Chapter 8: Ocean Sediments
Guide to Reading and Learning
Sediments accumulate in layers on the seafloor and, in doing so, preserve a historical record of climate change and other events such as volcanic eruptions and meteor impacts. Because we cannot perform controlled experiments that last thousands or millions of years, reading this record is virtually our only way of investigating what the Earth’s past has been and, therefore, of being able to predict what the future might be if we change variables like carbon dioxide concentration in the atmosphere.
We begin Chapter 8 by describing the large range of sizes and compositions of the many different types of particle that can be found in marine sediments. Then we visit each of the four major types of material that make up these particles and learn how they are transported to, and deposited in, the oceans. Most of you will be familiar with the concept that rivers carry particles of eroded rock and soil to the oceans. However, did you know that most eroded rock particles that are found in sediments of the deep oceans are not transported by rivers but instead through the atmosphere by winds?
Did you know that the remains of living marine organisms can not only be found in ocean sediments but, in fact, in many locations comprise most of the particles in the sediments? Or, did you know that some rocks on land are composed almost entirely of the remains of a single type of marine organism that was deposited in the oceans long ago, compacted, and then uplifted by tectonic processes to form the rock formations where they are found? The species that comprise most of the particles in ocean sediments are microscopically small plankton. In this chapter we provide a brief introduction to the various types of plankton that are so abundant in the oceans that they can accumulate in layers tens of meters or more thick in seafloor sediments. Pay attention to these organisms for, as we find in later chapters, they form the foundations of life in the oceans.
Aside from eroded particles from land and remains of marine organisms, other particles found in marine sediments are either precipitated from seawater or are dust particles created by meteorites as they burn up on entry into the Earth’s atmosphere. Although these sources are smaller than eroded rock and biological remains, they are disproportionately more important in other ways. For example, did you know that metal-rich sediments, whose existence was unknown until the late twentieth century, are now found to lie in numerous locations along the oceanic ridges and elsewhere, and that these sediments could provide an economically viable commercial source of certain rare metals? Did you know that these metal-rich sediments are deposited around hydrothermal vents that were discovered only in 1977, and that these vents support entire ecosystems of numerous species that depend on chemosynthesis, or that these vent ecosystems were completely unknown until 1977—a discovery that was as unexpected as finding life on Mars would be?
Most meteorites burn up in the atmosphere to form small particles of dust. Did you know that in the Earth’s history there have been a number of impacts of much bigger objects too large to burn up, and that one of these impacts is thought to be the likely cause of the mass extinction that saw the end of the dinosaurs? Ocean sediments contain a record of this and other impacts that can be read using the information that this chapter includes on the origin, characteristics, and transport routes of ocean sediment particles.
Chapter 8 Essential to Know
Critical Concepts used in this chapter
8.1 Sediments and Biogeochemical Cycles
- Sediments cover most of the seafloor and are important as sinks and sources in the biogeochemical cycles of elements in seawater.
8.2 Classification of Sediments
- All sediments are mixtures of particles from different sources and of different composition. However, they may be classified by their general characteristics.
Classification by Grain Size
- Sediments may be classified by grain size
- Sediment particles range from boulders to grains so small that they cannot be distinguished except under the most powerful microscopes. However, most sediments deposited at a specific location and time consist primarily of grains that are within a narrow range of particle sizes and are said to be well sorted.
- Sediments are generally classified as gravel, sand, or mud (which can be subdivided into silt and clay) in descending order of their predominant grain size.
Classification by Origin
- Sediments may be classified by the origin of the majority of their particles.
- Sediment particles may be derived from land (lithogenous), biological processes (biogenous), chemical precipitation from sweater (hydrogenous), or derived from meteorites (cosmogenous).
- The distribution of sediment particles from these different origins within ocean sediments at different locations is determined by many factors including grain size, location of origin, susceptibility to decomposition or dissolution in seawater, and the mechanisms of particle transport.
8.3 Lithogenous Sediments
- Lithogenous sediment particles are produced by wind and water erosion and weathering of terrestrial rocks. During weathering, easily dissolved minerals are removed, leaving mostly siliceous minerals including quartz, feldspars, and clay minerals. Clay minerals are layered structures of silicon, aluminum, and oxygen atoms, some containing iron and other elements. They are carried to the oceans by rivers, glaciers, and winds, or eroded from coastlines by waves.
