Sediments, Sedimentation, and Paleoceanography -Types of sediments -Distribution of ocean sediments and Processes of sedimentation -Paleoceanography
Sediments Sources of sediment: 1. living organisms (biogenic: calcareous, siliceous, organic matter) 2. the land surface (terrigenous) 3. the atmosphere (wind transports terrigenous and cosmogenous) 4. the ocean (precipitates minerals- hydrogenous or authigenic) Sediments classified according to: 1. particles size (pebbles, clay, etc.) 2. Location (neritic, pelagic) 3. Origin (biogenic, lithogenic, cosmogenic, hydrogenic) 4. Chemistry (siliceous, calcite, organic, etc.)
Particle Size Poorly sorted many different particle sizes Well sorted nearly uniform particle size
Location Marine sediments are classified as: (i) neritic (coastal) poorly sorted; from erosion of rocks on land, transported to ocean by rivers. (ii) pelagic (deep ocean) fine grained, increasing thickness with distance from mid-ocean ridge
Origin and Chemistry Sediments also classified according to source and chemistry: 1. Lithogenous (terrigenous) from rocks 2. Biogenous from marine organisms 3. Hydrogenous from sea water 4. Cosmogenous from space
Hydrogenous Sediments Derived from precipitation of minerals from sea water by chemical reactions. Occurs in open ocean and in vicinity of hydrothermal vents. Cosmogeneous Sediments Material from space Small iron rich meteorites Material formed from meteoric impacts (tektites). Mn precipitates in red clay Pitted composite silicate/ Fe-oxide cosmic spherule.
Lithogenous Sediments Consist of particles from rocks eroded on land by weathering Transported to ocean as dust (via atmos), ash (volcanoes), glaciers (boulders, cobbles, ice rafted debris) and rivers. Dominate most neritic sediments (except where biological productivity is very high) Pelagic lithogenous sediments are abyssal red clays (rich in iron) from dust they dominate in regions of low productivity (eg central gyres) Dust storm off Saharan desert Glacial sediments
Map of major consituents of marine sediments
Biogenous Sediments Shells, coral fragments, and hard skeletons (test, frustules) of phytoplankton and zooplankton. -Tests can be calcareous (CaCO 3 ) or siliceous (SiO 2 ) Organic matter -In addition to tests, cellulose, carbohydrates, proteins, fats and nucleic acids A sample from a sediment trap showing mostly large particles consisting of foraminifera, diatoms, fecal pellets, and marine snow
Plankton Tests Plankton tests (cell walls) form part of sediment: - Silica Tests: diatoms (phytoplankton) and radiolarians (zooplankton) have silica tests - Calcium Carbonate tests coccolithophores (phytoplankton) and foraminifera (zooplankton) pteropods (zooplankton) have calcium carbonate tests Other zooplankton are heterotrophic and consume phytoplankton, expelling fecal pellets composed of tests and organic matter.
Phytoplankton Dinoflagellates Reminder- not all phytoplankton have cell walls (tests) that are preserved in ocean sediments, but they can make up organic matter cyanobacteria These groups of phytoplankton have cell walls (tests) that are preserved in ocean sediments Coccolithophorids Diatoms Silicoflagellates
Ooze Pelagic sediments > 30% biogenous material are called oozes Calcareous ooze or Siliceous oozes Ooze type and distribution depends on: 1. Supply of organisms from above 2. Rate of dissolution 3. Depth of deposit 4. Dilution by other sediment Different oozes identified by the Challenger expedition
Siliceous Oozeooze of siliceous organisms (>30% siliceous) Diatomaceous ooze: tests of diatoms (phytoplankton) Radiolarian ooze- tests of radiolarians (zooplankton)
Diatoms
Radiolarians (These are rapidly evolving organisms that have been around for a long time and are often used for chronostratigraphies)
Silica Silica is undersaturated everywhere so dissolves at all depths, but more slowly in deep, cold ocean. Silica is only preserved below regions of very high productivity (rapid burial): - diatoms thrive at high lats - radiolarians thrive in tropics
Diatomaceous ooze Radiolarian ooze Radiolarian ooze Diatomaceous ooze
Calcareous Ooze Dominant in pelagic sediments: - coccolithophorid ooze - tests of coccolithophorids (phytoplankton, < 20μm) - pteropod ooze tests of pteropods (zooplankton snails, 1-10mm) - foraminiferan ooze tests of foraminifera (zooplankton, 30μm-1mm) Dissolves more readily with greater depth and in the presence of higher CO2- the Carbonate Compensation Depth (CCD)
Coccolithophores
Foraminifera
Planktonic foraminifera
Pteropods Limacina Cavolinia
Zooplankton Most zooplankton (except foraminifera, radiolarians and pteropods) don t leave hard parts in the sediment (but can leave other organic matter). But many zooplankton play an important role in the transport of different sediment types to the ocean floor
Particles Sinking Rates Particle size influences: (i) the rate at which particles sink (Stokes Law) (ii) horizontal displacement before reaching ocean floor Stokes Law: V=(2/9)[g(ρ 1 ρ 2 )/μ]r 2 (cm s -1 ) Shape factor for sphere g=acceleration due to gravity=981 cm s -1 ρ 1 =particle density=2.62 g cm -3 ρ 2 =density of sea water=1.028 g cm -3 μ=dynamic viscosity of sea water=1.3x10-2 g cm -1 s -1 r=particle radius V=2.67X10 4 r 2 for r <=0.0125cm
Stokes Law Vertical velocity of particles are related to: V g( ρ ρ ) = negative buoyancy V r 2 1 2 V 1/ μ The size, shape, and density of particles are the most variable
Horizontal Displacement Depends on strength of current and particle size: Even small particles appear to be deposited close to source, suggesting clumping.
