Marine Sedimentation - Chapter 4

Introduction to Marine Ecology & Oceanography

Marine Sedimentation - Chapter 4

Materials of the Sea Floor
Objectives:

  • To better understand the marine and terrestrial processes that determine the distribution of sediments and mineral resources on the sea floor.

  • To appreciate the role that shell-building organisms play in the accumulation of important sediment types and in moderating the chemistry of the oceans.

Overview of Sedimentary Materials:

  • Variety of sedimentary materials:

    • Rock, mineral, and organic particles transported by wind, water, or ice.

    • Minor contributions from outer space in the form of tiny meteorites.

    • Pelagic realm of the sea is covered by microscopic shells or "tests" of marine organisms.

    • Organic oozes composed of various materials:

    • Calcium carbonate

    • Opaline silica

    • Mixed calcareous-siliceous sediments

Classification of Marine Sediments

Marine sediments are classified into five broad groups based on the origin or source of the particles:

  1. Biogenous deposits:

    • Particles derived from biological processes.

    • Composed of shell matter, tests, and other remnants of life.

    • On deep-sea floors, they are referred to as oozes.

  2. Lithogenous deposits:

    • Inorganic sediments composed of rock and mineral fragments of terrestrial origin.

    • Terms "lithogenous" and "terrigenous" are often interchangeable.

      • "Terrigenous" usually refers to deposits on continental shelves and slopes.

      • "Lithogenous" refers to pelagic sedimentation away from shore.

    • Classified by particle size, from finest clays to very coarse sand and gravel.

    • Table 5-1 provides classifications by grain size.

  3. Hydrogenous deposits:

    • Form by chemical precipitation from seawater.

  4. Volcanogenic deposits:

    • Particles erupted from volcanoes.

    • Classified by size: volcanic ash (fine), lapilli, blocks and bombs (very large).

  5. Cosmogenous deposits:

    • Mainly micrometeorites that do not combust completely during atmospheric entry.

    • Rare and accumulate only in the deepest ocean basins far from land.

Biogenous Sediments

  • Biogenous deposits, or oozes, consist of more than 30% skeletal debris.

  • Divided into taxonomic and chemical groups:

    • Siliceous ooze: (SiO2 · nH₂O, opal)

    • Radiolarian ooze: Abundant marine protozoa (50-100 micrometers) found mainly in equatorial waters.

    • Diatom ooze: Single-celled plants (10-500 micrometers) prevalent in polar regions with nutrient-rich waters.

    • Calcareous ooze: (CaCO3, limestone, and chalk)

    • Foraminiferal ooze: Sand-sized skeletons of marine protozoans (1-300 micrometers).

    • Coccolith ooze: Tiny plates and fragments (1-20 micrometers) from one-celled plants.

    • Pteropod ooze: Shells of small pelagic gastropods (1-2 millimeters).

  • Factors affecting biogenous ooze formation:

    • Biological production in surface waters.

    • Dilution with other sediments on the sea floor.

    • Destruction of remains by scavengers or chemical solution.

  • Calcareous ooze does not form below 4500 meters due to dissolution under high pressure, referred to as the snow line or carbonate compensation depth (CCD).

    • The depth level varies with geological time affecting ocean productivity.

Lithogenous Sediments

  • Comprises approximately 38% of the deep-ocean floor and 28% of the total sea floor.

  • Sources include atmospheric dust, volcanic ash, and fine-grained rock debris.

  • They accumulate slowly, estimated at about 1 millimeter per 1000 years.

  • Terrigenous deposits:

    • Major source from large rivers, particularly from:

    • Ganges

    • Yellow

    • Yangtze rivers (contribute 1/4 of global terrigenous sediment).

    • The 20 largest rivers contribute approximately 75% of terrigenous sediment, mostly flowing into the Atlantic Ocean.

  • Characteristics of terrigenous deposits vary by location; each area represents a unique environment.

Hydrogenous Sediments

  • Manganese nodules form in areas with slow sedimentation (e.g., red clay areas).

    • Composed mostly of iron and manganese oxides, with some economic interest for copper, nickel, and cobalt.

    • Estimated to cover about 10% of the deep Pacific Ocean floor.

Suspended Sediment Transportation

  • Fine silt and clay remain suspended due to slow settling rates.

  • Biological filtering by organisms contributes to the transport and deposition of sediments as fecal pellets.

  • Nearly 90% of terrigenous sediments settle within a few hundred kilometers of their source.

  • Pelagic clays are typically wind-borne or volcanic dust.

Bottom-Transported Pelagic Sediments

  • Coarser materials reach abyssal depths via turbidity currents, which can reach speeds over 20 kilometers per hour.

  • These currents lead to the formation of turbidites characterized by a grading from coarse to fine deposits.

Interpretation of Marine Sediments

  • Sediments act as records of historical weathering processes and can eventually become sedimentary rocks.

  • Understanding sediment compositions helps locate economically valuable materials, such as oil and gas deposits.

  • Methods for studying sedimentary deposits include:

    • Acoustic methods

    • Sediment characteristic analysis from cores and grab samples

  • Original sediment layering is often disrupted by biological activity.

  • Preservation occurs in anoxic environments, which can lead to the preservation of organic matter in future oil source rocks.

Sample Collection

  • Methods of collection:

    • Dredges, weighted tubes, or samplers with spring-loaded jaws for capturing bottom materials.

  • Core samples are preferred for displaying sedimentary histories, with layers indicating conditions at each deposition period.

Sample Location and Analysis

  • Core sample descriptions in Table 5-2 categorize sediments based on the percentage of calcium carbonate and depth.

  • Figure 5-7 indicates hypothetical locations and depths for core samples, demonstrating the relationship between ocean basins and sedimentation processes.

Definitions

  • Anoxic: Without oxygen; basic condition of water or sediment lacking free oxygen due to organic matter oxidation.

  • Antarctic bottom water (AABW): Cold, saline, dense water mass formed around Antarctica, sinking to the deep ocean floor.

  • Carbonate compensation depth (CCD): Depth in the ocean where carbonate solution equals supply, varying with biological productivity and ocean circulation.

  • Minerals: Naturally occurring, inorganic, crystalline substances within a specific chemical composition.

  • Ooze: Deep-sea sediment with over 30% skeletal remains from microscopic organisms.

  • Reservoir rocks: Coarse sediment layers with high porosity that can store hydrocarbons.

  • Rocks: Aggregates of minerals categorized into three types: igneous, sedimentary, and metamorphic.

  • Sediment: Loose fragments transported through various processes.

  • Slide (mass movement): Sediment moving downslope, often triggered by external forces.

  • Source rocks: Organic-rich sediments capable of producing hydrocarbons over time.