Transport by Rivers
- Most rivers slow down as they near the ocean, and many flow slowly over flat coastal plains so that larger particles are generally deposited in river valleys and only small particles are carried out to the oceans. However, during storm runoff events large amounts of larger particles can be resuspended from the river bed and carried to the ocean, where they accumulate in sediments near the river mouth.
- Rivers that flow across active subduction zone margins are generally short due to the coastal mountain ranges and carry relatively small quantities of suspended sediment because they drain only small area of land.
- Approximately 90% of all lithogenous sediments reach the oceans through rivers and 80% of this input is derived from Asia. The largest amounts are carried by four rivers: the Ganges, Brahmaputra, and the Irrawaddy empty into the Bay of Bengal, and the Indus discharges into the Arabian Gulf. In the area where these rivers discharge to the oceans extensive deep sediment deposits extend to the deep sea floor in the form of abyssal fans.
- Most of the other rivers that transport large quantities of suspended sediments to the oceans empty into marginal seas.
Erosion by Glaciers
- Glaciers erode very large amount of rock from their valleys, producing particles ranging from large boulders to extremely fine particles. The eroded rock is bulldozed, dragged, and carried down the glacier’s valley and deposited at its lower end.
- Many high-latitude glaciers empty directly into the oceans or end near where the glacial valley meets the sea. Icebergs that break off glaciers can carry their eroded rock long distances into the oceans to be released as the iceberg melts
- Glaciers release their eroded rock at their lower ends. Much of this material then washes out to the ocean, especially the very fine-grained material called glacial flour (because it remains suspended for long periods and can give the water a milky appearance in lakes or fjords where the glacier empties).
Erosion by Waves
- Waves continuously erode coastlines.
- Sediment particles released to the oceans by coastal wave erosion are similar to riverborne suspended sediments but often have a larger proportion of unweathered mineral grains.
- Wave erosion creates particles of all sizes. Wave action then sorts the particles, transporting small ones offshore and leaving larger ones on or close to the shore.
Transport by Winds
- Dust particles can be carried very long distances through the atmosphere. For example, Sahara Desert sand grain dust is transported across the Atlantic Ocean, where it can be easily identified on air filters placed at the coastline in Florida.
- Dust particles in the atmosphere are eventually deposited on the ocean surface and sink slowly to the seafloor.
- Fine wind-blown dust particles fall over all parts of the oceans at a relatively uniform rate. Although this rate is slow, atmospheric dust accumulate continuously and is a major component of sediments that are remote from land and have very slow accumulation rates of other types of particles.
- Explosive volcanic eruptions of subduction zone volcanoes can inject very large quantities of fragmented rock into the atmosphere. Historical records suggest that large eruptions such as the one that created Long Valley caldera in California can inject hundreds of cubic kilometers of pulverized rock into the atmosphere.
- The largest particles injected into the atmosphere by volcanic eruptions rain out near the eruption site, but smaller particles can stay in the atmosphere for years and are distributed around the world before they eventually rain out onto the land or ocean.
Transport by Landslides
- Landslides on coastlines can carry lithogenous materials of a wide range of particle sizes into the oceans, where they are incorporated in the sediments.
- Slumps and turbidity currents on the continental slopes can carry lithogenous sediments from the continental shelf into trenches or out over the abyssal plain where there is no trench.
8.4 Biogenous Sediments
- Almost all ocean life depends on photosynthesis performed by microscopic organism called phytoplankton.
- Most phytoplankton are consumed by larger organisms called zooplankton that excrete organic-rich fecal material. This material is often in the form of fecal pellets, which are larger than the individual phytoplankton that compose them and sink much faster. Marine species larger than zooplankton also produce fecal pellets.
- Much of the organic matter in fecal pellets is decomposed by bacteria and other organisms or consumed by detritus feeders as it sinks through the water column or after it has been deposited on the seafloor. As a result, most ocean sediments contain little organic matter.
- Biogenous particles are predominantly the solution-resistant silica or calcium carbonate hard parts of microscopic phytoplankton and zooplankton.
- The two major factors affecting the accumulation rate of biogenous particles in sediments are the rate of production of the particles in the overlying water column and the rate of decomposition or dissolution of the particles.
- Biogenous particles can dominate sediments in areas where the productivity is high in the overlying water, but not where the hard parts are dissolved before they can accumulate in the sediments.
