Rivers are the major source of sediments supplied to the oceans. Muds (silt- and clay-sized sediments) are carried in suspension by moving water and begin to settle out soon after the river water meets the ocean, though finer clay particles remain in suspension for years, allowing them to be conveyed far out into the ocean before settling to the bottom. The dissolved load is also an important contributor to deep-sea sedimentation, for it contains PO4---, NO3-, and other nutrients needed for plant growth, as well as Ca++, HCO3-, and H4SiO4, from which pelagic organisms build their shells and skeletons.
Most of the sediment particles transported by rivers are deposited relatively near their mouths. Thus, by examining a map of the world, one can predict with substantial accuracy where most river-borne sediments are found on continental shelves and margins and in the deep sea. An overview of plate tectonics allows one to further predict the distributions of terrigenous sediments in the deep sea. Submarine canyons along the trailing margins of the North and South Atlantic and Northern Indian Ocean deliver great quantities of terrigenous sediments to deep sea fans and abyssal plains. On the other hand, the deep basins and deep-sea trenches that border much of the Pacific Ocean capture most terrigenous sediments before they reach the deep sea.
Sediment gravity flows occur when sediment is transported under the influence of gravity and sediment motion moves the accompanying interstitial fluid. Sediments are transported by a variety of mechanisms including suspension, saltation, traction, upward granular flow, direct interaction between grains, and the support of grains by a cohesive fluid. There are four main types of sediment gravity flows, in increasing order of importance:
The incredible speed and power of turbidity currents was revealed by submarine cable breaks following an earthquake at Grand Banks, off Nova Scotia, Canada, on November 19, 1929. The quake triggered a turbidity current which progressively broke several telegraph cables over a 13-hour period, as the current traveled down the continental slope and continental rise, and out across the abyssal plain to more than 720 km from its source. On the continental slope, velocity of the turbidity current exceeded 40 km/hr. After the cable break, a turbidite layer up to 1 m thick covered an area of at least 100,000 km2.
Turbidites, which are the distinctive sediment deposits left by turbidity currents, are characterized by graded bedding, moderate sorting and well-developed primary sedimentary structures, as first described by Bouma. Pelagic sediment layers typically lie between individual turbidites. However, because the coarser sands settle out first while the finest muds travel farthest, the texture, sedimentary structures, and thickness of an individual turbidite changes from near the source to its periphery. Proximal turbidites resemble debris flows in that they are massive, with poorly developed sedimentary structures, weak grading, and little interbedded pelagic sediment or terrigenous mud, because the erosive force of the proximal turbidity flow removed previously deposited finer sediments. Classical turbidites, showing complete Bouma sequences Figure 8.8 , are typically intermediate in distance from the source. Distal turbidites, which are most distant from the source area, consist of thin; fine-grained layers that often exhibit well developed cross-lamination.
Submarine canyons are the major conduits for movement of terrigenous sediments from river deltas and continental shelves down the continental margin to the deep sea. Submarine canyons themselves have been cut and sculpted by the erosive power of submarine gravity flows. During glacial advances when sea level was as much as 100 m or more lower, rivers delivered more sediment directly to the continental margins, so submarine canyons undoubtedly transported more sediment and eroded more rapidly.
Grain flows are probably the most common mechanism of downslope transport in submarine canyons and result in massive, relatively well-sorted channel deposits in the deep-sea fans at the mouths of these canyons. Turbidity currents are more sporadic events, but they carry much larger volumes of sediments and spread them far beyond the submarine fans onto the abyssal plains.
Major river deltas on continental margins typically merge downslope into massive abyssal cones, where sedimentation rates can be meters to 10's of meters per 1000 years, depending upon sea level and denudation rates in the source region. The Atlantic has seven major abyssal cones off the St. Lawrence, Hudson, Mississippi, Amazon, Orange, Congo, and Niger Rivers. The largest cones in the world have been built by the Amazon, Ganges-Bramaputra and Mississippi Rivers. The most massive of these is the Bengal Cone, which is 3000 km long, up to 1000 km wide and up to 12 km in thickness. The Bengal Cone is produced by redistribution of sediment from the Ganges and Bramaputra Rivers, whose source waters are in the Himalayas. The present rates of sediment influx into the Bay of Bengal indicate denudation rates in the Himalayas of up to 70 cm per 1000 years.
Abyssal cones generally grade seaward into extensive abyssal plains, which are formed by accumulations of turbidites up to 1 km thick. The vast abyssal plains of the North and South Atlantic Oceans, the Aleutian Abyssal Plain in the northeast Pacific, and others are built of layer upon layer of turbidites, often interbedded with pelagic sediments. Sedimentation events are sporadic, but averaged over time; accumulation rates on abyssal plains may be 10's of cm to more than a meter per 1000 years. Interestingly, abyssal plains are not extensive in the northern Indian Ocean, despite voluminous sediment supplies, because of topographic restriction.
Sediments that drape upper and middle continental slopes around the world are known as hemipelagic sediments. They grade from predominantly terrigenous muds into biogenic oozes. Even where biogenic constituents predominate, hemipelagic sediments typically have a dark color, which is imparted by the terrigenous component. The composition of the terrigenous muds reflects weathering intensity in the sedimentary provenance. The terrigenous muds, which were delivered to the ocean by rivers or by direct runoff from land, remained in suspension and were carried out to the continental margin by surface currents of by sediment-gravity flows.
Accumulation rates of hemipelagic sediments can be quite high, up to 10-30 cm/1000 years. Two factors account for these rates, proximity to terrigenous sediment sources and proximity to terrestrial nutrient sources. Nutrients stimulate biological productivity, including either carbonate or siliceous sediment production.
