Biogenic Sediments

Biogenic sediments, which are defined as containing at least 30% skeletal remains of marine organisms, cover approximately 62% of the deep ocean floor. Clay minerals make up most of the non-biogenic constituents of these sediments. While a vast array of plants and animals contribute to the organic matter that accumulates in marine sediments, a relatively limited group of organisms contribute significantly to the production of biogenic deep-sea sediments, which are either calcareous or siliceous oozes.

Distributions and accumulation rates of biogenic oozes in oceanic sediments depend on three major factors:

The abundances and distributions of the organisms that produce biogenic sediments depend upon such environmental factors as nutrient supplies and temperature in the oceanic waters in which the organisms live. Dissolution rates are dependent upon the chemistry of the deep ocean waters through which the skeletal remains settle and of the bottom and interstitial waters in contact with the remains as they accumulate and are buried. The chemistry of deep-sea waters, is, in turn, influenced by the rate of supply of both skeletal and organic remains of organisms from surface waters. It is also heavily dependent upon the rates of deep ocean circulation and the length of time that the bottom water has been accumulating CO2 and other byproducts of biotic activities.

Carbonate Oozes

Production

Most carbonate or calcareous oozes are produced by the two different groups of organisms. The major constituents of nanofossil or coccolith ooze are tiny (less than 10 microns) calcareous plates produced by phytoplankton of the marine algal group, the Coccolithophoridae or by an extinct group called discoasters. Foraminiferal ooze is dominated by the tests (shells) of planktic protists belonging to the Foraminiferida. Most foraminiferal tests are sand-sized (>61 mm in diameter), so many foraminiferal oozes are bimodal in particle-size distribution, because they are made up of sand-sized foraminiferal tests and mud-sized coccolith plates.

Discoasters, coccoliths and foraminiferal tests are all made of the mineral calcite. Pteropod ooze is produced by the accumulation of shells of pteropods and heteropods, which are small planktic mollusks. As these shells are composed of the mineral aragonite, pteropod oozes are more easily dissolved, so are restricted to relatively shallow depths (less than 3,000 m) in tropical areas.

Carbonate oozes are the most widespread shell deposits on earth. Nearly half the pelagic sediment in the world's oceans is carbonate ooze . Furthermore, foraminifera and coccolithophorids have been major producers of pelagic sediment for the past 200 million years. As a result, these are arguably among the most important and scientifically useful organisms on Earth. Because their larger size makes them easier to identify and work with, this is particularly true for the foraminifera. Their fossils provide the single most important record of Earth history over the past 200 million years. That history is recorded not only by the evolution of species and higher taxa through that time, but is also preserved in the chemistry of the fossils themselves. The field of Paleoceanography owns much of its existence to biostratigraphy, isotope stratigraphy and paleoenvironmental analyses that utilize fossil foraminifera.

The distributions and abundances of living planktic foraminifera and coccolithophorids in the upper few hundred meters of the ocean depends in large part on nutrient supply and temperature. Coccolithophorids, because they are marine algae, require sunlight and inorganic nutrients (fixed N, P, and trace nutrients) for growth. However, most coccolithophorid species grow well with very limited supplies of nutrients and do not compete effectively with diatoms and dinoflagellates when nutrients are plentiful. Furthermore, both high nutrient supplies and cold temperatures inhibit calcium carbonate production to some degree. For these reasons, diversities (number of different kinds) of coccolithophorids are high and production rates of coccoliths are moderate even in the most nutrient-poor regions of the subtropical oceans, the subtropical gyres. Production of coccoliths is higher in equatorial upwelling zones and often along continental margins and in temperate latitudes where nutrient supplies are higher, though diversities decline. In very high nutrient areas, such as upwelling zones in the eastern tropical oceans (i.e., meridional upwelling), polar divergences and near river mouths, production of coccoliths is minimal.

Even though planktic foraminifera are protozoans rather than algae, their distributions, diversities, and carbonate productivity are quite similar to those of coccolithophorids. Many planktic foraminifera, especially the spinose species that live in the upper 100 m of temperate to tropical oceans host dinoflagellate symbionts which aid the foraminifera by providing energy and enhancing calcification. Having algal symbionts is highly advantageous in oceanic waters where inorganic nutrients and food are scarce, so a diverse assemblage of planktic foraminifera thrives along with the coccolithophorids in the nutrient-poor subtropical gyres. Greater abundances of fewer species thrive in equatorial upwelling zones and along continental margins, so rates of carbonate shell production are higher. And similar to coccolithophorids, few planktic foraminifera live in very high nutrient areas, such as upwelling zones in the eastern tropical oceans, polar divergences and near river mouths, so production of carbonate sediments is minimal in these areas. Finally, planktic foraminifera require deep oceanic waters to complete their life cycles, which they cannot do in neretic waters over continental shelves.

Cool temperatures work together with higher nutrient supplies to reduce diversities of coccolithophorids and planktic foraminifera, and ultimately to shift the ecological community to organisms that do not produce carbonate sediments. A 10o C drop in temperature is physiologically similar to doubling nutrient supply, which is why the pelagic community in an equatorial upwelling zone resembles that of a temperate oceanic region, while the pelagic community of an intensive meridional upwelling zone resembles subpolar to polar communities.