Regional Variations of Biogenous Particle Production
- In high latitudes diatoms are the dominant photosynthetic organisms. They have siliceous hard parts and they dominate the inputs of biogenous material to the sediments.
- At lower latitudes, many of the dominant photosynthetic organisms have no hard parts so inputs of biogenous material to sediments are limited except in certain regions where coccolithophores grow in abundance. Coccolithophores have calcium carbonate hard parts that can become the dominant particle in sediments beneath areas of high coccolithophore productivity if the water depth is not great enough so that the calcium carbonate is dissolved (see “Dissolution of Biogenous Particles”)..
- Some zooplankton, small free-floating animals that eat phytoplankton, also have hard parts that can contribute to, or dominate, sediments in some areas. Foraminifera and pteropods have calcareous hard parts, whereas radiolaria have silica hard parts.
- Radiolaria are abundant in those tropical waters that have high productivity and can dominate sediments in these areas.
Dissolution of Biogenous Particles
- Calcium carbonate particles dissolve more easily at higher pressures (depths), whereas silica particles dissolve very slowly at all depths and their dissolution rate actually decreases with increasing depth.
- Thin siliceous hard parts may be dissolved before they reach the seafloor but not thicker hard parts. For example, hard parts from diatoms will accumulate in the sediments whatever the depth if they are abundant in the overlying water.
- The upper layers of seawater are generally saturated or supersaturated with calcium carbonate, so calcareous material does not dissolve. However, calcium carbonate solubility increases with increasing pressure (depth) and decreasing temperature. Because deep water in the oceans is cold, the calcium carbonate dissolution rate increases substantially with increasing depth.
- There are two types of calcium carbonate hard parts: calcite and aragonite. Some types of animal have calcite hard parts (e.g., pteropods) and others aragonite (e.g., foraminifera). Aragonite dissolves more easily than calcite so pteropod hard parts are totally dissolved at shallower depths than foraminifera hard parts. Where pteropods are more abundant than foraminifera, sediments at shallow depths may consist primarily of pteropod remains, in deeper water the pteropods are dissolved and sediments may be dominated by foraminifera remains, and at even deeper depths both forms of calcium carbonate are totally dissolved and the sediments contain no calcium carbonate.
Carbonate Compensation Depth
- The depth below which all calcium hard parts are dissolved before they can accumulate in sediments is called the carbonate compensation depth, or CCD.
- No calcium carbonate–containing particles survive to be accumulated in surface sediments below the CCD.
- The CCD varies between oceans and historically over time.
- Deep waters of the oceans are formed by sinking of cold surface water in certain regions near the poles. The cold water is saturated with carbon dioxide but as pressure increases the saturation solubility increases and deep water dissolves additional carbon dioxide released by respiration and decomposition of organic matter as the water flows through the ocean basins.
- The CCD is affected not only by changes in temperature and pressure but also by changes in dissolved carbon dioxide concentration. Adding dissolved carbon dioxide to water lowers the pH and increases the solubility of calcium carbonate. As a result, the CCD is shallower in those area of the oceans where the deep water is older (a longer period of time since it left the surface). The CCD is shallower in the Pacific Ocean than in the Atlantic Ocean and is shallower in the South Atlantic Ocean than in the North Atlantic Ocean, reflecting the formation of deep water in an area near Greenland and the flow of this water southward to the south polar region and then around Antarctica and north into the Pacific Ocean. No deep water is formed in the North Pacific Ocean.
Carbonate Compensation Depth and the Greenhouse Effect
- Changes in the mean CCD over time can have a major effect on the distribution of carbon dioxide between the atmosphere, oceans, and ocean sediments and so are important for greenhouse effect studies.
- The higher concentration of carbon dioxide in surface ocean waters that has resulted from human use of fossil fuels will eventually reach the deep waters and may increase pH sufficiently to significantly reduce the CCD. This would cause more calcium carbonate to dissolve and the excess carbon dioxide could be released to the atmosphere when the water returns to the surface, providing a positive feedback to the enhanced greenhouse effect.
8.5 Hydrogenous Sediments
- Hydrogenous sediments are precipitated from seawater predominantly as manganese and phosphorite nodules in certain areas near hydrothermal vents and in certain shallow tropical areas where conditions permit calcium carbonate to precipitate.
- Heat flowing up through thin mantle, especially at the oceanic ridges, drives hydrothermal vents that discharge hot water. The mechanism involved is not known but is thought to be convection of water heated within the rocks and sediments and replacement of this water by percolation of seawater into the rocks and sediments from areas surrounding the vents.