Clay minerals are aluminum silicates of varying complexities and stabilities. They occur as platy, lath-shaped or needle-like crystals, usually less than 4 µ in diameter. Their most striking property is cohesion, the tendency for constituent particles to stick together. Freshly deposited clay sediments contain much water and resemble cream or are jelly-like. Under pressure, clay sediments loose water and behave plastically, flowing under moderate stresses. Under very high pressure, clay sediments become sedimentary rocks such as shales that contain negligible water and are impermeable to fluids.
Mineralogies of clays often reflect their origin to a substantial degree. There are four major classes of clay minerals in marine sediments; three reflect the relative degree of chemical weathering in the source region, while the fourth indicates volcanic origin.
Clay-sized particles that have been primarily mechanically broken down and transported by ice, wind or very cold water have their cation suites relatively intact, including quite reactive cations such as Fe++. The most common of these unstable clay minerals is chlorite, which is found in high concentrations only at high latitudes where weathering processes are predominantly physical. Only 13% of the clay minerals in the oceans are chlorite Figure 8.9.
Illite is the most common clay mineral, often composing more than 50 percent of the clay-mineral suite in the deep sea. Illites are indicative of mechanical rather than chemical weathering, but are more stable than mica minerals. Illites are characteristic of weathering in temperate climates or in high altitudes in the tropics, and typically reach the ocean via rivers and wind transport.
Kaolinites are recrystallization products of intense chemical weathering, and therefore are mostly found in low latitudes. Kaolinite is common throughout the equatorial Atlantic, but less so in the Pacific for lack of source. Maximum concentrations of kaolinite in deep-sea sediments are found off equatorial West Africa. High concentrations in the eastern Indian Ocean result from wind weathering of extensive "fossil" kaolinite-rich laterites in arid western Australia. These laterites formed under wetter paleoclimatic conditions. Like chlorites, kaolinites make up only about 13% of the clay minerals in the deep sea.
The fourth major group of clay minerals are the montmorillonites or smectites, which are chemical alteration products of volcanic material. Smectites are most common in areas where sedimentation rates are low and volcanic sources are nearby. Source material can be either windblown volcanic ash or volcanic glass on the sea floor. Smectites are most common in the South Pacific where they make up about 50 percent of the clay-mineral suite.
Clays are present in virtually all marine sediments, though their proportions may be minor. In open ocean regions, remote from terrigenous sources, accumulation rates of deep-sea clays are on the order of a few mm per 1000 years. In pelagic sediments where clay minerals are the dominant constituent, sediments are typically bright red to chocolate brown in color and are known as red or brown clays. The color results from coatings of iron oxide on the sediment particles. The red clays were first described and mapped during the Challenger expedition (1872-1876). Accessory constituents include silt- or clay-sized grains of quartz, feldspar and pyroxene minerals, meteoric and volcanic dust, fish bones and teeth, whale ear bones, and manganese micro-nodules.
Changes in the size distribution of quartz grains that reach the deep sea can reflect changes in intensity of high-altitude winds that transported the eolian dust. Composition of the clay minerals, as well as the types of biogenic material reflect climatic conditions of the source region. Biotic constituents may also indicate relative age. Deep-sea sediments in both the North Atlantic and the North Pacific contain substantial proportions of windblown sediments; clay minerals in both regions are predominantly illites. Accumulation rates of windblown sediments in the deep sea are typically up to a few mm/1000 years.
Sediment is scoured from land by the mechanical action of ice; 1-2% of the volume of this ice is typically sediment. The composition of the rock material is relatively unaltered as it is transported by ice and ultimately dropped as the ice melts. Thus, "drop stones" indicate both source and distance transported. Around Antarctica, most icebergs form at the inner margins of the Ross and Weddell Seas, and are carried into the Circumpolar Current system. North of the Antarctic Convergence, where water temperatures warm above 0o C, icebergs melt, and so ice-rafted sediments seldom reach beyond 40o S. In the North Atlantic, the iceberg limit is roughly the boundary between very cold polar waters and temperate waters. The extent of ice rafting was much greater during glacial advances, particularly in the North Atlantic Figure 8.11.
Glacial marine sediments include coarse, poorly-sorted debris and a silt fraction composed of rock flour; they typically contain little or no carbonate or biogenic material. Around Antarctica, there is a zonal distribution of sediment facies Figure 8.12 . Along the inner continental shelf, deposits are subglacial till, gravels, and sands, with some biogenic material. The outer continental shelf deposits are similar, but more characterized by sands and silts that grade into the pelagic clays of the abyssal regions. These clays contain occasional ice-rafted detritus. The pelagic clays grade northward into siliceous biogenic oozes. Glacial-marine sedimentation rates are low around Antarctica, in part because the climate is so cold and dry that the dry-base glaciers carry minimal sediment loads. In addition, the very cold, slowly accumulating and slowly moving permanent ice cover on the Antarctic continent seems to protect the continent from erosion more than it erodes.
Glacial-marine sedimentation rates vary widely, depending upon climate in the source region. The North Atlantic Ocean, south of Iceland, receives about 60 percent of global ice-rafted deposition. Higher snowfall and warmer, faster-moving glaciers on Greenland result in sediment delivery rates nearly 30 times faster than those of Antarctic glaciers. For similar reasons, ice-rafted sedimentation in the Norwegian Sea is volumetrically comparable to that of the circum-Antarctic, despite the huge difference in source areas. The North Pacific and the Arctic Ocean together receive roughly similar volumes of glacial marine sediments as the Norwegian Sea and the Antarctic region individually. Arctic glacial sediments tend to be silts and clays, reflecting eroded permafrost soils that are carried in river pack ice into the Arctic Ocean.