Comprehensive description and chemical composition of ferromanganese crusts collected by USGS and others over the World Ocean

Ferromanganese oxide crusts are members of a family of manganese and iron oxide precipitates that are widely distributed in the lithosphere. They can be defined as encrustations forming largely on hard substrates that are at least periodically kept free from detrital sedimentation by strong currents, steep slopes, or both. In near-shore or hydrothermal environments where metal supply is much greater than it is in most of the open ocean, crusts may also form on soft sediments. The pioneering German "Midpac I" investigation of cobalt-rich ferromanganese oxide phases in the Central Pacific in 1981 lead to the development of interest in the processes causing cobalt enrichment in seamount "nodules" and crusts and the recognition of the possible resources in cobalt rich crusts within 200 miles of U.S. and territorial coasts. Crusts pose specific problems because unlike nodules they tend to occur in a highly localized manner. They also generally adhere to hard substrates in such a fashion that they cannot easily be separated from underlying substrate during recovery; And finally, their elemental composition varies from nearly pure manganese or iron oxides (both frequently hydrothermal in origin) to the fairly consistent admixtures that characterize cobalt-rich crusts. The main effort of this study has been to collect and evaluate all available samples and information, to analyze new samples by means of well-controlled and defined analytical procedures, and to present all information through the end of 1987 in a consistent and complete form. As a part of this effort, all data were classified and normalized for mapping purposes so that information from diverse sources could be compared over larger geographic areas. For resolving the difficult problem of contamination and water treatment a basic procedure was chosen that normalizes data to a constant Fe+Mn content in order to draw ocean wide conclusions on the distribution and chemical properties of ferromanganese crusts. Investigators who wish to use other means of dealing with the comparability problem have the complete basic information in the published database to create their own models.