Characterization of Seafloor Massive Sulfides Recovered from the Ultraslow-Spreading Gakkel Ridge


DOEI Funded Research: 2008


In the summer of 2001, 26 kilograms of massive sulfide, formed from complex interactions among hot (ca. 350ºC) sulfide- and metal -rich hydrothermal fluid and cold (ca. 2ºC) sulfate-rich seawater, were recovered in a dredge from the ultraslow-spreading Gakkel Ridge at 82º53’N, 6º15’W. Subsequently, a camera tow was carried out, and shimmering water and abundant biological activity was observed in the same area, suggesting that the vent field intersected by the dredge may still be active (Edmonds et al., 2003). This was the first major discovery of seafloor sulfides on an ultraslow-spreading ridge. Two things make it particularly note-worthy. First, it was thought that hydrothermal activity along a ridge with such a slow spreading rate (~6 to 13 mm/yr full spreading rate) would be rare. Other data collected on the cruise, however, challenged that assumption. As part of the program, Miniature Autonomous Plume Recorders (MAPRs), which record, as a function of depth, temperature and light scattering (as an indicator of suspended particles), were used to conduct a reconnaissance of possible hydrothermal plumes present along the ridge. The results indicate the presence of at least 9 to 12 discrete areas of active venting, and thus a much higher than predicted frequency than that predicted for the spreading rate (Edmonds et al., 2003). There is thus considerable interest about the nature of the hydrothermal activity along the Gakkel Ridge, and with how it compares to venting along faster spreading ridges. The second note-worthy aspect of the recovery of massive sulfide, however, is that it was from a ridge located in the high Arctic, with perennial sea ice cover, and thus from an area that is extremely difficult to access.

The goal of this study is to fully characterize the Gakkel Ridge massive sulfides. Given the difficult access, and the apparent abundance of hydrothermal activity, it is important to learn all that we can from the material recovered to date. Sample types recovered in the dredge include copper-rich massive sulfide, iron-rich massive sulfide, iron-oxide crusts, and fine grained zinc-rich massive sulfide. Petrographic studies will be done to estimate, based on mineral textures, the conditions (temperature, pressure, oxygen and sulfur fugacity) at which the deposits formed. Geochemical and mineralogical studies will be done to allow comparison of the Gakkel Ridge sulfides to sulfide deposits from known seafloor vent fields on faster spreading ridges. These data will provide much needed information about at least one type of mature hydrothermal system on an ultraslow-spreading ridge. Based on past studies of seafloor massive sulfide deposits, the data should provide us with insight into the composition of the fluids that formed the deposits, the styles of mixing that occurred as it was forming, and the maturity of the deposit, which will provide information about the duration of fluid flow. Seafloor hydrothermal activity transfers significant energy and mass from Earth’s interior to the oceans, and affects the physical and chemical properties of ocean crust, ocean chemistry, the formation of metal-rich sulfide deposits, and biological activity in the deep ocean. It is thus important to understand its impacts along all types of spreading ridges.