Chapter 4 Marine Sediments

Chapter Overview

• Marine sediments contain a record of Earth history.
• Marine sediments provide many important resources.
• Marine sediments have origins from a variety of sources.

Marine Sediments

• Provide clues to Earth history
  – Marine organism distribution
  – Ocean floor movements
  – Ocean circulation patterns
  – Climate change
  – Global extinction events
• Texture – size and shape of particles
• Sediment origins
  – Worn rocks
  – Living organisms
  – Minerals dissolved in water
  – Outer space
• Sediments lithify into sedimentary rock

Classification of Marine Sediments

Marine Sediment Collection

• Early exploration used dredges.
• Modern exploration
  – Cores – hollow steel tube collects
  sediment columns
  – Rotary drilling – collects deep
  ocean sediment cores

• National Science Foundation (NSF) – formed
  Joint Oceanographic Institutions for Deep
  Earth Sampling (JOIDES) in 1963
  – Scripps Institution of Oceanography
  – Rosenstiel School of Atmospheric and
  Oceanic Studies
  – Lamont-Doherty Earth Observatory of
  Columbia University
  – Woods Hole Oceanographic Institution
• Deep Sea Drilling Project (DSDP) – 1968
  – Glomar Challenger drilling ship
  – Core collection in deep water
  – Confirmed existence of sea floor spreading
• Ocean floor age
• Sediment thickness
• Magnetic polarity
• DSDP became Ocean Drilling Project (ODP)
  in 1983
  – JOIDES Resolution replaced Glomar
  Challenger
• Integrated Ocean Drilling Program (IODP)
  – Replaced ODP in 2003
  – Chikyu – new exploration vessel in 2007
• Expedition to Japan Trench after 2011 earthquake

Paleoceanography and Marine Sediments

• Paleoceanography – study of how ocean, atmosphere, and land interactions have produced changes in ocean chemistry, circulation, biology, and climate
  – Marine sediments provide clues to past changes.
• Lithogenous sediment (lithos = stone, generare = to produce) is derived from preexisting rock material that originates on the continents or islands from erosion, volcanic eruptions, or blown dust. Note that lithogenous sediment is sometimes referred to as terrigenous sediment (terra = land, generare = to produce).

Marine Sediment Classification

• Classified by origin
• Lithogenous – derived from land
• Biogenous – derived from organisms
• Hydrogenous or Authigenic – derived from water
• Cosmogenous – derived from outer space

Lithogenous Sediments

• Eroded rock fragments
  from land
• Also called terrigenous
• Reflect composition of
  rock from which derived
• Produced by weathering
  – Breaking of rocks into smaller pieces
• Small particles eroded and transported
• Carried to ocean
  – Streams
  – Wind
  – Glaciers
  – Gravity
• Greatest quantity around continental margins
• Reflect composition of rock from which derived
• Coarser sediments closer to shore
• Finer sediments farther from shore
• Mainly mineral quartz (SiO2)

Lithogenous Quartz and Wind Transport

Grain Size

• One of the most important sediment properties
• Proportional to energy of transportation and
  deposition
• Classified by Wentworth scale of grain size

Wentworth Scale of Grain Size

Texture and Environment

• Texture indicates environmental energy
  – High energy (strong wave action) – larger
  particles
  – Low energy – smaller particles
• Larger particles closer to shore

Sorting

• Measure of grain size uniformity
• Indicates selectivity of transportation process
• Well-sorted – all same size particle
• Poorly sorted – different size particles mixed together

Sediment Distribution

• Neritic
  – Shallow-water deposits
  – Close to land
  – Dominantly lithogenous
  – Typically deposited quickly
• Pelagic
  – Deeper-water deposits
  – Finer-grained sediments
  – Deposited slowly

Neritic Lithogenous Sediments

• Beach deposits
  – Mainly wave-deposited quartz-rich sands
• Continental shelf deposits
  – Relict sediments
• Turbidite deposits
  – Graded bedding
• Glacial deposits
  – High-latitude continental shelf
  – Currently forming by ice rafting

Pelagic Deposits

• Fine-grained material
• Accumulates slowly on deep ocean floor
• Pelagic lithogenous sediment from
  – Volcanic ash (volcanic eruptions)
  – Wind-blown dust
  – Fine-grained material transported by deep ocean currents
• Abyssal Clay
  – At least 70% clay sized particles from
  continents
  – Red from oxidized iron (Fe)
  – Abundant if other sediments absent

Biogenous Sediment

• Hard remains of once-living organisms
• Two major types:
  – Macroscopic
• Visible to naked eye
• Shells, bones, teeth
  – Microscopic
• Tiny shells or tests
• Biogenic ooze
• Mainly algae and protozoans

Biogenous Sediment Composition

• Two most common chemical compounds:
  – Calcium carbonate (CaCO3)
  – Silica (SiO2 or SiO2·nH2O)

Silica in Biogenous Sediments

• Diatoms
  – Photosynthetic algae
  – Diatomaceous earth
• Radiolarians
  – Protozoans
  – Use external food
• Tests – shells of microscopic organisms
• Tests from diatoms and radiolarians generate siliceous ooze.

