Transport and release of transition elements during early diagenesis: Sequential leaching of sediments from MANOP Sites M and H. Part I. pH 5 acetic acid leach

Sediments from MANOP sites M and H in the eastern tropical Pacific Ocean can be partitioned into operationally defined phases by means of a sequential leach procedure. This paper reports the results of the first leach in the sequence, an acetic acid solution buffered as p H 5 with sodium acetate. This leach is designed to remove carbonate-bound and sorbed cations. The only cation bound in a consistent ratio to calcium in the carbonate is strontium. The molar ratio is 2 × 10^?3. In contrast, transition metals are sorbed onto the surfaces of other sedimentary particles. The proportions sorbed range from 2 to 10% of the total manganese, about 10% of the total nickel and copper, and less than 1% of the total iron. The pool of sorbed metals in surface sediments is sufficiently large and the rate of biological stirring is sufficiently rapid for this metal reservoir potentially to dominate the porewater reservoir in supplying metals to ferromanganese nodules. A simple model for nodule growth based upon transfer of metals through this reservoir suggests that only 1% of the sedimentary sorbed metals within a radius of 2 to 9 cm is required to support typical nodule growth rates.