- Hydrothermal vents have been found on the oceanic ridges in many locations throughout the oceans and are probably quite common. The vents support communities of unique organisms that depend on chemosynthetic primary production by microbial organisms in the vent water as the ultimate source of their food..
- The temperature and composition of the water discharged by hydrothermal vents varies. However, most vent plumes have no oxygen, substantial sulfide concentrations, and high concentrations of iron, manganese, and other metals (including copper, cobalt, lead, nickel, silver, zinc) that have soluble sulfides.
- Once discharged into the surrounding ocean water, metal sulfides are oxidized and precipitate as a rain of fine particles of their hydrous oxides. Some particles sink to the seafloor to form metal-rich sediments in areas surrounding the vents and others are transported and deposited far from the vent to contribute to sediments elsewhere.
- Test-mining of metal-rich hydrothermal sediments has occurred in the Red Sea, where restricted circulation has allowed large concentrations of such sediments to accumulate.
Undersea Volcano Emissions
- Hydrothermal vents have recently been found to exist on the flanks of hot-spot and magmatic-arc volcanoes where, in some instances, they discharge their fluids at much shallower depths (even sometimes within the photic zone) than at the oceanic ridges.
- Manganese nodules are dark brown, rounded lumps of rock, often larger than a potato, that are found lying on the abyssal ocean floor in high abundance in some locations.
- Manganese nodules form by precipitation of minerals from seawater and are usually formed initially around a large sediment particle such as a shark tooth. The minerals build in concentric layers around the nodule in a manner similar to tree rings, and occasional disturbance by marine organisms is thought to be necessary for the nodules to grow on all sides and in order that they not be buried by new sediments.
- Manganese nodules consist mostly of manganese oxide and iron oxide but also have high concentrations of other metals, including copper, nickel and zinc. The source of these minerals is not known but may be particles transported from hydrothermal vents.
- Manganese nodules are potentially commercially valuable, especially in the central Pacific Ocean where they are most abundant.
Phosphorite Nodules and Crusts
- Phosphorite nodules containing up to 30% phosphorus form in limited areas of the continental slope and some seamounts.
- They grow very slowly (1–10mm per 1000 yr) and their formation apparently requires low dissolved oxygen concentrations in the overlying bottom water and a large supply of phosphorus carried to the sediments by sinking detritus as a result of high productivity in the overlying surface waters.
- Phosphorite nodules grow only by accumulation on their underside, where phosphorus is released by decomposition of detritus in the sediment..
- Many limestone rocks lack fossils. In some limestone rocks, the fossils have been decomposed, but some other limestone rocks consist of calcium carbonate precipitated directly from seawater.
- Conditions that permit calcium carbonate precipitation must have been widespread in the past but are now found in very limited regions, such as the Bahamas.
- Both high temperature and high productivity appear to be necessary for calcium carbonate precipitation to occur, as these conditions cause the pH to rise.
- Calcium carbonate is precipitated around suspended sediment particles to form rounded grains called ooliths.
- In marginal seas with arid climates, evaporation may increase salinity so high that salts precipitate progressively from the seawater—first calcium and magnesium carbonate, then calcium sulfate, and finally sodium chloride.
- Evaporites form in very few areas today but evaporite formation must have been more common at times in the past. For example, the Mediterranean Sea has several layers of evaporite sediments, some more than 100 m thick, indicating that the Mediterranean may have evaporated almost to dryness several times when sealevel fell and the connection with the Atlantic Ocean was broken.
8.6 Cosmogenous Sediments
- Cosmogenous sediment particles are derived from meteors and meteorites and are relatively rare, although they may amount to tens of thousands of tonnes per year spread over the entire oceans.
- There are two types of particles—iron-rich and silicate-rich—derived from different types of meteorite. Both form spherical particles called cosmic spherules, as the material is melted in the atmosphere and then solidifies in droplets.
8.7 Sediment Transport, Deposition, and Accumulation
- Large particles sink more rapidly and need higher current speeds to resuspend them than smaller particles do.
- Large particles are not transported far by ocean currents but smaller particles can be carried long distances.
- Orbital velocities in waves are much higher than ocean current speeds and waves resuspend sand-sized particles and move them long distances along the coast. However, when these particles are carried offshore to deeper water where wave orbit velocities are lower and wave motion does not extend to the seafloor, the particles are deposited and only the smallest particles are transported further by ocean currents.
