MARINE SEDIMENTS Pre-Lab Exercise

Use the information below to answer the pre-lab questions.

This information is adopted from materials developed by Professor Alan Trujillo, Palomar College

The sea floor is composed of basalt that originates at mid‑ocean ridges. However, the sea floor is covered in most places by layers of sediment. Sediment is any accumulation of loose material. Examples are sand lying on a beach, mud at the bottom of a lake, or gravel on a riverbed.

Rivers and glaciers carry large amounts of sediment off the continents to the ocean. Likewise, wind blows fine particles from the land out to sea. Organisms in the surface waters provide a continuous supply of skeletal material that rains down onto the sea floor. Other sediments are formed in place by chemical reactions. The types of sediment on the ocean floor are determined by factors such as its distance from land, climate, water temperature, biological productivity, and water depth.

Because marine sediments accumulate under specific conditions, an understanding of modern sediment distributions helps us interpret events in the geologic past, particularly for the last 200 million years of Earth's history (the maximum age of the ocean floor). Sediment cores are long cylinders of sediment brought up by ocean floor drilling. Oceanographers use these cores to look at the layers of sediment that have accumulated on the sea floor over time. Collectively, studies of cores reveal information about past climate, composition of atmospheric gases, evolution of animal and plant species, and movement of surface and deep-water currents. Because of the keen interest in understanding changes in Earth’s atmosphere today, the study of marine sediments is a very important area of research in oceanography.

Classification of Marine Sediments

We classify marine sediments by their source. The four main types of sediment are lithogenous, biogenous, hydrogenous and cosmogenous (Table 1 below). In this lab, you will primarily examine lithogenous, biogenous, and hydrogenous sediments. All three types of sediment are important for a number of reasons. For example, lithogenous sediments can tell us about changing plate tectonic activity, such as the uplift of mountains, which increases the amount of sediment that rivers deliver to the ocean. Biogenous sediments can tell us about environmental conditions in ocean surface waters; for example, changes in surface seawater temperature may cause shifts in the types of planktonic organisms that accumulate on the sea floor, telling us about the coming and going of ice ages. Hydrogenous sediments are economically important, providing substances ranging from the salt on your table to various metals in your computer.

Lithogenous Sediment

Lithogenous sediments (lithos = rock, generare = to produce) are sediments derived from erosion of rocks on the continents. A look at the “Sources” section of Table 1 (below) illustrates the diverse ways in which sediments from the continents enter the marine environment. Rivers and glaciers deliver large amounts of sediment the continental shelves. Turbidity currents transport some of this material down submarine canyons to the continental rise and further onto the abyssal plains. Along tectonically active margins, turbidity currents can carry sediments into a trench, where thesediments may be either subducted or accreted (added on) to the adjacent plate.

An example of lithogenous sediment: this river delivers sand and silt to the ocean, shown by the light colors in the water beyond the river mouth. From Essentials of Oceanography, 11^th Edition, Trujillo and Thurman, © 2014, Pearson Prentice Hall, Inc.

Although most lithogenous sediments accumulate along the continental margins, winds may blow small particles (clay, silt, and volcanic ash, for example) far out to sea. These particles settle slowly through the water and accumulate on the ocean floor. These small particles accumulate very slowly, at rates averaging 1 millimeter (0.04 inch) per 1000 years, which is equivalent to the size of the thickness of a dime. When these tiny particles settle in areas where little other material is being deposited (usually in the deep-ocean basins far from land), they form a sediment called abyssal clay.

Table 1. Classification of the four main types of marine sediments showing composition, sources, and main locations found. From Essentials of Oceanography, 11^th Edition, Trujillo and Thurman, © 2014, Pearson Prentice Hall, Inc.

Biogenous Sediment

Biogenous sediments (bio = life, generare = to produce) are sediments made from the skeletal remains of once-living organisms. These hard parts include a wide variety of particles such as shells of microscopic organisms (called tests), coral fragments, sea urchin spines, and pieces of mollusc shells.

