Marine Sediments Notes

Chapter 4: Marine Sediments

Overview

• Marine sediments contain a record of Earth's history.

• Marine sediments provide a variety of important resources.

• Marine sediments have a variety of origins.

Marine Sediments

• Eroded rock particles and fragments transported to the ocean.

• Deposits settle through the water column.

• Oceanographers study sediments to decipher Earth's history.

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.

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

Classification of Marine Sediments (TABLE 4.1)

Lithogenous

• Composition: Rock fragments, quartz sand, quartz silt, clay, volcanic ash

• Sources: Rivers, coastal erosion, landslides, glaciers, turbidity currents, wind-blown dust

• Main locations found: Continental shelf, continental shelf in high latitudes, continental slope and rise, ocean basin margins, deep-ocean basins

Biogenous

• Composition:

  • Calcareous ooze (microscopic): Calcium carbonate $(CaCO_3)$, Coccolithophores (algae), Foraminifers (protozoans)

  • Shell coral fragments (macroscopic): Macroscopic shell-producing organisms, Coral reefs

  • Siliceous ooze (microscopic): Silica $(SiO<em>2 H</em>2O)$, Diatoms (algae), Radiolarians (protozoans)

• Sources:

  • Warm surface water

  • Shallow low-latitude regions

  • Cold surface water, volcanic eruptions

• Main locations found:

  • Low-latitude regions; sea floor above CCD; along mid-ocean ridges and the tops of volcanic peaks

  • Continental shelf; beaches

  • Shallow low-latitude regions

  • High-latitude regions; sea floor below CCD; upwelling areas where cold, deep water rises to the surface, especially that caused by surface current divergence near the equator

Hydrogenous

• Composition: Manganese nodules (manganese, iron, copper, nickel, cobalt), phosphorite (phosphorous), oolites $(CaCO_3)$, metal sulfides (iron, nickel, copper, zinc, silver), evaporites (gypsum, halite, other salts)

• Sources: Precipitation of dissolved materials directly from seawater due to chemical reactions

• Main locations found: Abyssal plain, continental shelf, shallow shelf in low-latitude regions, hydrothermal vents at mid-ocean ridges, shallow restricted basins where evaporation is high in low-latitude regions

Cosmogenous

• Composition: Iron-nickel spherules, tektites (silica glass), space dust, iron-nickel meteorites

• Sources: Meteors

• Main locations found: In very small proportions mixed with all types of sediment and in all marine environments, localized near meteor impact structures

Marine Sediment Collection

• Early exploration used dredges.

• Modern exploration:

  • Cores: hollow steel tube collects sediment columns

  • Rotary drilling: collects deep ocean sediment cores

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.

Lithogenous Sediments Transportation

• Small particles eroded and transported.

• Carried to ocean by streams, wind, glaciers, and gravity.

• Greatest quantity around continental margins.

Lithogenous Sediments Composition and Distribution

• Reflect composition of rock from which derived.

• Coarser sediments closer to shore.

• Finer sediments farther from shore.

• Mainly mineral quartz $(SiO_2)$.

Wentworth Scale of Grain Size for Sediments (TABLE 4.2)

• Grain Size, Particle Name, Grain Size, Example, Energy of Depositional Environment

• Above 256 mm, Boulder, Coarse-grained, Coarse material found in streambeds near the source areas of rivers, Highest energy

• 64 to 256 mm, Cobble, Coarse-grained

• 4 to 64 mm, Pebble, Gravel

• 2 to 4 mm, Granule, Gravel

• 1/16 to 2 mm, Sand, Fine-grained, Beach sand

• 1/256 to 1/16 mm, Silt, Fine-grained, Feels gritty in teeth

• 1/4096 to 1/256 mm, Clay, Fine-grained, Microscopic; feels sticky, Lowest energy

Sediment Texture

• Grain size sorting: indication of selectivity of transportation and deposition processes

• Textural maturity: increasing maturity if

  • Clay content decreases

  • Sorting increases

  • Non-quartz minerals decrease

  • Grains are more rounded (abraded)

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, and fine-grained material transported by deep ocean currents.

Pelagic Deposits and Silica

• Silica-secreting organisms live in sunlit surface waters; siliceous ooze only accumulates beneath areas where productivity is high.

• Where the rate of supply of siliceous tests is less than the rate at which silica dissolves, no siliceous ooze accumulates.

