In the Pacific, barite is found in radiolarian oozes beneath the equatorial upwelling zone. In the Atlantic, elevated barite concentrations are found on the mid-ocean ridges in areas of low sedimentation rates and where there is an abundance of ferromanganese or iron oxide from hydrothermal sources.
Glauconite deposits occur from 65o N to 80o N, but are most common on lower latitude outer shelves and slopes from 20-700 m water depth. Glauconite forms from micaceous minerals or muds of high iron content where sedimentation rates are relatively low. Associated sediments are mainly calcareous, with a high proportion of fecal pellets.
The most important phosphatic mineral is microcrystalline carbonate fluorapatite. Phosphatic nodules and crusts typically form along continental shelves, upper continental slopes and on oceanic plateaus beneath upwelling surface waters and where bottom currents limit accumulation of detrital sediments. Typical areas of phosphatic deposition are the continental margins of Peru, Chile, and southwest and northwest Africa. Phosphorite nodules or crusts average 18% phosphate. Conglomerates of phosphatized limestone pebbles and megafossils in a matrix of glauconite may have up to 15% phosphate.
Marine phosphates and phosphorite deposits are also found associated with anoxic sediments. Phosphorite may form by replacement of carbonate by phosphate. Upwelling occurs in the southern Caribbean in the surface waters above the Cariaco Basin, resulting in export of organic matter to bottom sediments. Phosphate precipitation is occurring along the rim of the basin where anoxic water from the trench mix with oxygenated waters from above. Phosphate may also be adsorbed by hydrous iron minerals, aluminum oxides and clay minerals. This accounts for phosphate concentrations of 1-2% in some iron-rich, clay or zeolite sediments in the deep sea.
As seawater percolates into hot, volcanic rocks, seawater sulfate reacts with reduced iron. Where the hot solutions are forcibly expelled from the rocks (vents and fumeroles), metal sulfides precipitate as crusts and chimneys up to several meters high ridges Figure 8.24 . Localized accumulation rates can be a meter per year. Deposits rich in Fe, Mn, Cu, and Zn can occur where there is hydrothermal activity on the sea floor. One of the most spectacular examples of ridge-crest metalliferous deposits was discovered in the Red Sea in 1963. Rather than localized vents, metals are concentrated in deep, brine-filled basins. Manganese micronodules (less than 1 cm in diameter), nodules (1-10 cm in diameter) and crusts or coatings form in sediments or on exposed hard surfaces in the deep sea ridges Figure 8.25 . These oxides are brown-black agglomerations of manganese and iron oxides in fine-grained silicates or iron oxide-rich groundmasses in detrital and biogenic grains. Accessory metals include Ni, Cu, K. Ca, and Co. Elemental distribution patterns within nodules are variable and depend both on the environment of deposition and the nature of the mineral phases they contain. Where redox potential is lower, nodules are more iron rich; in well-oxidized deep-sea settings, nodules are richer in Mn.
A nodule commonly forms around a nucleus such as a shark's tooth or volcanic fragment. Nodules grow in concentric layers that may represent changes in seawater composition during growth. Rates of nodule growth are 1-4 mm/106 years. They commonly occur where sedimentation rates are less than 5 mm/1000 years. Apparently, sporadic movement by benthic organisms burrowing through the sediments is sufficient to keep most nodules at the sediment surface, where they can grow. The greatest area of manganese nodule development occurs in the Pacific, where 75% of the equatorial and North Pacific deep sea floor is covered with nodule patches Figure 8.26 . Fields of nodules develop in areas swept clean of fine detrital sediments by bottom currents. Where nodules cover 100% of the sediment surface, the area is called a manganese nodule pavement. In some cases, nodules join to form a solid surface. Such pavements are found on deep plateaus including the Blake Plateau in the western North Atlantic and the Agulas Plateau south of South Africa.
