Modern deposition of manganese along the Dead Sea shore

Manganese-rich encrustations along the modern shoreline of the Dead Sea are of three kinds: (a) Black laminae, ranging in thickness from 0.1 mm to 4 mm, alternating with aragonite layers which form continous hard crusts on pebbles and boulders (b) Thin black laminae in laminated muds found in very shallow waters. (c) Thin black laminae, alternating with aragonite and gypsum, surrounding partially submerged tree trunks.

No identifiable X-ray diffraction pattern was obtained in the black layers. SEM studies revealed discrete, shapeless manganese-rich aggregates embedded in an aragonitic matrix. The black laminae consist of approximately 75% aragonite, 15% insoluble residue (mostly quartz), and about 10% manganese-rich fraction. Chemical analyses of the isolated manganese-rich fraction gave up to 50% manganese, and less than 1% iron.

Profiles of dissolved manganese in interstitial water show a two-fold enrichment of manganese as compared with the overlying water, with a particularly high concentration (18 mg/l) at the sediment—water interface. This is interpreted to be due to release of manganese from solid phases in the reduced sediments, and upward migration of dissolved manganese in interstitial water, and oxidation to insoluble tetravalent manganese at the interface with the oxygen-containing water.

The manganese-rich crusts were formed below the water table and were deposited no later than 350 years and possibly sooner.