July 1,2002 to June 30, 2003
This document summarizes science results of the first year of the above referenced project between July 2002 through June 2003. This project is one part of a multiinvestigator project entitled the Ring of Fire program led by Dr. Robert Embley of NOAA-PMEL Newport, OR. For our part of the science program, the autonomous underwater vehicle ABE (Autonomous Benthic Explorer) developed here at Woods Hole Oceanographic Institution (WHOI) was used to map the summit of Explorer Ridge (49°46'N 1300 16'W) in the northeast Pacific off the west coast of Canada and United States (Figure 1). Located north of the more well-known Juan de Fuca Ridge, Southern Explorer Ridge (SER) is a medium-rate spreading center that is kno\vn to host a large hydrothermal vent complex known as "Magic Mountain". Previous Jives by Canadian submersible Pisces IV in the 1980's and ROV ROPOS in the 1990's documented the existence of the active "Magic Mountain" vent area and many areas of extinct sulfide chimneys within the rift valley. Magic Mountain was classed then as one of the largest sulfide structures known on the modern seafloor [Tunnicliffe et al., 1986; Scott et al., 1990] and yet it has not been revisited for some time and has not been imaged with modern and state of the art mapping systems available today.
The Mapping Program and the Autonomous Benthic Explorer (ABE)
The mapping program was conducted during a cruise of the research vessel Thomas Thompson that left Seattle on June 28th and returned to Victoria, British Columbia Canada, July 11th. Dr. Robert Embley was chief scientist on the leg. For the highresolution mapping we utilized the autonomous underwater vehicle ABE (Figure 2). ABE is a fully autonomous underwater vehicle that navigates itself around an acoustic transponder net previously deployed on the seafloor. ABE drives along 0n a preplanned dive track and is fully autonomous in that it makes its own decisions in avoiding the seafloor, maintaining its altitude and depth and understanding where it is in the world. ABE carries a number of mapping systems including a mechanically scanned 675 khz pencil-beam sonar (Imagenex), a CTD, optical backscatter, redox potential and a 3 axis fluxgate magnetometer. For this particular project ABE was also equipped with a 200 khz multibeam sonar system, the Simrad SM2000. This latter sensor system provided an order of magnitude improvement in the density of coverage seafloor depth estimates (i.e. pings), which translates into a much finer resolution bathymetric map than previously had been possible.
ABE completed 7 full dives, averaging approx. 18 hours bottom malJping time per dive and mapped an area approx. 7 by 3.5 km along the summit of Southern Explorer Ridge (Figure 3 and 4). Other science programs during the cruise included collecting EM300 ship-based swath bathymetry data over a large area of Explorer Ridge and CTD tow-yos to locate any active hydrothermal activity. The overall goal of the cruise leg and this project was to identify and locate hydrothermal activity on this part of the midocean ridge system and to "rediscover" the Magic Mountain vent field in order to establish its relationship to other vents systems now known to exist further south along the Juan de Fuca Ridge. The specific science goal of this project was to collect high-resolution bathymetric data along with magnetic field data in order to define the hydrothermal systems present at the summit of Explorer Ridge.
High Resolution Magnetic Mapping
The magnetic properties of ocean crust are sensitive to the alteration and thermal environments associated with hydrothermal vent systems [Ade-Hall et al.., 1971; Johnson and Pariso, 1987; Watkins and Paster, 1971]. Hydrothermal alteraL"ton can rapidly destroy the magnetic minerals within the extrusive crust, creating non-magnetic pipe-like upflow zones beneath focused vent sites and demagnetized zones along fluid pathways, such as faults and subsurface permeability zones [e.g. Tivey and Johnson, 2002; Tiveyet al.,1993; 2003]. Likewise, the elevated temperatures beneath vent fields can lead to thermal demagnetization of the magnetic minerals. Thermal demagnetization is potentially a more dynamic response, if for example, vent temperatures change systematically over time. While magnetic studies are inherently non-unique, systematic patterns in the magnetization contrast can provide important insight into the degree of homogeneity or heterogeneity of the subsurface crust. The magnetic measurements can also be collected on a spatial scale that is relevant to the process of fluid flow through the crust ranging from the individual vent chimneys on a scale of meters to the rift valley scale of several hundreds of meters. Magnetic mapping was therefore undertaken at SER to assist in delineating areas of crustal hydrothermal alteration. Previous high resolution magnetic field mapping over the Endeavour Ridge segment of the Juan de Fuca Ridge, just to the south of SER, revealed the presence of tightly constrained magnetization lows (typically 100 m diameter) that correlated spatially with areas of both active and inactive hydrothermal activity [Tivey and Johnson, 2002].
The new magnetic field mapping at SER shows similar relationships, although with some important differences (Figure 5). Magnetization lows are primarily concentrated in the rift valley with a few lows off-axis associated with major faults. The lows appear to be more elongate along the rift valley and broader in plan view (several 100's of meters) compared to Endeavour. Previous SER mapping found extensive hydrothermal deposits along the rift valley although most were relict deposits. Thus, it appears that hydrothermal activity in the rift valley has been extensive, which has influenced the magnetization pattern. Unlike Endeavour Ridge, the active Magic Mountain venting site was found "outside" of the rift valley, adjacent to the east valley wall (Figure 6). This suggests fluid pathways exist through the uplifted wall of the rift valley. The active venting Magic Mountain area and rift valley wall has reduced magnetization, although not as strongly defined as the active vent sites at Endeavour. This suggests that either Magic Mountain is too young to have formed an alteration halo, although ROV ROPOS observations suggest that the vent system is relatively mature, or that the fluid flow and alteration is simply more broadly distributed along the wall. Many relict hydrothermal chimneys were found in the area of the Magic Mountain site suggesting a broad area of upflow and thus the latter hypothesis is preferred.