If surface production was the only factor controlling accumulation rates of carbonate oozes, deep-sea sediment patterns would be quite simple. Carbonate oozes would cover the seafloor everywhere except

Rates of accumulation would be on the order of 3-5 cm/1000 years in the open ocean and 10-20 cm/year beneath equatorial upwelling zones and along most continental margins.

Dissolution

Over much of the ocean floor, carbonate accumulation rates are controlled more by dissolution in bottom waters than by production in surface waters. Dissolution of calcium carbonate in seawater is influenced by three major factors: temperature, pressure and partial pressure of carbon dioxide (CO2). The easiest way to understand calcium carbonate (CaCO3) dissolution is to recognize that it is controlled, in large part, by the solubility of CO2:

CaCO3 + H20 + CO2<====> Ca++ + 2HCO3-

The more CO2 that can be held in solution, the more CaCO3 that will dissolve. Since more CO2 can be held in solution at higher pressures and cooler temperatures, CaCO3 is more soluble in the deep ocean than in surface waters. Finally, as CO2 is added to the water, more CaCO3 can dissolve. The result is that, as more CO2 is added to deep ocean water by the respiration of organisms, the more corrosive the bottom water becomes to calcareous shells.

The rain of organic matter from surface waters through time increases the partial pressure of CO2 in bottom water, so the longer the bottom water has been out of contact with the surface, the higher its partial pressure of CO2. Beneath high-nutrient surface waters, primary production exceeds what is utilized in the surface mixed layer. Excess organic matter falling through the water column accumulates on the bottom, where organisms feed upon it and oxidize it to CO2.

The depth at which surface production of CaCO3 equals dissolution is called the calcium carbonate compensation depth (CCD). Above this depth, carbonate oozes can accumulate, below the CCD only terrigenous sediments, oceanic clays, or siliceous oozes can accumulate. The calcium carbonate compensation depth beneath the temperate and tropical Atlantic is approximately 5,000 m deep, while in the Pacific, it is shallower, about 4,200-4,500 m, except beneath the equatorial upwelling zone, where the CCD is about 5,000 m. The CCD in the Indian Ocean is intermediate between the Atlantic and the Pacific. The CCD is relatively shallow in high latitudes.

Surface waters of the ocean tend to be saturated with respect to CaCO3; low latitude surface waters are usually supersaturated. At shallow to intermediate seafloor depths (less than 3000 m), foraminiferal tests and coccolith plates tend to be well preserved in bottom sediments. However, at depths approaching the CCD, preservation declines as smaller and more fragile foraminiferal tests show signs of dissolution. The boundary zone between well preserved and poorly preserved foraminiferal assemblages is known as the lysocline.

The preservation potential of the various kinds of carbonate shells and skeletons differs. Pteropod shells are aragonite, a less stable form of CaCO3. Pteropod shells dissolve at depths greater than 3,000 m in the Atlantic Ocean and below a few hundred meters in the Pacific. Calcitic planktic foraminiferal tests, especially small tests of juvenile spinose foraminifera, dissolve more readily than coccoliths, which are also made of calcite. Pelagic sediments from relatively shallow depths in low latitudes are often dominated by pteropods shells, at intermediate depths by foraminiferal tests, below the lysocline and above the CCD by coccoliths, and below the CCD by red clays.

: Regional changes in the depths of the lysocline and CCD result, in part, from changes in CO2 content of bottom waters as they "age". In modern oceans, deep ocean circulation is driven by formation of bottom waters during the freezing of sea ice. Seawater, due to its salt content, can cool below -1o C before ice begins to form. When sea ice forms, the salt is excluded and is left behind in the seawater. Water in the vicinity of the freezing sea ice becomes more saline and therefore more dense. As a result, large-scale sea ice formation creates very dense water masses that sink to the bottom of the ocean to form deep bottom water. During the Antarctic winter, the freezing of sea ice in the Weddell Sea produces Antarctic Bottom Water (AABW), which sinks to the sea bottom and spreads northward into the South Atlantic. During the Arctic winter, sea ice formation in the Norwegian and Greenland Seas produce North Atlantic Deep Water (NADW), which sinks to the bottom of the North Atlantic and flows southward. AABW is slightly more dense than NADW, so when they meet, AABW flows beneath NADW. As the NADW and AABW spread eastward into the Indian and Pacific Oceans, they mix to become Deep Pacific Common Water (DPCW). The "youngest" bottom waters are in the Atlantic, the "oldest" are in the North Pacific.

When seawater is at the surface, it equilibrates with the atmosphere with respect to O2 and CO2. From the time a water mass sinks from the surface until it comes back to the surface, respiration by organisms in the water column and on the bottom use up O2 and add CO2. As a result, the longer bottom water is away from the surface, the more corrosive it is to CaCO3.