The manganese comes from terrestrial sources by wind and water transport. In the water column, plankton extract manganese from solution, then carry it to the bottom. Manganese is also scavenged from seawater and deposited on the bottom by organic aggregates. Local deep-water sources of manganese may be interstitial waters leaching sediments rich in Mn and Fe near basaltic rocks. Near mid-ocean ridges, nodules may derive their Fe, Mn and accessory minerals from volcanic sources, as noted above.
: Although there is economic interest in both metalliferous sulfide deposits and in manganese nodules, the costs of mining currently exceed the value of the minerals.
When the organic matter reaches the sea floor, it provides food for benthic filter-feeding and detritus feeding organisms, reducing the concentration of POC accumulating in the sediments relative to what reaches the sea floor. In the Panama Basin, which is an upwelling area, depth-stratified sediment trap studies indicate that approximately 5% of the particulate matter reaching the bottom are POC, yet TOC concentrations in the sediments are less than 2%. Utilizable organic matter is known as labile organic matter. The least degradable materials, which often include terrestrial cellulose brought to the deep ocean in gravity flows, are called refractory solid organic matter. In typical pelagic sediments, TOC concentrations are less than 1%.
Most organic carbon in sediments accumulates under conditions of high primary productivity in surface waters and low oxygen in bottom waters or interstitial pore waters. As a result of coastal upwelling and runoff from land that provide nutrients to phytoplankton communities in surface waters, combined with relatively rapid sedimentation rates in these regions, roughly 50% of all organic carbon burial occurs on continental shelves and margins.
Organic-rich sediments that accumulate where bottom waters are depleted of oxygen (anoxic) are called sapropels. Anoxic conditions develop either because of rapid influx of POC or because of stagnation of bottom waters. Though limited in extent in modern oceans, sapropels occur in a variety of settings, including semi-isolated basins with restricted bottom circulation and portions of continental margins or slopes that lie within the mid-water oxygen minimum zone and below upwelling zones.
Late Quaternary deep-water sediments in the Black Sea provide an example of restricted bottom circulation under which sapropels (ooze or sludge rich in organic matter) formed. From 23,000 to about 9,000 years ago, when sea level was 40 m or more lower than today, the Black Sea was completely isolated from the Mediterranean and was a large, freshwater lake which was aerobic thoughout. As sea level rose following the last glacial advance, seawater began to occasionally spill over the Bosphorus Sill into the Black Sea, filling the deeper parts of the basin with dense seawater. However, river runoff into the Black Sea kept surface waters fresh. Because of higher evaporation rates in the Mediterranean, most of the flow of water through the Bosphorus was freshwater from the Black Sea to the Mediterranean. The seawater filling the basin of the Black Sea was isolated from air beneath a layer of low density fresh water. Primary productivity in the surface waters rained organic matter into the deep waters, depleting all oxygen, so that by 7,000 years ago, anoxic conditions were fully developed. About 3,000 years ago, two-way circulation developed with the Mediterranean, driving turnover of the deep waters of the Black Sea and allowing deep sea marine faunas to become established.
Examples of modern sapropel formation within the oxygen minimum zone beneath upwelling high productivity surface waters can be found on the continental slope of the Arabian Peninsula and in the California borderlands. Upwelling in the northwest Indian Ocean provides sufficient surface productivity to provide an excess of organic matter to sediments on the continental slope of the Arabian Peninsula where the oxygen minimum zone intersects the slope. Off California, the combined effects of sluggish circulation in semi-isolated basins, continental margin depths within the oxygen minimum zone, and high surface water productivity all contribute to accumulation of laminated, organic-rich sediments in the Santa Barbara basin.
Anoxic sediments have been widespread in the past and are of great economic importance as source rocks for hydrocarbon deposits. Expansion and intensification of the oceanic oxygen minimum zone, probably during times of reduced thermohaline circulation, is one mechanism that seems to account for many sapropels. Deep basins connected only by shallow connections, which resulted in restricted bottom circulation, were especially common during early stages of continental rifting that formed the Atlantic basins Figure 8.27.