Near-surface electromagnetic, rock magnetic, and geochemical fingerprinting of submarine freshwater seepage at Eckernförde Bay (SW Baltic Sea)

AbstractSubmarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernförde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations. FigureFreshwater advection within pockmarks reduces the magnetic susceptibility of near-surface Eckernförde Bay sediments, traced by electromagnetic mapping complemented by rock magnetic and geochemical methods

[1]  J. King,et al.  SEDIMENTARY MAGNETISM, ENVIRONMENTAL MAGNETISM, AND MAGNETOSTRATIGRAPHY , 1991 .

[2]  M. Hovland.  Geomorphological, geophysical, and geochemical evidence of fluid flow through the seabed , 2003 .

[3]  I. J. Won,et al.  Magnetometers and electro-magnetomenters , 2004 .

[4]  I. J. Won,et al.  GEM‐3: A Monostatic Broadband Electromagnetic Induction Sensor , 1997 .

[5]  Roy Thompson,et al.  Quaternary Climates, Environments and Magnetism , 1999 .

[6]  G. Turner,et al.  Environmental Applications of Magnetic Measurements , 1980, Science.

[7]  D. Rey,et al.  Magnetic properties of sediment in the Ría de Arousa (Spain): dissolution of iron oxides and formation of iron sulphides , 2004 .

[8]  P. K. Harrington,et al.  Formation of pockmarks by pore-water escape , 1985 .

[9]  A. Roberts,et al.  Environmental magnetism: Past, present, and future , 1995 .

[10]  P. Dando,et al.  Spatial distribution and budget for submarine groundwater discharge in Eckernförde Bay (Western Baltic Sea) , 2004 .

[11]  R. Hyndman,et al.  Reduced magnetization produced by increased methane flux at a gas hydrate vent , 2005 .

[12]  M. Tivey,et al.  Direct inversion of potential fields from an uneven track with application to the Mid-Atlantic Ridge , 1995 .

[13]  A. Roberts,et al.  Magnetite dissolution, diachronous greigite formation, and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand , 2006 .

[14]  R. Evans,et al.  Impact of Low Salinity Porewater on Seafloor Electromagnetic Data: A Means of Detecting Submarine Groundwater Discharge? , 2001 .

[15]  M. Dillon,et al.  Rock magnetic signatures in diagenetically altered sediments from the Niger deep‐sea fan , 2006 .

[16]  C. Benech,et al.  Optimum depth of investigation and conductivity response rejection of the different electromagnetic devices measuring apparent magnetic susceptibility , 1999 .

[17]  B. Housen,et al.  Rock-magnetic signature of gas hydrates in accretionary prism sediments , 1996 .

[18]  B. Moskowitz,et al.  Depth Distribution of Magnetofossils in Near-Surface Sediments From the Blake/Bahama Outer Ridge, Western North Atlantic Ocean, Determined by Low-Temperature Magnetism , 2006 .

[19]  M. Whiticar,et al.  Pockmarks: Submarine vents of natural gas or freshwater seeps? , 1981 .

[20]  Michael Schlüter,et al.  Rhizon sampling of porewaters near the sediment‐water interface of aquatic systems , 2005 .

[21]  H. Müller.  Characterization of marine near-surface sediments by electromagnetic profiling , 2010 .

[22]  Silke Marczinek,et al.  Grundwasserströmung und -beschaffenheit im Einzugsgebiet der Eckernförder Bucht, Schleswig-Holstein , 2002 .

[23]  Peeter Pehme,et al.  Groundwater Geophysics: A Tool for Hydrogeology , 2011 .

[24]  John W. Geissman,et al.  Environmental Magnetism: Principles and Applications of Enviromagnetics , 2004 .

[25]  R. Kirsch.  Groundwater geophysics : a tool for hydrogeology , 2006 .

[26]  Volker Ratmeyer,et al.  The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems , 2004 .

[27]  W. Balsam,et al.  Gulf of Mexico sediment sources and sediment transport trends from magnetic susceptibility measurements of surface samples , 2006 .

[28]  David J. Dunlop,et al.  Rock Magnetism: Frontmatter , 1997 .