Diatomaceous Earth

• Siliceous ooze lithifies into diatomaceous earth.
• Diatomaceous earth has many commercial uses.

Calcium Carbonate in Biogenic Sediments

• Coccolithophores
  – Also called nannoplankton
  – Photosynthetic algae
  – Coccoliths – individual plates from dead organism
  – Rock chalk
• Lithified coccolith-rich ooze

• Foraminifera
  – Protozoans
  – Use external food
  – Calcareous ooze

Distribution of Biogenous Sediments

• Depends on three processes:
  – Productivity
• Number of organisms in surface water above ocean floor
  – Destruction
• Skeletal remains (tests) dissolve in seawater at depth
  – Dilution
• Deposition of other sediments decreases percentage of biogenous sediments

Pelagic Deposits

• Siliceous ooze
• Accumulates in areas of high productivity
• Silica tests no longer dissolved by seawater when buried by other tests

Neritic Deposits

• Dominated by lithogenous sediment, may contain biogenous sediment
• Carbonate Deposits
  – Carbonate minerals containing CO3
  – Marine carbonates primarily limestone
• CaCO3
  – Most limestones contain fossil shells
• Suggests biogenous origin
  – Ancient marine carbonates constitute 25% of all sedimentary rocks on Earth.

Carbonate Deposits

• Stromatolites
  – Fine layers of carbonate
  – Warm, shallow-ocean, high salinity
  – Cyanobacteria
• Lived billions of years ago
• Modern stromatolites live near Shark Bay, Australia

Calcareous Ooze

• CCD – Calcite compensation depth
  – Depth where CaCO3 readily dissolves
  – Rate of supply = rate at which the shells dissolve
• Warm, shallow ocean saturated with calcium carbonate
• Cool, deep ocean undersaturated with calcium carbonate
  – Lysocline – depth at which a significant amount of CaCO3 begins to dissolve rapidly

Calcareous Ooze and the CCD

• Scarce calcareous ooze below 5000 meters (16,400 feet) in modern ocean
• Ancient calcareous oozes at greater depths if moved by sea floor spreading

Distribution of Modern Calcium Carbonate Sediments

The map shows the percentage (by weight) of calcium carbonate in the modern surface sediments of the ocean basins. High concentrations of calcareous ooze (sometimes exceeding 80%) are found along segments of the midocean ridge, but little is found in deep-ocean basins below the CCD. For example, in the northern Pacific Ocean—one of the deepest parts of the world ocean—there is very little calcium carbonate in the sediment. Calcium carbonate is also rare in sediments accumulating beneath cold, high-latitude waters where calcareous-secreting organisms are relatively uncommon.

Environmental Conditions for Oozes

Hydrogenous Marine Sediments

• Minerals precipitate directly from seawater
  – Manganese nodules
  – Phosphates
  – Carbonates
  – Metal sulfides
• Small proportion of marine sediments
• Distributed in diverse environments

Manganese Nodules

• Fist-sized lumps of manganese, iron, and other metals
• Very slow accumulation rates
• Many commercial uses
• Unsure why they are not buried by seafloor sediments

Phosphates and Carbonates

• Phosphates
  – Phosphorus-bearing
  – Occur beneath areas in surface ocean of very
  high biological productivity
  – Economically useful as fertilizer
• Carbonates
  – Aragonite and calcite
  – Oolites

Metal Sulfides

• Metal sulfides
  – Contain:
• Iron
• Nickel
• Copper
• Zinc
• Silver
• Other metals
  – Associated with hydrothermal vents

Evaporites

• Evaporites
  – Minerals that form when seawater evaporates
  – Restricted open ocean circulation
  – High evaporation rates
  – Halite (common table salt) and gypsum

Evaporiative Salts in Death Valley

Cosmogenous Marine Sediments

• Macroscopic meteor debris
• Microscopic iron-nickel and silicate spherules (small globular masses)
  – Tektites
  – Space dust
• Overall, insignificant proportion of marine sediments

Marine Sediment Mixtures

• Usually mixture of different sediment types
• Typically one sediment type dominates in different areas of the sea floor.

Pelagic and Neritic Sediment Distribution

• Neritic sediments cover about ¼ of the sea floor.
• Pelagic sediments cover about ¾ of the sea floor.
• Distribution controlled by
  – Proximity to sources of lithogenous sediments
  – Productivity of microscopic marine organisms
  – Depth of water
  – Sea floor features

Pelagic Sediment Types

the proportion of each ocean floor that is covered by the pelagic deposits abyssal clay, calcareous ooze, and siliceous ooze. The world ocean (combined) pie chart shows that calcareous ooze is the most dominant sediment worldwide, covering about 45% of the deep-ocean floor. The world ocean pie chart also shows that abyssal clay covers about 38% and siliceous ooze about 8% of the world ocean floor area. If you examine the individual ocean pie charts, they show that the amount of ocean basin floor covered by calcareous ooze decreases in deeper ocean basins because they generally lie beneath the CCD. The dominant oceanic sediment in the deepest basin—the North Pacific—is abyssal clay . Conversely, calcareous ooze is the most widely deposited sediment in the shallower Atlantic and Indian Oceans. Note that siliceous oozes cover a smaller percentage of the ocean floor because regions of high productivity of organisms that produce silica tests are generally restricted to the equatorial region (for radiolarians) and the high latitudes such as near Antarctica and the far northern Pacific (for diatoms).

