Estimate of metal fluxes into the hydrogenetic ferromanganese crusts of the northwestern Pacific Ocean
|Location||International Geological Congress,oslo 2008|
|Holding Date||21 September 2008|
The recent geological study and exploration of ferromanganese crust deposits have shown wide-spread distribution at depths between about 500 and 6000 meters over inactive submarine volcanoes, tectonic escarpments, trench slopes, and abandoned rifts over the norhtwestern Pacific.
The earlier fine-scale berylium isotpe dating undertaken on 5-10 cm thick hydrogenetic crusts from different geological environments indicated that the crusts have been growning at rather constant rates of 4-7 mm/m.y. generally without significant time breaks (Usui et al., 2007). These uniform and constant growth rates suggest that the marginal basins around the Philippine Sea plate have been exposed to oxygenated bottom waters constantly since the middle Miocene or earlier. The growth rates of these crusts in the northwestern Pacific marginal basins are markedly slower than those on the seamounts over the Pacific plate.
Based on chemical composition, specific gravity, and isotopic age, we calculated the accumulation rates of metal elements into ferromanganese crusts. The chemical composition was analyzed with ICP/ES, the water content was determined by weight loss after dehydration, and the growth rate were measured with Be-10 profiles at few-millimeter intervals. The flux of manganese for the youngest part (during the Quaternary) of the crusts ranges from 0.05 to 0.1 mg/cm2/kyr that is more than one order of magnitude smaller than the total flux over the northern Pacifc sediments ranging 1-3 mg/cm2/kyr (Chester, 2003). The mode of variation in the metal flux was compared with areas, water depths, and growth periods.
The manganese flux shows the smallest variation among the analyzed samples, while the iron and aluminum fluxes show the largest variations which indicates probably terrigenous source diluting major elements manganese. The cobalt flux of the youngest part is again similar among samples, while its secular variation is quite variable among other elements. The variable cobalt flux is probably related with redox condition or supply of bottom waters that promote deposition of cobalt with manganese oxide. This preliminary description suggested that the areal and secular variations of metal fluxes can be potential tools for as paleoceanographic records.
Chester, R. (2003) Marine Geochemistry, Blackwell Publ. Usui, A., Graham, I., Ditchburn, R.G., Zondervan, A, Hishida, H., and Shibasaki, H., (2007) Island arc 16 (3), p. 420-430.