[29]  J. Deconinck,et al.  Early diagenesis of marine organic-matter and magnetic properties of sedimentary rocks: the role of iron limitation and organic-matter source organisms , 2002 .

[30]  W. Burnett,et al.  Measurement and significance of the direct discharge of groundwater into the coastal zone , 2001 .

[31]  E. Verwey,et al.  Electronic Conduction of Magnetite (Fe3O4) and its Transition Point at Low Temperatures , 1939, Nature.

[32]  R. Berner.  A New Geochemical Classification of Sedimentary Environments , 1981 .

[33]  T. Stieglitz,et al.  Quantifying submarine groundwater discharge in the coastal zone via multiple methods. , 2006, The Science of the total environment.

[34]  Douglas W. Oldenburg,et al.  Simultaneous 1D inversion of loop loop electromagnetic data for magnetic susceptibility and electrical conductivity , 2003 .

[35]  A. Roberts,et al.  Diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems , 2007 .

[36]  S. Hutchinson,et al.  Diagenesis of magnetic minerals in the intertidal sediments of the Yangtze Estuary, China, and its environmental significance. , 2001, The Science of the total environment.

[37]  A. Roberts,et al.  Multiple mechanisms of remagnetization involving sedimentary greigite (Fe3S4) , 2004 .

[38]  M. Whiticar.  Diagenetic relationships of methanogenesis, nutrients, acoustic turbidity, pockmarks and freshwater seepages in Eckernförde Bay , 2002 .

[39]  A. Roberts,et al.  Formation of iron sulfide nodules during anaerobic oxidation of methane , 2007 .

[40]  F. Oldfield,et al.  The influence of particle size variations on the magnetic properties of sediments from the north‐eastern Irish Sea , 1994 .

[41]  A. Reitz,et al.  Integrated Rock Magnetic and Geochemical Quantification of Redoxomorphic Iron Mineral Diagenesis in Late Quaternary Sediments from the Equatorial Atlantic , 2003 .

[42]  A. Kuijpers,et al.  New geological aspects for freshwater seepage and formation in Eckernförde Bay, western Baltic , 2002 .

[43]  A. E. Hoffman,et al.  Equilibrium Constants for a Gas-Condensate System , 1953 .

[44]  M. Schoonen,et al.  Magnetic properties of hydrothermally synthesized greigite (Fe3S4)—I. Rock magnetic parameters at room temperature , 1996 .

[45]  A. Roberts,et al.  Reductive diagenesis,magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: A new view , 2009 .

[46]  D. Heslop,et al.  Rock magnetic identification and geochemical process models of greigite formation in Quaternary marine sediments from the Gulf of Mexico (IODP Hole U1319A) , 2008 .

[47]  G. E. Archie.  The electrical resistivity log as an aid in determining some reservoir characteristics , 1942 .

[48]  J. Bloemendal,et al.  Rock magnetism of Late Neogene and Pleistocene deep‐sea sediments: Relationship to sediment source, diagenetic processes, and sediment lithology , 1992 .

[49]  W. Moore.  Large groundwater inputs to coastal waters revealed by 226Ra enrichments , 1996, Nature.

[50]  F. Vilas,et al.  Early diagenesis of magnetic minerals in marine transitional environments: geochemical signatures of hydrodynamic forcing , 2005 .

[51]  C. Booth,et al.  Use of mineral magnetic concentration data as a particle size proxy: a case study using marine, estuarine and fluvial sediments in the Carmarthen Bay area, South Wales, U.K. , 2005, The Science of the total environment.

[52]  E. Auken,et al.  A comparison of helicopter-borne electromagnetics in frequency- and time-domain at the Cuxhaven valley in Northern Germany , 2009 .

[53]  M. Fuller,et al.  Hysteresis properties of titanomagnetites: Grain-size and compositional dependence , 1977 .

[54]  L. Thomsen,et al.  A new 'schlieren' technique application for fluid flow visualization at cold seep sites , 2004 .

[55]  David J. Dunlop,et al.  Rock Magnetism: Fundamentals and Frontiers , 1997 .

[56]  J. King,et al.  A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments , 1982 .

[57]  M. Tivey,et al.  Crustal magnetization reveals subsurface structure of Juan de Fuca Ridge hydrothermal vent fields , 2002 .