Sea Floor Sediments Represent Surface Ocean Conditions

• Microscopic tests sink slowly from surface ocean to sea floor (10–50 years)
• Tests could be moved horizontally
• Most biogenous tests clump together in fecal pellets
  – Fecal pellets large enough to sink quickly (10–15 days)

Worldwide Marine Sediment Thickness

The map shows that areas of thick sediment accumulation occur on the continental shelves and rises, especially near the mouths of major rivers. The reason sediments in these locations are so thick is because they are close to major sources of lithogenous sediment. Conversely, areas where marine sediments are thinnest are where the ocean floor is young, such as along the crest of the midocean ridge. Since sediments accumulate slowly in the deep ocean and the sea floor is continually being created here, there hasn’t been enough time for much sediment to accumulate.
However, as the sea floor moves away from the mid-ocean ridge, it gets progressively older and carries a thicker pile of sediments.

Resources from Marine Sediments

• Both mineral and organic resources
• Not easily accessible
  – Technological challenges
  – High costs

Energy Resources

• Petroleum
  – Ancient remains of microscopic organisms
  – More than 95% of economic value of oceanic nonliving resources
• More than 30% of world’s oil from offshore resources
• Future offshore exploration will be intense
  – Potential for oil spills

Energy Resources

• Gas Hydrates
  – Also called clathrates
  – High pressures squeeze chilled water and gas into icelike solid
  – Methane hydrates most common
• Gas hydrates resemble ice but burn when lit
• May form on sea floor
  – Sea floor methane supports rich community of organisms
• Most deposits on continental shelf

• Release of sea floor methane may alter global climate.
• Warmer waters may release more methane.
• Methane release may cause underwater slope failure.
  – Tsunami hazard
• Gas hydrates may be largest store of usable energy.
• Rapidly decompose at surface pressures and temperatures

Other Resources

• Sand and gravel
  – Aggregate in concrete
  – Some is mineral-rich
• Evaporative salts
  – Gypsum – used in drywall
  – Halite – common table salt
• Phosphorite – phosphate minerals
  – Fertilizer for plants
  – Found on continental shelf and slope
• Manganese nodules
  – Lumps of metal
  – Contain manganese, iron, copper, nickel, cobalt
  – Economically useful

Distribution of Sea Floor Manganese Nodules

The map shows that vast areas of the sea floor contain manganese nodules, particularly in the Pacific Ocean.
Technologically, mining the deep-ocean floor for manganese nodules is possible. However, the political issue of determining international mining rights at great distances from land has hindered exploitation of this resource. In addition, environmental concerns about mining the deep-ocean floor have not been fully addressed. Evidence suggests that it takes at least several million years for manganese nodules to form and that their formation depends on a particular set of physical and chemical conditions that probably do not last long at any location. In essence, they are a nonrenewable resource that will not be replaced for a very long time once they are mined. Of the five metals commonly found in manganese nodules, cobalt is the only metal deemed “strategic” (essential to national security) for the United States. It is required to produce dense, strong alloys with other metals for use in high-speed cutting tools, powerful permanent magnets, and jet engine parts. Currently, the
United States must import all of its cobalt from large deposits in southern Africa. However, the United States has considered deep-ocean nodules and crusts (hard coatings on other rocks) as a more reliable source of cobalt. In the 1980s, cobalt-rich manganese crusts were discovered on the upper slopes of islands and seamounts that lie relatively close to shore and within the jurisdiction
of the United States and its territories. The cobalt concentrations in these crusts are about one-and-a-half times as rich as the best African ores and at least twice as rich as deep-sea manganese nodules. However, interest in mining these deposits has faded because of lower metal prices from land-based sources.

Other Resources

• Rare Earth elements
  – Assortment of 17 metals
  – Used in technology, e.g., cell phones, television screens, etc.
• Sea floor may hold more rare Earth element deposits than found on land.

Rare-earth elements—an assortment of 17 chemically similar metallic elements such as lanthanum and neodymium—are used in a variety of electronic, optical, magnetic, and catalytic applications. For example, rare-earth elements are used in a host of technological gadgets from cell phones and television screens to fluorescent light bulbs and batteries in electric cars. Demand for rare-earth elements has skyrocketed in recent years, with China supplying about 90% of the current world demand. Over millions of years, deep-sea hot springs associated with the mid-ocean ridge pulled rare-earth elements out of seawater and enriched them in sea floor muds. A recent study of rare-earth elements on the floor of the Pacific Ocean indicated that some locations are particularly enriched. For example, an area of the sea floor near Hawaii measuring 1 square kilometer (0.4 square mile) holds as much as 25,000 metric tons (55 million pounds) of rare-earth elements. Overall, estimates suggest
that the ocean floor might hold more rare-earth elements than all the known deposits on land