Marine manganese accretions : nodule-micronodule comparisons among major ocean basins

Abstract

Stable oscillatory reaction waves are proposed as a model for the formation of alternate Mn-rich and Fe-rich laminae in marine manganese accretions. High resolution electron microprobe traverses of coexisting nodules and micronodules from sediments of the Gulf of Mexico, FAMOUS (French American Mid-Ocean Undersea Survey) Area, Hatteras Abyssal Plain and Clarion-Clipperton Fracture Zone reveal an overall lamination pattern with 6 (mu)m spacing. These layers are of variable density, elemental composition and crystal size. Pacific nodules and micronodules have better developed laminae than Atlantic or Gulf of Mexico but chemical variation of similar scale is evident in all nodule crusts and micronodules. Within single micronodules Mn/Fe varies over 2 orders of magnitude (1-175) while Si/Al ranges from 1(TURN)10 . Layers of variable density are evident in both compositional contrast backscattered electron micrographs and in transmission electron micrographs of micronodules. The oscillatory reaction model calls for the precipitation of undifferentiated, non-stoichiometric Mn + Fe oxyhydroxide as the initial phase. Subsequent reordering reduces and remobilizes manganese which diffuses outward. Reprecipitation of a Mn-rich layer occurs when the Mn encounters sufficient dissolved oxygen. The surface created may then become the substrate for more undifferentiated non-stoichiometric oxyhydroxide. Autocatalytic precipitation of Mn-OH may also proceed to exhaust locally available manganese. Successive cycles of this system create a concentric, laminated structure. Seawater and pore fluids serve as the unlimited, dissolved metal sources while nodules and micronodules act as sinks to which the metals are removed. Thus, the manganese-iron oxyhydroxide system satisfies the essential conditions for a stable oscillatory system.