Regional variations in Mn,Ni, Cu,and Co of ferromanganese nodules from a basin in the Southeast Pacific

Deep-sea nodules from the Southeast Pacific (northern sector of the Nazca Plate) have been studied regarding their metal content, shapes, and sedimentary environment. The area investigated lies in the southern boundary region of the equatorial zone of high biological productivity. The nodules accumulate by two different growth processes: (1) early diagenetic growth by supply from pore water and (2) hydrogenetic growth by supply from near-bottom sea-water. These growth processes lead to different genetic types of nodules. Considering the $\text{Mn}\text{Fe}$ ratio, the examined basin can clearly be divided in three ferromanganese facies. The $\text{Mn}\text{Fe}$ quotient increases generally from south to north, whereas Ni and Cu concentrations show a distinct maximum in the mid facies at a $\text{Mn}\text{Fe}$ ratio of about 5. Computing the correlation of $\text{Mn}\text{Fe}$ quotient versus (Ni+Cu), hyperbolic regressions show the best coefficients of correlation and separate the quantity of nodules in two groups. The ascending and the descending hyperbola represent two geochemical ranges which are controlled by different accumulation mechanisms.