- Thus, large particles (which are primarily lithogenous) tend to collect in sediments of the continental shelf, whereas sediments accumulating far from land are generally very fine-grained and less likely to be dominated by lithogenous particles.
- The smallest clay-sized particles form cohesive sediments that make the particles difficult to resuspend. Fine-grained cohesive muds often form in coastal areas such as wetlands that are protected from waves.
- Turbidity currents are swift-moving (70 km·hr–1 or more) slumps of sediment similar to avalanches. They can carry coarse grain-sized sediments to and across the deep ocean floor adjacent to the continental shelves.
- Because large particles settle faster, turbidity currents leave layers of sediment called turbidite layers. In a turbidite layer, sediment grain size decreases upward toward the surface; finer-grained sediment layers appear above and below turbidite layers.
- Sediment accumulation rates are high near continents and much lower in the deep oceans far from land.
- All sediments are mixtures of particles from different origins. The composition of sediment at a given location is determined by the relative rates of accumulation of each type of material at that location.
- Sediment accumulation rates in nearshore areas range from about 100 cm per 1000 years to extremes of several meters per year at some river mouths. On continental shelves and in marginal seas the rates are generally 10–100 cm per 1000 years. Rates in the deep oceans remote from land are much lower, about 0.1 cm per 1000 years.
8.8 Continental Margin Sediments
- The continental shelves are covered in lithogenous sediments of larger grain sizes except in areas where currents are slow or production of biogenous particles is low.
- On many continental shelves lithogenous sediment accumulates continuously as new material is supplied by coastal erosion and riverborne sediments.
- On some continental shelves—such as that off the U.S. East Coast, where riverborne sediment is trapped in estuaries and lagoons and where currents speeds on the continental shelf are relatively high—little or no new sediment accumulates, especially on the outer part of the shelf, because inputs of lithogenous particles are low and particles of other types are generally small enough that they are transported away from these areas by currents.
- In areas where no new sediment accumulates, the seafloor is covered by relict sediments. Relict sediments are sediments laid down when sealevel was lower so that the deposition area was coastal and only covered by shallow water. Shells and other marine organism remains in relict sediment are often remains of species, such as some oysters that only live in very shallow coastal waters.
8.9 Distribution of Surface Sediments
- The distribution of sediment types in surface sediments (currently accumulating material) reflects the proximity of lithogenous sediment sources, the productivity of the overlying waters and the type of organisms that are abundant, the seafloor depth, and the depth of the CCD.
- Radiolarian oozes accumulate in a region of high productivity that extends in a band across the deep oceans at the equator. However, radiolarian oozes do not accumulate in areas where the sedimentation rate of lithogenous material is much larger than that of radiolarian particles, as in most areas near the continents and in the Atlantic Ocean, where lithogenous sediment accumulation is higher than in other oceans.
- Diatoms are the dominant siliceous biogenous material except near the equator, where radiolaria dominate. Thus, diatom oozes are found in the deep oceans in areas of high productivity but only where inputs of lithogenous particles are low and where the seafloor is deeper than the CCD (so that calcareous biogenous particles are dissolved before they accumulate in the sediments).
- In areas where the seafloor is shallower than the CCD and the inputs of lithogenous material are low, calcareous particles accumulate fast enough to be a major part of the sediments. These areas include oceanic plateaus, seamounts, and the flanks of the oceanic ridges
- In areas remote from land, deeper than the CCD, and where biological productivity in the overlying water is low, the only material that reaches the sediments in significant quantities is very fine-grained lithogenous particles transported large distances by currents and winds. These form slowly accumulating very fine-grained sediments called deep-sea clays, sometimes called red clays because the particles are reddish or brownish in color due to their iron oxide content.
Siliceous Red Clay Sediments
- In the deep basins of the North and South Pacific, South Atlantic, and southern Indian Oceans there are transitional areas where sediments grade progressively between deep sea clays and diatom oozes.
- Sediments carried to the oceans predominantly by glaciers accumulate in some areas of the Arctic Ocean, Bering Sea, and around Antarctica. They can contain pebbles and even larger particles, as icebergs originating from glaciers can carry these ice-rafted sediment particles far from land.
- Terrigenous sediments dominate in areas close to the mouths of rivers that carry large suspended sediment loads to the ocean, for example, in the northern Arabian Sea and the Bay of Bengal.
