The oxidation state of manganese in marine sediments and ferromanganese nodules

Abstract

Marine sediments and ferromanganese nodules from the Pacific Ocean have been analyzed for the O Mn ratio of solid manganese. We tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples. We choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences. The ferromanganese nodules that we analyzed were all from MANOP site H and had Mn Fe ratios that ranged from 5.6 to 70. These nodules were invariably highly oxidized with O Mn values ranging from 1.90 to 2.00. Our most precise analyses suggest that less than 1% of the total manganese is present as Mn(II). We also analyzed red clay and hemipelagic sediments from the eastern tropical Pacific (Baja borderland and MANOP site H) and carbonate ooze samples from the equatorial Pacific. These sediments are also highly oxidized ( O Mn = 1.90 to 2.00 ) except when Mn(II) appears in the interstitial water. As dissolved Mn(II) increases the value of the O Mn ratio in the solid phase decreases. The O Mn ratio decreases to values as low as 1.40. This decrease appears to be due to a decrease in oxidized manganese by reduction, however, an increase in reduced manganese in the solid sediments by adsorption or MnCO 3 formation can not be ruled out in all cases.