Sedimentation Basics
What are the two factors that control sedimentation?
Particle size distribution of the material.
Energy conditions at the site.
How is sediment typically sorted when in stable shelf conditions?
With increasing depth, grain size decreases
What are the two sources that deep sea sediment is derived?
- External sources; terrigenous particles that are transported off the shelf
- Internal sources; particles produced within the deep-sea water column
What causes water's high heat capacity, high surface tension, and the density differences between phases?
Hydrogen Bonds
What causes water's dipole structure?
The distribution of the ions makes different sides of the molecule slightly charged, with hydrogen being slightly positive and oxygen being slightly negative.
What are the two characteristics that are used to classify sediments?
Size and Mode of Formation
What is relic sediment?
Particles would not be deposited on those areas under the present time conditions due to past conditions, including sea level rise and glaciation events.
What are the three categories of processes that disperse sediment to the deep seafloor?
Bulk emplacement; buildup of sediment on the edge of the shelf/slope moves downward due to gravity. All types of sediment and grain sizes can be included
Pelagic sediment; sediment settling down the water column includes fine-grained material from terrigenous dust or biogenic material
Hydrogenous sediment: Two nodules to know are Ferromanganese and Phosphorite
How does the solubility of water relate to temperature, salinty and pressure
Solubility of gases in water decreases as temperature and salinity increase, but increases with higher pressure.
Why is seawater primarily made up of sodium and chloride?
This is due to the residence time of the elements in seawater or the average time that an ion spends in solution before it is removed.
- Chloride and Sodium have high residence times since they have low geochemical and biochemical reactivity.
- Lower residence time ions are more reactive.
What are the five modes of formation for sediment particles?
Terrigenous Sediment - Fine and coarse grains that are produced by the weathering and erosion of rocks on land.
Biogenous sediment - Fine and coarse grains that are derived from the hardest parts of organisms, such as shells and skeletal debris. Material is usually calcium carbonate or silica.
Hydrogenous sediment - Particles that are precipitated by chemical or biochemical reactions in seawater near the seafloor.
Volcanogenous sediment - Particles that are ejected from volcanoes.
Cosmogenous sediment - Very tiny grains that originate from outer space. Tend to be mixed into terrigenous and biogenous sediment
Why is most of the shelf sediment considered relic sediment now?
Due to the rise in sea level during the Holocene, there is no time for the shelf sediment to retain the new steady state conditions.
Describe each of these types of bulk transport:
- Slumps
- Slurries
- Ice rafting
- Slumps: Sediment mass that slides downward intact with no internal deformation, but most of the time changes the orientation of the original folding
- Slurries: Debris flows and mud flows, where the original internal structure is destroyed due to mixing, transporting shelf sediment to the deep ocean floor. Most of these slurries are transported by turbidity currents, forming deep-sea fan-like deposits with normal particle distribution and sorting.
- Ice rafting: Sediment entrained in glaciers and ice sheets is transported when icebergs are formed off those glaciers and ice sheets. Iceberg then melts and the sediment is released and settles on the sea floor
How do salinity and temperature change with the different latitudes?
Temperature: Decreases as we get to the poles. Highest at the equator
Salinity: High salinity at 20 - 30 degrees latitudes
- Evaporation -> increased salinity
- Ice formation -> increased salinity
- Freshwater Runoff/Precipitation -> decreased salinity
What are the five major solutes in seawater?
Major constituents, Nutrients, Gases, Trace Elements, and Organic Compounds
What would be the difference between sediment close to an input source vs far from an input source?
Sediment particles close to the input source would be more angular and less sorted.
-Due to high sedimentation rates and lower transport times, not allowing particles to be sorted and eroded.
Sediment particles far from the input source would be more rounded and more sorted.
- Due to low sedimentation rates and higher transport times, allowing particles to be sorted and eroded over time.
How do the energy conditions of the waves and tides influence shelf sedimentation?
From the shoreline, where high-energy conditions occur and reach the bottom, all the fine sediment is suspended and removed, allowing only gravel and sand to be deposited.
Describe the two types of pelagic sediment that can settle in deep-sea sediment.
Pelagic Clays: Composed of various minerals with kaolinite and chlorite in the clay fraction
- Kaolinite:chlorite ratio indicates the source regions of the pelagic material, as warm, moist acidic soils favor kaolinite formation and destroy chlorite.
Biological Oozes: 30% or more of biogenic material in typically inorganic mud, mainly consisting of skeletal debris from organisms.
- Calcareous Oozes: Settles as fecal pellets or particles. These calcium carbonate particles dissolve below 4-5 km, the carbonate compensation depth, depending on acidity, temperature, and pressure.
- Siliceous Oozes: Consists of remains of radiolarians and diatoms that grow hard silica parts and rapidly in nutrient-rich waters. These particles will deposit everywhere because they dissolve slowly, so they are more often found in high-productivity areas.
Why is there an oxygen minimum layer between the surface and deep water masses?
The surface layer can receive oxygen from diffusion and photosynthesis
Respiration from organisms and decomposition of photosynthesizing organisms both use oxygen and create a layer of minimal oxygen.
Deep water is composed of surface water from the polar surface, which results in a slow increase of oxygen the deeper you go.
How does the CO2 system shift to reestablish the equilibrium when disrupted?
- Reactions shift to the left to counter CO2 removal during photosynthesis, turning carbonate ions into CO2 and water, decreasing acidity
- Reactions shift to the right to counter the CO2 increase during respiration, turning CO2 into carbonate ions and H+ ions, increasing acidity.
CO2 + H2O <-> H2CO3 <-> HCO3- + H+ <-> CO32- + H+
What does the Hjulström diagram describe?
Using the diagram, what happens with these three sediment sizes at these energy conditions;
- Sand (0.1 mm)at 100 cm/s velocity?
- Silt (0.01 mm) at 10 cm/s velocity?
- Gravel (10 mm) at 1 cm/s velocity?
It describes the relationship between particle size and the current velocity, indicating what happens to a certain particle size under specific conditions.
- Erosion
- Transportation
- Deposition
How do different locations, specifically latitudes, impact the formation of shelf sediment?
- From the equator upward to the subtropics, we see biogenic sediment from coral reefs and a buildup of carbonate sediment from organisms.
- From middle to temporal latitudes, we see more river input, creating more terrigenous sediment.
- Northern latitudes are mostly composed of poorly sorted glacial deposits
Label the types of bottom sediment in the key shown on the world map.
1. Calcareous Oozes
2. Siliceous Oozes
3. Red Clay
4. Terrigenous Sediment
5. Glacial-Marine Sediment
6. Continental Shelf-Deposits
What is stratification, and the three water masses based on this phenomenon?
Layering of water masses due to density differences resulting from temperature and salinity. Lower temperature and higher salinity increase density.
- Surface layer; 2% of volume, about 100 m, and affected by weather/climate.
- Pycnocline layer; 18% of volume, transition zone, constant in low latitudes due to constant heating, mid latitudes, the pycnocline matches the halocline.
- Deep layer; 80% of volume, originates in high latitudes by cooling and sinking of surface water
What is the main source of salt in seawater, as well as some sinks that remove salt from seawater?
Source is primarily rivers and weathering rocks.
Sinks include:
- Evaporation of water leads to a brine and supersaturation and subsequent precipitation
- Formation of salt layers in the sediment (Red Sea)
- Sea spray by onshore winds
- Adsorption to clay minerals
- Formation of nodules
- Precipitated as organism shells
- Removal as fecal pellets