenberg and Elizabeth Trembath‐Reichert and Bradley Weymer and Yipeng Zhang and Ariel T Thomas},
doi = {10.1029/2020rg000706},
issn = {8755-1209},
year = {2021},
date = {2021-01-01},
journal = {Reviews of Geophysics},
volume = {59},
number = {1},
abstract = {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 review paper considers offshore freshened groundwater (OFG), which is water hosted in sediments and rocks below the seafloor, with a total dissolved solid concentration lower than seawater. We have compiled >300 records to demonstrate that freshened groundwater occurs offshore on most continents around the world and has a global volume of 1 × 106 km3. The majority of OFG was deposited when sea level was lower than today and is hosted in sandy sub‐seafloor layers that are located within 55 km of coasts in water depths less than 100 m. We present a range of geochemical, geophysical, and modeling approaches that have successfully been used to investigate OFG systems. We also propose approaches to address key scientific questions related to OFG, including whether it may be used as an unconventional source of potable water in coastal areas. Most known OFG is located at water depths of <100 m within 55 km of the coast, hosted in siliciclastic aquifers in passive margins Key gaps in knowledge include the extent and function of OFG systems, as well as the mechanism and timing of emplacement Isotopic tracers, jointly inverted geophysical data and 3‐D hydrological models can help address these knowledge gaps},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Chesley:2019fv,
title = {Crustal Cracks and Frozen Flow in Oceanic Lithosphere Inferred From Electrical Anisotropy},
author = {Christine Chesley and Kerry Key and Steven Constable and James Behrens and Lucy MacGregor},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2019/12/Chesley_et_al-2019-Geochemistry_Geophysics_Geosystems.pdf},
year = {2019},
date = {2019-12-01},
journal = {Geochemistry Geophysics Geosystems},
volume = {20},
number = {138},
pages = {21},
abstract = {Geophysical observations of anisotropy in oceanic lithosphere offer insight into the formation and evolution of tectonic plates. Seismic anisotropy is well studied but electrical anisotropy remains poorly understood, especially in the crust and uppermost mantle. Here we characterize electrical anisotropy in 33 Ma Pacific lithosphere using controlled‐source electromagnetic data that are highly sensitive to lithospheric azimuthal anisotropy. Our data reveal that the crust is ∼18–36 times more conductive in the paleo mid‐ocean ridge direction than the perpendicular paleo‐spreading direction, while in the uppermost mantle conductivity is ∼29 times higher in the paleo‐spreading direction. We propose that the crustal anisotropy results from subvertical porosity created by ridge‐parallel normal faulting during extension of the young crust and thermal stress‐driven cracking from cooling of mature crust. The magnitude of uppermost mantle anisotropy is consistent with recent experimental results showing strong electrical anisotropy in sheared olivine, suggesting its paleo‐spreading orientation results from sub‐Moho mantle shearing during plate formation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{freshwaterSR,
title = {Aquifer systems extending far offshore on the U.S. Atlantic margin},
author = {Chloe Gustafson and Kerry Key and Rob L Evans },
url = {https://doi.org/10.1038/s41598-019-44611-7},
doi = {10.1038/s41598-019-44611-7},
year = {2019},
date = {2019-06-19},
journal = {Scientific Reports},
volume = {9},
number = {1},
abstract = {Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{10.1093/gji/ggz253,
title = {Bayesian Joint Inversion of Controlled Source Electromagnetic and Magnetotelluric Data to Image Freshwater Aquifer Offshore New Jersey},
author = {Daniel Blatter and Kerry Key and Anandaroop Ray and Chloe Gustafson and Rob Evans},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2019/12/6FA32540-AE06-4DF8-93A2-E5FA1D3AAEC8.pdf},
issn = {0956-540X},
year = {2019},
date = {2019-01-01},
journal = {Geophysical Journal International},
abstract = {Joint inversion of multiple electromagnetic data sets, such as controlled source electromagnetic and magnetotelluric data, has the potential to significantly reduce uncertainty in the inverted electrical resistivity when the two data sets contain complementary information about the subsurface. However, evaluating quantitatively the model uncertainty reduction is made difficult by the fact that conventional inversion methods – using gradients and model regularization – typically produce just one model, with no associated estimate of model parameter uncertainty. Bayesian inverse methods can provide quantitative estimates of inverted model parameter uncertainty by generating an ensemble of models, sampled proportional to data fit. The resulting posterior distribution represents a combination of a priori assumptions about the model parameters and information contained in field data. Bayesian inversion is therefore able to quantify the impact of jointly inverting multiple data sets by using the statistical information contained in the posterior distribution. We illustrate, for synthetic data generated from a simple 1D model, the shape of parameter space compatible with controlled source electromagnetic and magnetotelluric data, separately and jointly. We also demonstrate that when data sets contain complementary information about the model, the region of parameter space compatible with the joint data set is less than or equal to the intersection of the regions compatible with the individual data sets. We adapt a trans-dimensional Markov chain Monte Carlo algorithm for jointly inverting multiple electromagnetic data sets for 1D Earth models and apply it to surface-towed controlled source electromagnetic and magnetotelluric data collected offshore New Jersey, USA, to evaluate the extent of a low salinity aquifer within the continental shelf. Our inversion results identify a region of high resistivity of varying depth and thickness in the upper 500 m of the continental shelf, corroborating results from a previous study that used regularized, gradient-based inversion methods. We evaluate the joint model parameter uncertainty in comparison to the uncertainty obtained from the individual data sets and demonstrate quantitatively that joint inversion offers reduced uncertainty. In addition, we show how the Bayesian model ensemble can subsequently be used to derive uncertainty estimates of pore water salinity within the low salinity aquifer.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Key:2016eq,
title = {MARE2DEM: a 2-D inversion code for controlled-source electromagnetic and magnetotelluric data},
author = {Kerry Key},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2018/02/GJI-Key-2016.pdf},
year = {2016},
date = {2016-01-01},
journal = {Geophysical Journal International},
volume = {207},
number = {1},
pages = {571--588},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Naif:2016ea,
title = {Porosity and fluid budget of a water-rich megathrust revealed with electromagnetic data at the Middle America Trench},
author = {Samer Naif and Kerry Key and Steven Constable and Rob L Evans},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2018/01/G3-Naif-2016.pdf},
year = {2016},
date = {2016-01-01},
journal = {Geochemistry Geophysics Geosystems},
volume = {17},
number = {11},
pages = {4495--4516},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Key:2013gl,
title = {Electrical image of passive mantle upwelling beneath the northern East Pacific Rise},
author = {Kerry Key and Steven Constable and Lijun Liu and Anne Pommier},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2018/01/Nature-2013-Key.pdf},
year = {2013},
date = {2013-03-01},
journal = {Nature},
volume = {495},
number = {7442},
pages = {499--502},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Naif:2013gh,
title = {Melt-rich channel observed at the lithosphere-asthenosphere boundary},
author = {Samer Naif and Kerry Key and Steven Constable and Rob L Evans},
url = {http://emlab.ldeo.columbia.edu/wp-content/uploads/2018/01/Nature-2013-Naif.pdf},
year = {2013},
date = {2013-03-01},
journal = {Nature},
volume = {495},
number = {7441},
pages = {356--359},
keywords = {},
pubstate = {published},
tppubtype = {article}
}