- The central basin of the Red Sea is the only area where hydrothermal sediments are known to dominate. However, small areas of hydrothermal sediments also are found around hydrothermal vents.
8.10 The Sediment Historical Record
- Sediments accumulate layer upon layer and preserve a history of changing deposition characteristics, although there is some mixing of sediments by bioturbation (churning of the upper layers of sediment by living organisms).
- The sediment historical record can provide information about changes in depth of the seafloor, temperature of the overlying water, productivity of the overlying water, and the CCD, among other things.
- Reading the sediment historical record is difficult because so many different factors, including bioturbation, affect it and because the age of each layer must be determined precisely.
Sediment Age Dating
- Ages of sediment layers are determined primarily by fossils and calibrated by radionuclide dating in sediments and rocks when possible. Some dating information can also be obtained from magnetic anomaly data and paleomagnetism.
- Physical and chemical changes called diagenesis occur in sediments over time as they are progressively buried.
- The pore waters (water trapped between the mineral grains) are depleted in oxygen due to continued decomposition of organic matter. Eventually sulfides form, as the oxygen in sulfate is used by decomposers in place of the depleted oxygen. This allows metals that have soluble sulfides but insoluble hydrated oxides (e.g., iron, manganese) to dissolve. Silica and calcium carbonate are also dissolved progressively.
- Oxygen diffuses slowly and is carried by bioturbation into sediments from the water above, whereas sulfides, dissolved silica, calcium, and carbonate ions formed from calcium carbonate dissolution can diffuse slowly upward within the sediment until they reach the oxygen diffusing down and are oxidized and precipitated. Pore waters are also squeezed out of the sediments as the sediments are compacted by the weight of the accumulated sediments above.
- Diagenesis is important because it can recycle nutrients from detritus in the sediments back to the water column.
Tectonic History in the Sediments
- Because the type of seafloor sediment differs depending on depth, distance from the continents, and latitude, changes on a particular piece of crust can be used to reveal various aspects of the tectonic history.
- For example, new crust near the oceanic ridges is shallower than the CCD, and sediments will contain calcium carbonate. However, as the crust moves away from the oceanic ridge, cools, and sinks isostatically, it descends below the CCD and newer sediment will not contain calcium carbonate.
Climate History in the Sediments
- The past 170 million years of the Earth’s climate history is preserved in sediments, primarily in biogenous particles.
- Different species grow in different temperature ranges. Thus, if we assume ancient species have similar temperature requirements to closely related species today, the remains of these species can tell us about the temperature range that existed at the place and time they were formed.
- Past climate can also be determined from certain isotope ratios on biogenous particles.
- Oxygen consists primarily of two naturally occurring isotopes: O-16 and O-18. O-16 containing water evaporates slightly faster than O-18 containing water. When world climate is cold and there are more glaciers and ice sheets O-16 is transferred preferentially to this ice and the ratio of O-16 to O-18 in seawater goes down. Because the ratio of oxygen isotopes in marine organisms carbonate shell material is primarily determined by the ratio of the isotopes in the surrounding water, changes in the isotope ratio of calcareous sediments can be used to determine past climate temperatures.
- Isotopes of an element also react at slightly different rates at different temperatures. The isotope ratios of most elements are relatively uniform throughout the oceans at any one point in history. The ratios of isotopes taken up by different species vary but, within a single species, the ratio depends only on the ratio in seawater and the temperature. As a result, isotope ratio differences between biogenous remains of a single species deposited at the same time but in different locations can reveal the temperature difference between the locations at the time the sediments were deposited.
Support for Extinction Theories in the Sediments
- About 65 million years ago the last dinosaurs and more than half of all marine species became extinct. Other extinctions have occurred during the Earth’s history.
- Evidence gathered from 65 million-year-old sediments supports a theory that this extinction was caused by a giant meteorite impact in the oceans near what is now the Yucatán Peninsula in Mexico.
Evidence of Impact at Chicxulub
- The Yucatán Peninsula was about 500 m underwater 65 million years ago.
- Magnetic surveys show a 180 km diameter impact crater buried in the region now called Chicxulub.
- 65 million-year-old sediments in this and surrounding areas have a thick layer of unusual materials.
