Claire Pontbriand


My research addresses the inter-related processes of volcanism, faulting, and hydrothermal flow at two different sites on the global Mid-Ocean Ridge (MOR) system.  My primary emphasis will be to use seismological and statistical methods to study hydrothermal circulation at the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge at 26°08’N, which is the largest known active hydrothermal system on the seafloor (Rona, 1984; Humphris, et al., 1995; Tivey, et al., 1995; Becker, et al., 1996; Tivey, et al., 2003; Canales, et al., 2007; deMartin, et al., 2007; Sohn, 2007).  My research seeks to understand subsurface processes associated with fluid flow, and the geometry/extent of the secondary circulation system responsible for anhydrite deposition. I present results from a deployment of five Ocean Bottom Seismometers (OBS) in a small-aperture network (~200 m) around the perimeter of the active mound, which provide the first-ever observations of naturally occurring hydraulic fracturing events within a seafloor massive sulfide deposit.  I have generated a hypocenter catalog for more than 72,000 microearthquakes observed beneath/within the active mound over a 9-month period, and I use these results to constrain the depth and lateral extent of secondary recharge in the hydrothermal system.  I also plan to conduct statistical analyses with the microearthquake catalog and simultaneously acquired records of exit-fluid temperature to constrain spatio-temporal patterns of subsurface flow, and the role of faulting/fracturing in the construction and maintenance of a massive hydrothermal deposit.

My secondary project is focused on understanding the unique modes of volcanic activity associated with ultra-slow spreading on the 85°E segment of the Gakkel Ridge (9 mm yr-1 full rate).  The Gakkel Ridge, which is the slow spreading end-member in the global MOR system, presents significant challenges for deep-sea research owing to its location several kilometers beneath the permanent Arctic ice pack. The 85°E segment is of special interest owing to previous observations of a major earthquake swarm in 1999 (Muller & Willfried, 2000) and a large hydrothermal plume over the segment during the AMORE expedition in 2001 (Edmonds, et al., 2003; Michael, et al., 2003; Baker, et al., 2004). My research uses samples and images acquired from the site with a purpose-built camera and sampling system (CAMPER) during the AGAVE expedition in 2007 to constrain the modes of volcanic accretion at this remote site, including what appears to be ubiquitous explosive activity during the 1999 earthquake swarm. Explosive activity at this site is of special interest owing to the fact that the seafloor depths in the axial valley where volcaniclastic material was observed/sampled are much greater (4000+ m) than the critical point of seawater (~2600 m), which means that magma fragmentation cannot be triggered by lava-seawater interaction (White, et al., 2003). The alternate possibility that fragmentation was triggered by magmatic volatiles is at odds with the widely-held belief that volatile concentrations in mid-ocean ridge basalts (MORBs) are much too low to fragment magmas at deep-sea pressures.  The unique dataset generated during the CAMPER dives at this site in 2007 provides a new perspective on volcanic accretion at ultra-slow spreading ridges, as well as volatile processes in deep-sea eruptions. My research integrates high-definition seafloor digital imagery with high-resolution bathymetric data and analyses of lava samples and volcaniclasts to develop a new model of volcanic accretion at this enigmatic site.