Characterization of Seafloor Massive Sulfides Recovered from the Ultraslow-Spreading Gakkel Ridge
DOEI Funded Research: 2008
Abstract
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.