The most important type of biogenous sediment comes from the tests of one-celled microscopic algae and protozoans living in the surface waters of the oceans. When these tests comprise greater than 30% of the particles in the sediment, the sediment is called an ooze. Oozes are most abundant beneath open ocean areas where nutrients are available to enhance productivity. In these areas, oozes accumulate at an average rate of 1 centimeter (0.4 inch) per 1000 years. Oozes are generally absent on the continental margins where lithogenous sediments dominate.

Two Types of Oozes

There are two major types of ooze based on the composition of the tests: calcareous ooze is made of calcium carbonate (CaCO₃); siliceous ooze is made of silica (SiO₂ ) or opal (SiO₂ ^. nH₂O).

About 48% of all deep-ocean sediment is calcareous ooze. This sediment is composed of the tests of protozoans called foraminifers (or “forams” for short), and tiny algae called coccolithophores, which produce tiny plates called coccoliths (Figure 1). These calcite-secreting organisms are most productive in warm surface waters where seawater is saturated with calcium carbonate.

50 microns a.	b.
Figure 1. Examples of common microscopic calcite-secreting organisms. (a) A test from a single-celled protozoan called a foraminifer. (b) A test from a single-celled algae called a coccolithophore, which has individual plates called coccoliths. The bars indicate scale; 1 micron equals 1 millionth of a meter or 0.00004 inch.

Deeper in the ocean, changes in temperature, pressure, and water chemistry cause calcareous tests to dissolve. At a certain depth, the tests dissolve faster than they accumulate, so calcareous oozes do not form below this depth; this depth is called the calcite compensation depth (CCD) (Figure 2). The depth of the CCD varies from one ocean basin to another, but on average occurs at approximately 4500 meters (2.8 miles) below sea level. The result is that calcareous oozes accumulate in areas above 4500 meters in the middle and low latitudes, usually on the mid-ocean ridges. In fact, there is a strong correlation between the locations of mid-ocean ridges and the distribution of calcareous ooze. As these sediments are buried, they are subjected to increasing pressure and heat, which causes the calcareous ooze to harden into chalk.

Figure 2. Schematic profile view of the ocean showing the calcite compensation depth (CCD). Above the CCD, calcite is stable and does not dissolve. Below the CCD, ocean conditions cause calcite to dissolve rapidly. From Essentials of Oceanography, 11^th Edition, Trujillo and Thurman, © 2014, Pearson Prentice Hall, Inc.

About 14% of all deep-ocean sediments are siliceous oozes. Siliceous ooze is made from the tests of another protozoan, radiolarians (or “rads” for short), and algae called diatoms (Figure 3). These organisms are most abundant in regions of high productivity, which are commonly associated with high levels of nutrients and cold surface water. Siliceous oozes are typically found on the deep-sea floor where calcareous oozes are absent. Two major zones where siliceous oozes accumulate are in polar regions and beneath the zone of equatorial upwelling. Hardened deposits of diatom-rich siliceous ooze and clay are referred to as diatomaceous earth, which is used in a wide variety of industrial applications, including making filters, abrasives, and heat-resistant insulators.

a.	b.
Figure 3. Examples of common microscopic silica-secreting organisms. (a) A test from a single-celled protozoan called a radiolarian. (b) A test from a single-celled algae called a diatom. The bars indicate scale; 1 micron equals 1 millionth of a meter or 0.00004 inch.

Not all silica from siliceous microorganisms winds up as siliceous ooze. In some cases, minor amounts of silica are deposited with calcareous ooze. Although the exact method of formation is unclear, during burial siliceous material combines to form hard rounded lumps or nodules called chert nodules. For example, the white cliffs of Dover (England) are made of chalk and also contain abundant chert nodules. Chert, a microcrystalline form of silica, is so hard that it is often used as a whetstone to sharpen knives.

Hydrogenous Sediment

Hydrogenous sediments are created from chemical reactions in seawater. Under special chemical conditions, dissolved materials in seawater precipitate (form solids). Many types of hydrogenous sediments have economic value.