• Where the rate of supply of siliceous tests is greater than the rate at which silica dissolves, siliceous ooze accumulates.

• Abyssal clay

Pelagic Deposits: 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 the naked eye (shells, bones, teeth)

  • Microscopic: tiny shells or tests, biogenic ooze, mainly algae and protozoans

Biogenous Sediment Composition

• Calcium carbonate

• Silica

Silica in Biogenous Sediments

• Diatoms: photosynthetic algae; diatomaceous earth.

• Radiolarians: protozoans; use external food.

Silica in Biogenous Sediments Details

• Tests from diatoms and radiolarians generate siliceous ooze.

• Siliceous ooze lithifies into diatomaceous earth.

Calcium Carbonate in Biogenic Sediments

• Coccolithophores:

  • Also called nannoplankton

  • Photosynthetic algae

  • Coccoliths: individual plates from dead organism

  • Rock chalk: lithified coccolith-rich ooze

• Foraminifera

Distribution of Biogenous Sediments

• Productivity

• Destruction

• Dilution

Neritic Deposits

• Dominated by lithogenous sediment, may contain biogenous sediment

• Carbonate Deposits

  • Carbonate minerals containing $(CO_3)$

  • Marine carbonates primarily limestone – $(CaCO_3)$

  • Most limestones contain fossil shells suggesting biogenous origin.

  • Ancient marine carbonates constitute 25% of all sedimentary rocks on Earth.

Carbonate Deposits: Stromatolites

• Fine layers of algae and carbonate.

Calcareous Ooze and the CCD

• CCD: Calcite compensation depth

  • Depth where $(CaCO_3)$ 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 $(CaCO_3)$ begins to dissolve rapidly

• Ocean pressure increases and the properties of seawater change below the CCD, affecting where calcite dissolves and where it is deposited.

Calcareous Ooze and the CCD Details

• Above the CCD, calcite is stable and not dissolved.

• Below the CCD, ocean conditions cause calcite to dissolve rapidly.

• Conditions below the CCD: Lower temperature, higher pressure, high $(CO_2)$, low pH (more acidic)

Sea Floor Spreading and Sediment Accumulation

• Calcite-secreting organisms live in warm surface waters.

• Silica-secreting organisms live in cold surface waters created by upwelling and associated with high productivity.

• Biogenous and fine-lithogenous particles settle toward ocean floor.

• 1 Calcareous ooze deposited on the MOR above the CCD.

• 2 Calcareous ooze is covered and protected.

• 3 Sea floor spreading moves calcareous ooze beneath the CCD into deep water.

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

• Contain: Iron $(Fe)$, Nickel $(Ni)$, Copper $(Cu)$, Zinc $(Zn)$, Silver $(Ag)$, other metals…

• Associated with hydrothermal vents

Evaporites

• Minerals that form when seawater evaporates.

• Restricted open ocean circulation.

• High evaporation rates.

• Halite (common table salt) and gypsum.

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

• Distribution controlled by

  • Proximity to sources of lithogenous sediments – can this vary?

  • Productivity of microscopic marine organisms – can this vary?

  • Depth of water – can this vary?

  • Sea floor features – can these vary?

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 [Doo-Doo!]

• Fecal pellets large enough to sink quickly (10-15 days)

Worldwide Marine Sediment Thickness

• Thick sediment accumulates near mouths of major rivers and on continental shelves, especially.

• Sediment is thinnest where the ocean floor is young along mid-ocean ridges.

Resources from Marine Sediments

• Energy resources

  • Petroleum: mainly from continental shelves

  • Gas hydrates

• Sand and gravel (including tin, gold, and so on)

• Evaporative salts

• Phosphorite

• Manganese nodules and crusts

Energy Resources: Gas Hydrates

• Also called clathrates

• High pressures squeeze chilled water and gas into icelike solid

• Methane hydrates most common

Energy Resources: Gas Hydrates Details

• 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

Energy Resources: Gas Hydrates and Climate Change

• Release of sea floor methane may alter global climate.

• Warmer waters may release more methane.

• Methane release may cause underwater slope failure, leading to Tsunami hazard.

Energy Resources: Gas Hydrates Potential

• Gas hydrates may be largest store of usable energy in organic carbon form.

• Rapidly decompose at surface pressures and temperatures