The comparative behavior of elements with close chemical properties is very interesting in the study of natural processes. Mo and W may be an example of such elements. However, the behavior of tungsten in the ocean is studied very poorly. Average weight ratio Mo/W in the crust is about 1:1. For dissolved elements in river waters (Turekian, 1969) this ratio is increased up to 33:1. Oceanic water is even more depleted in tungsten comparatively with its close analog - molybdenum (Sohrin et al., 1987): the ratio Mo/W in seawater is about 940. The aim of this work is to obtain reliable data on W contents in sediments and ferromanganese nodules of Pacific Ocean, and to compare geochemical behavior of Mo and W in the processes of oceanic sedimentation and nodule formation.
Sediments (14 samples) and nodules (11 samples) from North Pacific (traverse from Japanese coast to Mexican coast through Hawaiian islands) were analyzed. Molybdenum and tungsten contents were determined by ICP-MS on the isotopes 95Mo, 97Mo, 183W. Tungsten contents in Fe-Mn nodules range from 7 to 80.7 ppm with average value 49 ppm. Molybdenum contents vary from 32.6 to 777 ppm (average 442 ppm). Our data on tungsten contents are close to those reported by D. Cronan (1976) for Pacific nodules (average W content is about to 60 ppm). Strong correlation Mo-Mn and W-Mn is observed in nodules, that may be evidence that molybdenum and tungsten are connected mainly with manganese oxides. Sediments contain 0.7 - 4.9 ppm of tungsten (2.7 ppm in average) and 1.2 - 11 ppm of molybdenum (for one sample Mo content increases up to 20 ppm; average is 6.9 ppm). The alteration of Mo to W ratio in sedimentary processes is shown in Table 1.
The concentration of tungsten in waters of open ocean is 3 times lower than that in river waters and 4-11 times lower than in coastal waters (Chen and Yang, 1984). Average concentration ratios K = C(nodules) / C(seawater) for Mo and W differ by two orders of magnitude, and are equal to 4.5×104 and 4.7×106 correspondingly. It is probable that during sorption onto Fe and Mn oxyhydroxides tungsten enriches solid phase with greater distribution factor than molybdenum. It is supported by experimental data (Takematsu et al., 1985).
Chen, L. & Yang, S., Haiyang Xuebao. 6, 701-708 (1984).
Cronan, D.S., In Chemical Oceanography (Riley, J.P. & Chester, R., eds.) 5, 217-263 (Academic Press, London, 1976).
Sohrin, Y., Isshiki, K. & Kuwamoto, T., Mar. Chem. 22, 95-103 (1987).
Takematsu, N., Sato, Y., Okabe, S. & Nakayama, E., Geochim. Cosmochim. Acta 49, 2395-2399 (1985).
Turekian, K.K., In Handbook of Geochemistry (Wedepohl, K, ed.) 297-323 (Springer-Verlag, Berlin, 1969).
Table 1. The alteration of molybdenum to tungsten ratio in sedimentary environment.
Earth crust River water Oceanic Particulate matter Fe-Mn Pelagic
water in the ocean nodules sediments
Mo/W ~1 33 940 ? 4.5-12 1.5-4