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By Sally Philpot, Siân E. Boyd, Hubert L. Rees, Emma Bee, Carla Houghton, Ceri James, Silvana N. R. Birchenough, S. Limpenny David, A. Coggan1 Roger, Koen Vanstaen

Every year approximately 22 million tonnes of marine sand and gravel are dredged form British waters. The eastern English Channel (EEC) contains substantial reserves, a proportion of which have been identified by the aggregates industry for potential future exploitation. The main aim of this study is to produce broadscale marine habitat maps of the EEC area. The UK aggregates industry has formed a consortium of representatives (the eastern English Channel Association), and have sponsored an initial baseline survey and follow-up sampling programmes as part of a Regional Environmental Assessment (REA) covering a patchwork of aggregate prospecting areas. To complement this effort, a large-scale geophysical survey was conducted in 2005 to place the REA in a wider spatial context and, thereby, to help assess the environmental significance of any future impacts arising from commercial aggregate extraction over the broader region of the EEC.

Sep 24, 2006

By Richard H. T. Garnett

The first, profitable, marine placer mining started with tin 100 years ago but no major industry developed until the mid-20th. Century. Dredging at sea yielded cassiterite in both South Thailand and Indonesia, and remains an important state-run industry in the latter country. Malaysia and Russia have also contributed production, and the coastal waters of Tasmania and England have been explored. Marine placer deposits of gold have been examined in New Zealand, Chile, Argentina, Russia and West Africa. Philippino deposits were dredged during the 1930s. Fifty years later, at Nome in Alaska, gold was recovered by dredging from the offshore, and the technical/economic benefits of a seabed-deployed crawler were first demonstrated. Off the coast of Namibia placer diamonds were recovered by airlift mining during the 1960’s, but De Beers effectively started the existing marine diamond industry in 1989. The Wirth drill and a crawler continue to be used by the company, and plans recently have been announced for operations in South African waters. Other commodities such as mineral sands, platinum group minerals, and chromite have been investigated offshore in several worldwide localities.

Aug 24, 2006

By Peter A. Rona

The recent discovery of the first black smoker-type hydrothermal venting and massive sulfide mineral deposits at a site in the rift valley of the slow-spreading Mid-Atlantic Ridge near latitude 26°N, longitude 45°W (Rona et al., 1986, Nature, 321: 33-37), demonstrates that a complete series of hydrothermal mineral deposit types exists at slow-spreading oceanic ridges (half-rate = 2 cm/y), similar to the series previously known at faster-spreading oceanic ridges (half-rate > 2 cm/y). The largest hydrothermal mineral deposit known at a seafloor spreading center is a stratiform massive sulfide body with bulk dry weight of about 100 x 106 metric tons in the Atlantis II Deep at the slow-spreading axis of the Red Sea. This observation invalidates the concept that size of a hydrothermal deposit is directly proportional to rate of seafloor spreading. Instead, the occurrence, grade and size of a hydrothermal deposit formed at a seafloor spreading center is controlled by anomalous chemical and physical conditions which may occur at extremely localized sites at the full range of spreading rates. The basic hydrothermal process involving subseafloor hydrothermal convection driven by magmatic heat sources appears to be similar at slow- and faster-spreading centers. However, differences exist in the periodicity of magmatic cycles that energize hydrothermal circulation (104 y at slow-spreading centers; 103 y at faster-spreading centers); in the distribution of hydrothermal sites along a spreading axis (estimated 100 km along slow-spreading centers; 10 km along faster-spreading centers); and in residence time

Jan 1, 1986

By Mayumy Amparo Cabrera-Ramírez

Polymetallic nodules are a widespread resource in the Pacific Ocean, particularly in the areas of Clarion-Clipperton (Cronan, 2000). The most abundant mineral phases are formed by iron and manganese oxides, with minor elements such as nickel, copper, cobalt, molybdenum and rare earths elements (Cronan, 1977; Cronan and Moorby, 1981; Iyer and Sharma, 1990; Mangini et al. 1990; Banerjee et al. 1991). In the Clarion-Clipperton Zone (CCZ), several authors mention that there are three main mineral species of manganese oxides, which have been recognized and which are associated with different processes of formation: todorokita (Mn2+, Ca, Mg) Mn34+ O7. H2), vernadite dMnO and birnessite enriched in Fe and (Na, Ca) 0.5 (Mn4+, Mn3+) 2O4. 1.5 H2O. In nodules of hydrothermal origin have been found todorokite, birnessite, vernadite, goethite and pyrolusite. Although there are many oxides and hydroxides of manganese and iron in amorphous and microcrystalline phases that make up the bulk of the nodule (Frondel et al. 1960; Cronan 1977; Dymond 1981; Calvert and Piper, 1984; Aplin and Cronan, 1985; Martin and Lallier, 1993; Achurra et al. 2009), identifying specific mineral phases is difficult due to the intergrowth of different phases and associated detrital material.

Jan 1, 2011

By C. W. Devey, K. S. Lackschewitz

The mineralogy and geochemistry of hydrothermally altered wall rocks from different modern basaltic- and felsic-hosted hydrothermal fields at seafloor spreading centers and convergent margins have been studied to determine the characteristics of different types of alteration and to calculate the magnitudes of elemental exchanges between seawater and wall rock.

Aug 24, 2006

By Sang Bum Chi

In order to better predict and manage benthic disturbance by deep seabed manganese nodules mining, we needed to conduct a benthic disturbance experiment at the selected area, which has a similar environmental condition as the preservation reference area. Therefore, we have performed environmental baseline studies from 2003--not only to select the benthic environmental impact testing site but, also to detect any environmental changes before and after mining activity. Since then, a large dataset of various parameters ranging from physical (e.g. temperature, salinity, current, weather), chemical (e.g. dissolved oxygen, nutrients, dissolved and particulate organic carbon), biological (e.g. bacteria, plankton, benthos), geological (e.g. Mn-nodule abundance, sediment shear strength) and others (e.g. bathymetric profile and slope, mass flux) have been accumulated. Using this dataset, descriptive statistics and cross-correlations were performed to select the benthic environmental impact testing site. Outcome of this analysis suggests that the potential benthic impact testing site would be a rectangular-shape area (10 km × 10 km) and located at 10°27'~10°33'N, 131°53'~131°58'W, which is latitudinally the same as the preservation reference area.

Jan 1, 2011