Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 143 



Induced Polarization for Core Analysis 



Another interesting possibility now being in- 

 vestigated is to use IP at sea to assay full-length 

 vibracore samples. Many techniques can assist ge- 

 ologists and mineral prospectors in identifying 

 promising areas for mineral accumulations. Never- 

 theless, to determine precisely what minerals are 

 present and in what quantities, it is still necessary 

 to do laborious, expensive site-specific coring. More- 

 over, once a core is obtained, it often takes many 

 hours to analyze its constituents, and much of this 

 work must be done in shore-based laboratories. 



To explore a prospective offshore mine site 

 thoroughly, hundreds or even thousands of core 

 samples would be needed. Geologists need analyti- 

 cal methods that would enable them to quickly iden- 

 tify and characterize deposits. USGS researchers 

 have begun to insert IP electrodes into unopened 

 vibracores to determine the identity and propor- 

 tion of polarizable minerals present. Such a pro- 

 cedure can be done in about 20 minutes and can 

 therefore save considerable time and expense. If 

 the analysis showed interesting results, the ship 

 could immediately proceed with more detailed cor- 

 ing (shore-based analysis of cores precludes revisit- 

 ing promising sites on the same voyage). 



Geochemical Techniques 

 Water Sampling 



Measurement of geochemical properties of the 

 water column is a useful exploration method for 

 detecting sulfide-bearing hydrothermal discharges 

 at active ridge crests. ^^ Some techniques have been 

 developed for detecting geochemical anomalies in 

 the water column 500 kilometers (310 miles) or 

 more from active vent sites. Used in combination 

 with geophysical and geological methods, these 

 techniques help researchers "zero in" on hydro- 

 thermal discharges. Other geochemical methods are 

 used to sense water column properties in the im- 

 mediate vicinity of active vent sites. 



Reconnaissance techniques include water sam- 

 pling for particulate metals, elevated values of dis- 

 solved manganese, and the helium-3 isotope. Iron 

 and manganese adsorbed on weak acid-soluble par- 



ticulate matter have been detected 750 kilometers 

 (465 miles) from the vent from which they were is- 

 sued. Total dissolvable manganese is detectable sev- 

 eral tens of kilometers from active hydrothermal 

 sources. Methane, which is discharged as a dis- 

 solved gas from active vent systems, can be detected 

 on the order of several kilometers from a vent site.''^ 

 Analysis of water samples for methane has the 

 advantage that it can be done aboard ship in less 

 than an hour. Analysis for total dissolvable man- 

 ganese requires about 10 hours of shipboard time. 



At a distance of 1 kilometer or less from an ac- 

 tive vent, the radon-222 isotope and dissolved me- 

 tals also may be detected. The radon isotope pro- 

 duced by uranium series decay in basalt, reaches 

 the seafloor through hydrothermal circulation and 

 can be sampled close to an active vent. Helium-3 

 derived from degassing of the mantle beneath 

 oceanic crust and entrained in subseafloor hydro- 

 thermal convection systems may be detectable in 

 the vicinity of active vents. Other near-field water 

 column measurements which may provide evidence 

 of the proximity of active vents include measure- 

 ments of light scattering due to suspended partic- 

 ulate matter, temperature, thermal conductivity, 

 and salinity. Light scattering and temperature ob- 

 servations proved to be very useful in identifying 

 hydrothermal plumes along the southern Juan de 

 Fuca Ridge.'''' 



Geochemical properties of the water column are 

 measured using both deep-towed instrument pack- 

 ages and "on-station" sampling techniques. For 

 example, NOAA's deep-towed instrumented sled, 

 SLEUTH has been used to systematically survey 

 portions of the Juan de Fuca Ridge. Measurements 

 made by SLEUTH sensors over the ridge crest 

 were supplemented by on-station measurements up 

 to 100 kilometers off the ridge axis.^^ Similar sur- 

 veys have been made over the Mid-Adantic Ridge^' 

 and elsewhere. The sensitivity and precision of in- 

 struments used to acquire geochemical information 

 continues to improve. Perhaps as importantly. 



''Rona, "Exploration for Hydrothermal Mineral Deposits," pp. 

 7-37. 



"Ibid. 



'■'E.T. Baker, J.W. Lavelle, and G.J. Massoth, "Hydrothermal 

 Particle Plumes Over the Southern Juan de Fuca Ridge," Nature, 

 vol. 316, July 25, 1985, p. 342. 



«»Ibid. 



"P. A. Rona, G. Klinkhammer, T.A. Nelsen, J.H. Trefey, and 

 H. Elderfield, "Black Smokers, Massive Sulfides, and Vent Biota at 

 the Mid-Atlantic Ridge," Nature, vol. 321, May 1, 1986, p. 33. 