- Within this layer, the deepest sediments are rounded, very coarse-grained material called tektites, which are glassy material formed when rocks are melted by meteorite impacts. Above that, there is a layer of coarse-grained sediment that contains fossilized terrestrial plant matter and then progressively finer-grained material until, at the top of the anomalous layer, marine sediments normal for this region reappear.
- This evidence can best be explained by a meteorite impact that caused a mega-tsunami that swept onto the continents and back, perhaps sloshing back and forth for days. This tsunami may have been caused by the impact itself or by an estimated magnitude 11 massive earthquake that the impact generated
Evidence of Other Impacts
- There is evidence of a number of craters similar to the Chicxulub crater and associated anomalous sediment layers in various parts of the world that were possibly created by meteorite impacts at various times in the Earth’s past. Some of these may also be associated with mass extinctions.
- A crater of this nature found at the mouth of Chesapeake Bay was apparently created approximately 35 million years ago.
Critical Concept Reminders:
CC.1 Density and Layering in Fluids (p. 180)
- Fluids, including the oceans, are arranged in layers sorted by their density. Heat sources under the seafloor can heat and reduce the density of any water present, and this heated water rises through the seafloor and up into the water column. To read CC1 go to page 2CC.
CC.2 Isostasy, Eustasy, and Sea Level (p. 193)
- Earth’s crust floats on the plastic asthenosphere. Sections of crust rise and fall isostatically as temperature changes alter their density. Oceanic crust cools progressively after it is formed and sinks because its density rises. Thus, the seafloor becomes deeper with increasing distance from an oceanic ridge. To read CC2 go to page 5CC.
CC.3 Convection and Convection Cells (p. 180)
- Fluids that are heated from below, including water within sediments and cracks in the seafloor, rise because their density is decreased. This establishes convection processes. Heated water rising through the seafloor is replaced by colder water that seeps or percolates downward into the seafloor to replace it. To read CC3 go to page 10CC.
CC.4 Particle Size, Sinking, Deposition, and Resuspension (pp. 171, 172, 174, 176, 183)
- Suspended particles (either in ocean water or in the atmosphere) sink at rates primarily determined by particle size: large particles sink faster than small particles. Once deposited, particles can be resuspended if current (or wind) speeds are high enough. Generally large particles are more difficult to resuspend, although very fine particles may be cohesive and also difficult to resuspend. Sinking and resuspension rates are primary factors in determining the grain size characteristics of beach sands and sediments at any given location. To read CC4 go to page 12CC.
CC.6 Salinity, Temperature, Pressure, and Water Density (p. 180)
- Sea water density is controlled by temperature, salinity, and to a lesser extent pressure. Density is higher at lower temperatures, higher salinities, and higher pressures. Heated water discharged by hydrothermal vents has a high enough salinity in some areas of very limited mixing that it is more dense than the overlying seawater and collects in a layer next to the seafloor. To read CC6 go to page 16CC.
CC.7 Radioactivity and Age Dating (p. 192)
- Some elements have naturally occurring radioactive (parent) isotopes that decay at precisely known rates to become a different (daughter) isotope. Measuring the concentration ratio of the parent and daughter isotope can be used to calculate the age of the various materials since they were first formed, but only if none of the parent or daughter isotope are gained or lost from the sample during this time period. This condition is usually not met in sediments and radioisotope age dating is difficult so other dating methods including variations in fossil assemblages, and magnetic field properties are used extensively. To read CC7 go to page 18CC.
CC.8 Residence Time (pp. 170, 173)
- The residence time of seawater in a given segment of the oceans is the average length of time the water spends in the segment. In restricted arms of the sea or where residence time is long, very fine grained particles from glaciers and rivers can become concentrated in the water. Residence time is also a major factor in determining the change in concentration of an element in seawater if its inputs to the oceans change. To read CC8 go to page 19CC.
CC.9 The Global Greenhouse Effect (pp. 174, 179, 194)
- The oceans play a major part in studies of the greenhouse effect as the oceans store large amounts of carbon dioxide both in solution and as carbonates in sediments, formed at shallow enough depths that the carbonates are not dissolved. To read CC9 go to page 22CC.
CC.14 Photosynthesis, Light, and Nutrients (pp. 175, 182)
- Chemosynthesis and photosynthesis are the two processes by which simple chemical compounds are made into the organic compounds of living organisms. Many sediments have high concentrations of particles that originate from photosynthetic organisms or species that consume these organisms. To read CC14 go to page 46CC.