Hydrogenous sediments include evaporites, meaning any type of sediment that forms from the evaporation of seawater. As seawater evaporates, the ions that remain behind can become so concentrated that they will combine with one another to form crystals that precipitate. The two most common types of evaporates are gypsum and halite. Gypsum is hydrous calcium sulfate (CaSO₄·2H₂O), and is mined worldwide to make fertilizer, plaster, and cement. Halite is sodium chloride (NaCl), which is common table salt. When you salt your food, you are eating the evaporated remains of ancient ocean water!

Manganese nodules are another type of hydrogenous sediment. They form marble-size to tennis ball-size lumps of iron and manganese oxide that lie scattered across the deep sea floor where sedimentation rates are particularly low. Although they contain large amounts of manganese, these nodules are economically most important for their cobalt, nickel, and chromium. Their formation is not well understood; however, we do know that they form as concentric layers (like an onion), adding layers of iron and manganese minerals slowly over time. The rates of formation are on the order of 1 to 10 millimeters (0.04 to 0.4 inch) per million years.

Figure 4. Manganese nodules, a type of hydrogenous sediment. From Essentials of Oceanography, 11^th Edition, Trujillo and Thurman, © 2014, Pearson Prentice Hall, Inc.

The beach sands in some tropical areas are composed of another type of hydrogenous sediment called oolites (oo = egg, ite = stone). Oolites are sand-sized grains made of calcium carbonate precipitated out of seawater in warm, tropical regions, such as in the Bahamas. Oolites need to roll back-and-forth to form , so they form only in shallow areas where waves cause back-and-forth motion on the seabed. The back-and-forth motion causes the grains to accrete layer after layer, somewhat like a snowball, so that each oolite has a spherical shape with a layered, onion-like internal structure.

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Describing Sediment Characteristics

Figure 5 shows a hypothetical distribution of sediment types across a passive continental margin and adjacent ocean basin. Note that those sediments found close to the continent along the continental shelf are known as neritic sediments and are largely lithogenous. Those sediments found further from the continent are known as pelagic sediments and are often dominated by biogenous particles.

Figure 5. Schematic view of the distribution of various sediment types across an idealized passive margin and adjacent ocean basin. Notice that with increasing distance from the continent that the grain size of lithogenous sediments decreases. From Essentials of Oceanography by Trujillo and Thurman, © Pearson Prentice Hall, Inc.

Lithogenous sediments exhibit characteristics that reflect the processes involved in their transportation and deposition. These characteristics are described as the sediment's texture, which includes the size and shape of the particles. For example, grain size is the size of the individual particles (Table 2), whereas rounding describes how angular versus how smooth the particles are (Table 3).

Grain Size	Term	Size Designation	Example
Coarse-grained	Gravel	Greater than 2mm	Large rock fragments
	Sand	0.062‑2mm	Most beach sand
	Silt	0.004‑0.062mm	Gritty; usually quartz
Fine-grained	Clay	Less than 0.004mm	Microscopic, flat particles

Table 2. Lithogenous sediment grain sizes and common examples.

Table 3. Terms used to describe the rounding of sediment grains.

Grain size and rounding (see the tables on the previous page) can both indicate the energy of the environment in which the sediment accumulated. By “energy of the environment,” we mean the ability of water, wind, or gravity to move the sediment particles. For example, a beach is a “high-energy” environment because of the breaking waves and fast currents, whereas the deep ocean floor is a “low-energy” environment because water movement is sluggish. In a high-energy environment such as a beach, small grains are washed away leaving mostly larger grains behind, and vigorous wave activity on the beach rolls the grains around to make them smooth. In contrast, low-energy environments often have smaller grain sizes, and possibly more angular grains. Note that these generalizations apply best to lithogenous sediments. Caution should be used when applying such characteristics to biogenous sediments because, for example, some biological shells may start out round and thus are not useful indicators of environmental energy.