Carbonate Sedimentation Worldwide

The depth of the CCD and the pattern of carbonate sedimentation in any part of the world's ocean reflects the influences of surface production of organic matter, surface production of carbonates, and the corrosiveness of the bottom water to CaCO3.

Because coccolithophorids and planktic foraminifera thrive in temperate to subtropical oceans where surface nutrient supplies are very limited, these organisms produce a continual rain of CaCO3 to the sea floor. In equatorial upwelling zones, organic productivity is elevated enough to stimulate higher rates of production of calcareous and siliceous skeletal remains, but not enough to export excess organic matter to the deep ocean where its respiration would increase corrosiveness of bottom waters to CaCO3.

In more intensive upwelling zones, especially in the eastern tropical Pacific and the Antarctic divergence, and off major river deltas, high nutrient supplies stimulate high rates of organic productivity by diatoms and dinoflagellates, often to the exclusion of coccolithophorids and planktic foraminifera, which reduces CaCO3 production. At the same time, the rain of organic matter to the ocean floor supports abundant deep-sea life whose respiration adds significantly to the CO2 in bottom waters. The result is substantial shoaling of the lysocline and CCD in these regions. The greater corrosiveness of AABW compared to NADW at approximately the same "age" is caused by upwelling-induced high organic productivity at the Antarctic divergence, which exports excess of organic matter into AABW.

Pelagic sediments in the Atlantic and Indian Oceans are predominantly calcareous oozes. In the Pacific Ocean, where the CCD is deeper, red clays dominate, especially in the North Pacific. Carbonate oozes delineate shallower regions in the south Pacific, including the East Pacific Rise and the complex topography to the southwest.

Siliceous Oozes

Biogenic siliceous oozes have two major and two minor contributors. Silicoflagellates are a minor group of marine algae that also construct opalline silica skeletons. Sponge spicules are also an important biogenic source of opalline silica in neretic waters, but are of minor importance in the deep sea.

Silica is undersaturated throughout most of the world's oceans. As a result, extraction of silica from seawater for production of silica shells or skeletons requires substantial energy. Furthermore, for siliceous sediments to be preserved, they must be deposited in waters close to saturation with respect to silica and they must be buried quickly. Young seawaters that are highly undersaturated with respect to H4SiO4 are far more corrosive to SiO2 than are old seawaters that have been dissolving and accumulating H4SiO4 over hundreds to thousands of years.

Seawaters around volcanic islands and island arcs tend to have higher concentrations of H4SiO4 in solution and therefore are more conducive to silica production in surface waters and silica preservation in sediments. Siliceous sediments are most common beneath upwelling zones and near high latitude island arcs, particularly in the Pacific and Antarctic. More than 75% of all oceanic silica accumulates on the sea floor between the Antarctic convergence and the Antarctic glacial marine sedimentation zone. Accumulation rates of siliceous oozes can reach 4-5 cm/1,000 years in these areas.

Conditions favoring deposition of silica or calcium carbonate are different . Silica solubility increases with decreasing pressure and increasing temperature. Silica is undersaturated in the oceans, but it is less undersaturated in deep water. Carbonate solubility increases with depth, and bottom waters become more undersaturated in calcium carbonate. The patterns of carbonate and silica deposits reflect different processes of formation and preservation, resulting in carbonate oozes that are poor in biogenic silica and vice versa.

The diatoms are extremely important primary producers that benefit physiologically from rich supplies of dissolved inorganic nutrients. Under such conditions, their growth rates far exceed other phytoplankton and they can rapidly produce both organic matter and siliceous sediments. They thrive in areas of intensive upwelling and near terrestrial sources of dissolved nutrients, including silica. Silicoflagellates show similar distributions. On the other hand, because both groups require substantial nutrient resources for growth, they are never abundant where nutrients are scarce, and so are insignificant primary and sediment producers in subtropical gyres. Diatom oozes, which contain more than 30% diatom frustules, are found beneath the Antarctic divergence, off the Aleutian island arc in the far North Pacific, and beneath areas of intensive meridional upwelling such as the eastern tropical Pacific. These oozes contain a significant percentage of radiolarian and silicoflagellate skeletons as well. Diatom-rich muds are common on continental shelves and margins where runoff from land contributes terrigenous muds as well as nutrients that stimulate diatom production.

Radiolaria, being protists, are slightly less dependent on the most nutrient-rich areas of the oceans. They are important contributors to siliceous oozes around the Antarctic, but radiolarian oozes (> 30% radiolarian skeletons) are primarily in the tropical Pacific beneath the equatorial upwelling zone and below the CCD. Above the CCD in this region, the sediments are calcareous with a significant siliceous component.

After burial, most siliceous oozes remain unconsolidated, but a fraction dissolve and reprecipitate as chert beds or nodules. Chert is cryptocrystalline and microcrystalline quartz, which is very hard and impermeable. Chert beds are very difficult to drill, which has frustrated ocean drillers since the early days of the Deep Sea Drilling Project (DSDP). The abundance and widespread distribution of chert beds of Eocene age, discovered by the DSDP, indicate important changes in deep-sea chemistry over the past 50 million years.