Particulate concentrations and vertical fluxes of sedimentary material and chemical elements, composition of particulate and sinking material in the water column of the Bear Island Trough, Norwegian Sea, July 1995

CTD and nephelometric sounding data are considered along with parameters of the near-bottom currents and particulate fluxes measured by a subsurface mooring station in the northern part of the Bear Island Trough. It is shown that the near-bottom current is characterized by highly variable parameters, while distribution of suspended particulate matter demonstrates surface and bottom maxima. Horizontal and vertical fluxes of sedimentary material in the nepheloid layer are studied.

Observations and rare earth element composition of manganese nodules during Atlantis cruise AT266, Blake Plateau

An area of about 22,000 km² on the northern Blake Plateau, off the coast of South Carolina, contains an estimated 2 billion metric tons of phosphorite concretions, and about 1.2 billion metric tons of mixed ferromanganese-phosphorite pavement. Other offshore phosphorites occur between the Blake Plateau and known continental deposits, buried under variable thicknesses of sediments. The phosphorite resembles other marine phosphorites in composition, consisting primarily of carbonate-fluorapatite, some calcite, minor quartz and other minerals. The apatite is optically pseudo-isotropic and contains about 6% [CO3]**2- replacing [PO4]**3- in its structure. JOIDES drillings and other evidence show that the phosphorite is a lag deposit derived from Miocene strata correlatable with phosphatic Middle Tertiary sediments on the continent. It has undergone variable cycles of erosion, reworking, partial dissolution and reprecipitation. Its present form varies from phosphatized carbonate debris, loose pellets, and pebbles, to continuous pavements, plates, and conglomeratic boulders weighing hundreds of kilograms. No primary phosphatization is currently taking place on the Blake Plateau. The primary phosphate-depositing environment involved reducing conditions and required at least temporary absence of the powerful Gulf Stream current that now sweeps the bottom of the Blake Plateau and has eroded away the bulk of the Hawthorne-equivalent sediments with which the phosphorites were once associated.

(Table 1) Mineral phosphorus contents in bottom sediments and interstitial waters of the Southeast Atlantic

Concentrations of mineral phosphorus in interstitial waters from sediments of the Southeast Atlantic generally increases from the ocean bed to the continental slope and shelf. In diatomaceous oozes of the Southwest Africa shelf, phosphorus concentration in fresh interstitial waters reaches 2.5 mg/l in absence of phosphorite concretions and 0.1-0.7 mg/l in their presence. After prolonged storage of samples concentration of dissolved mineral phosphorus sometimes increases up to 7-8 mg/l. The key factor regulating phosphorus content of solid and liquid phases of unaltered sediments are content and composition of organic matter.

(Figure 1 and 2) Phosphorus pools in the investigated sediments quantified by SEDEX sequential extraction and distribution of recovered 33P spike between sedimentary P pools after incubation

Phosphorus is an essential nutrient for life. In the ocean, phosphorus burial regulates marine primary production**1, 2. Phosphorus is removed from the ocean by sedimentation of organic matter, and the subsequent conversion of organic phosphorus to phosphate minerals such as apatite, and ultimately phosphorite deposits**3, 4. Bacteria are thought to mediate these processes**5, but the mechanism of sequestration has remained unclear. Here, we present results from laboratory incubations in which we labelled organic-rich sediments from the Benguela upwelling system, Namibia, with a 33P-radiotracer, and tracked the fate of the phosphorus. We show that under both anoxic and oxic conditions, large sulphide-oxidizing bacteria accumulate 33P in their cells, and catalyse the nearly instantaneous conversion of phosphate to apatite. Apatite formation was greatest under anoxic conditions. Nutrient analyses of Namibian upwelling waters and sediments suggest that the rate of phosphate-to-apatite conversion beneath anoxic bottom waters exceeds the rate of phosphorus release during organic matter mineralization in the upper sediment layers. We suggest that bacterial apatite formation is a significant phosphorus sink under anoxic bottom-water conditions. Expanding oxygen minimum zones are projected in simulations of future climate change**6, potentially increasing sequestration of marine phosphate, and restricting marine productivity.

(Table 2, page 1376) Chemical analyses of manganese nodules and crusts from the South-western Pacific

Phosphate deposits which apparently formed during the Miocene climatic optimum are widespread on the Chatham Rise and Camp bell Plateau, and on seamounts in the north Tasman Sea. They formed under oxidising conditions by the phosphatisation of older or contemporaneous foraminiferal oozes (Campbell Plateau and Chatham Rise) and coral limestones (Tasman Sea). The phosphorites of the rise and plateau were formed where current activity was sufficiently strong to prevent normal sedimentation, and now form lag deposits. After the Miocene, phosphorite formation ceased and was followed by manganese oxide deposition where conditions were highly oxidising on the eastern Campbell Plateau and north Tasman seamounts, and by glauconite formation in the much less oxidising environments of the western Campbell Plateau and the Chat ham Rise. The manganese deposits are not volcanogenic, as was formerly thought, but formed by slow precipitation from well oxygenated sea water.