In addition to magnetization lows, areas of strong magnetization appear to correlate with the many volcanic constructional features seen in this area of Explorer Ridge. This presumably implies greater volume of magnetic material rather than enhanced magnetization, although further work needs to be done to investigate these correlations. These volcanic domes dominate the morphology of the rift axis, but they are also heavily dissected by pervasive faulting (Figure 4). Further work needs to be done to investigate the relationships between the magnetization highs and lows with various seafloor morphological features mapped on the seafloor.
Publications resulting from this project
Tivey, M.A., D. Fornari, H. Schouten, A. Bradley, D. Yoerger, H.P. Johnson, R. Embley, W. Chadwick, T. Shank, S. Hammond, High-resolution magnetic field and bathymetric imaging using autonomous underwater vehicles, remotely operated vehicles and submersibles, EGS-AGU-EUG Spring Meeting, 5, 2858, Abstract EAE03-A02858, Nice, France, May 2003.
Tivey, M.A., R. Embley, W. Chadwick, A. Bradley and D. Yoerger, HighResolution Magnetic Field Mapping Over Explorer Ridge - NOAA Ocean Exploration Program, EOS Trans. ACU, 83 (47), TIIC-1267 (Fell ~1eeting) 2002.
Embley, R.W., E.T. Baker; J. Baross; A.E. Bates; Y.C. Beaudoin; A.M. Bradley; D.A. Butterfield; W.W. Chadwick Jr.; B.L. Cousens; K.M. Gillis; M. Jakuba; K. Juniper; R.J. Leveille; M. Lilley; J.E. Lupton; S.C. Merle; K. Nakamura; A. Metaxas; C.L. Moyer; J.E. Resing; S.D. Scott, M.A. Tivey; V. Tunnicliffe; A. Williams-Jones; D.R. Yoerger, Rediscovery and Exploration of Magic Mountain, Explorer Ridge, NE Pacific, EOS Trans. AGU 83 (47), TIIC-1264 (Fall Meeting) 2002.
Jakuba, M., D. Yoerger, W.W. Chadwick, A. Bradley, and R.W. Embley, Multibeam sonar mapping of the Explorer Ridge with an autonomous underwater vehicle, EOS Trans. AGU, 83 (47), TIIC-1266, 2002.
Jakuba, M., and D. Yoerger, High-resolution Multibeam Sonar Mapping with the Autonomous Benthic Explorer (ABE), Proceedings of the DUST conference, Durham, NH, August 2003.
Ade-Hall, J.M., H.C. Palmer, and T.P. Hubbard, The magnetic and opaque petrological response of basalt to regional hydrothermal alteration, Geophys. J. R. astr. Soc., 24, 137-174, 1971.
Braunmiller, J. and J. Nabelek, Seismotectonics of the Explorer region, J. Geophys Res., 107, 2208, DOl: 10: 1029/2001JB000220, 2002.
Johnson, H.P., and J.E. Pariso, The effects of hydrothermal alteration on the magnetic properties of oceanic crust: Results from drill holes CY-2 and CY-2A, Cyprus Crustal Study Project, in Cyprus Crustal Study Project: Initial Report, Holes CY-2 and CY-2A, edited by P.T. Robinson, I.L. Gibscn, and A. Panayiotou, pp. 283-293, Geological Survey of Canada, 1987.
Scott, S. D., R. L. Chase, M. D. Hannington, P. J. Michael, T. F. McConachy, and G. T. Shea, (1990): Sulphide deposits, tectonics and petrogenesis of Southern ER, Northeast Pacific Ocean, in Ophiolites: Ocean Crustal Analogues; proceedings ofthe symposium "Troodos 87", 1. Malpas, E.M. Moores, A. Panayiotou, C. Xenophontos (eds), 719-733.
Tivey, M.A., P.A. Rona, and H. Schouten, Reduced crustal magnetization beneath the active sulfide mound, TAG hydrothermal field, Mid-Atlantic Ridge 26°N, Earth Planet. Sci. Lett., 115, 101-115, 1993.
Tivey, M.A. and H.P. Johnson, Crustal magnetization reveals subsurface structure of Juan de Fuca Ridge hydrothermal fields, Geology, 30, 979-982, 2002.
Tunnicliffe, V., M. Botros, M. E. de Burgh, A. Dinet, H. P. Johnson, S. K. Juniper, and R. E. McDuff, Hydrothermal vents of ER, northeast Pacific, Deep-Sea Res., 33,401-412, 1986.
Watkins, N.D., and T.P. Paster, The magnetic properties of igneous rocks from the ocean floor, Phil. Trans. R. Soc. Lond., Ser. A, 268, 507-550, 1971.