SEISMIC IMAGES OF SEAWATER: FROM THE MANTLE TO THE OCEANS

Water is arguably the most important compound on Earth – it supports life, regulates our climate, and profoundly influences processes from weathering to the behavior of subduction zone earthquakes. In the past decade, several developments in active-source seismology have offered new approaches to illuminating the role of seawater in processes on our planet. Here I highlight two of those advances, both triggered by papers published in 2003 (Ranero et al., Nature; Holbrook et al., Science): the use of reflection and refraction seismology to constrain the amount of water bound in hydrous minerals in oceanic lithosphere, and the discovery that marine seismic reflection data can image thermohaline fine-structure within the ocean. Seismic images of the crust and upper mantle of oceanic lithosphere just outboard of some subduction trenches show reflections interpreted as penetrative bending-related normal faults, in association with suppressed upper mantle P-wave velocities indicative of pervasive serpentinization (up to 25% in places). Seawater apparently uses the normal faults as pathways to the upper mantle, where it causes serpentinization. The chemically bound water then enters the subduction zone with the downgoing plate, where it ultimately dewaters and rises to influence processes from plate boundary friction to decompression melting beneath the arc. The second development, “seismic oceanography,” produces surprisingly clear images of ocean structure from reflections returned from subtle temperature boundaries within the water column. This has opened up a new avenue for visualizing ocean processes including fronts, eddies, and ocean mixing, which are important players in the Earth’s climate engine.