How are small sediments aggregated and rapidly transported to the seafloor? Marine snow (aggregates of many different particles) Fecal pellets (copepods, salps, other zooplankton) Tests are relatively large and dense and often large so sink rapidly on their own) A sample from a sediment trap showing mostly large particles consisting of foraminifera, diatoms, fecal pellets, and marine snow
Marine snow Marine snow: An aggregate of many different sources of organic and inorganic matter, much of which is sticky (often from gelatinous zooplankton) and thus the aggregate grows as it sinks and collides with other particles
From zooplankton
Understanding sedimentation and paleoceanography requires understanding marine biology and marine biologists
Carbonates: coccolithophorids, pteropods, foraminifera Carbonate Compensation Depth (CCD): A Marine Snowline (but not to be confused with marine snow) Like snowfall, there is a rain of carbonates from the ocean surface to the seafloor. Snow only remains on the ground where it doesn t melt. Carbonates remain on the seafloor where they don t dissolve
The Lysocline and CCD Dissolution of CaCO 3 varies with depth and temperature. CaCO 3 dissolves more rapidly in cold, deep water: undersaturated and slightly more acidic conditions due to high CO 2 content. Lysocline - depth at which CaCO 3 first starts to dissolve. Carbonate compensation depth (CCD) depth where rate of dissolution = rate of supply from above. No CaCO 3 can be deposited below the CCD. CCD ~4500-5000m depending on location.
Distribution of CO 2 and O 2 Photosynthesis: CO 2 + H 2 O CH 2 O + O 2 decreases dissolved CO 2 and increased dissolved O 2 in surface water. Respiration CH 2 O + O 2 CO 2 + H 2 O increases dissolved CO 2 and decreased dissolved O 2 in deep water. One consequence is decreasing ph in deep water.
The Carbonate System An increase in CO 2 from fossil fuels will affect both the CCD in the future and coral reefs The CCD is affected by the amount of carbonate ions and pressure (depth)
Rate of Deposit Neritic: 1. Highly variable deposition rates 2. Major rivers (Ganges, Yangtse, Yellow and Brahmaputra ~25% of land derived sediments) 3. Also varies with ocean productivity 4. Can be ~ metres per year! Pelagic: 1. Slow deposition rates ~1 cm per 1000 yrs 2. ~500-600m thick
Summary of Sediments
Sediment accumulation on the seafloor varies with: -the type of ocean productivity above that part of the seafloor, -the depth (and thus dissolution) of carbonates and silica, -transport from the continents, -the history of that part of the seafloor (sediment
Sediment accumulation on the seafloor varies with: -the type of ocean productivity above that part of the seafloor, -the depth (and thus dissolution) of carbonates and silica, -transport from the continents, -the history of that part of the seafloor (sediment
Snapshot view of the changes in the seafloor with distance from the spreading center Map of primary productivity View of how the types of sediment accumulated on a given spot on the seafloor change with time as the seafloor moves over different regions of ocean productivity, in this case the productive equatorial region (which affects the type of sedimentation), and changes its depth from the sea surface (which affects dissolution)
Joides Resolution Ocean drilling deep beneath the ocean floor can recover hundreds of meters of sediment to reveal past changes in tectonic history and climate history
Map of core locations Tectonic history of core locations Southern Chile: High sedimentation rates from terrigenous fluxes via rivers Nazca Ridge: Low sedimentation - far from shore, low biological productivity, dissolution of calcite Equatorial regions: Moderate sedimentation rates varying with biological productivity and distance from continents
An interesting bit of history from ~40 cm of sediment near the Gulf of Mexico. Remains of an asteroid impact marking the end of the Cretaceous (and dinosaurs) ~65 million years ago. Accompanied by extinction and consequential speciation of microorganisms (foraminifera, radiolarians, diatoms, coccolithophorids).
Piston coring is easier and less expensive than ocean drilling and can recover many meters of sediment
Box coring can recover the sediment surface (without disturbance) and a greater quantity of sediment Santa Barbara Basin Sediments are deposited in an anoxic basin. The sediments are annually laminated from seasonal variations in sedimentation from terrigenous flux from the Ventura river and ocean productivity
Oxygen isotopes in planktonic and benthic foraminifera Oxygen isotopes: -Most oxygen atoms have 8 protons and 8 neutrons (oxygen 16-99.8%) -Some oxygen atoms have 8 protons and 9 neutrons (oxygen 17-0.04%) -Or 8 protons and 10 neutrons (oxygen 18-0.21%) Protons (black) Neutrons (green) planktonic foraminifera -Measure the isotope composition of the Oxygen (O) in the Calcium Carbonate (CaCO 3 ) test of the foraminifera benthic foraminifera Temperature of the water in which the foraminifera made its test Isotope composition of the water, which follows a similar pattern to salinity, and thus is affected by evaporation and precipitation
The effect of building more ice on continents (through evaporation and precipitation of mostly 16 O) and the coincidence of colder temperatures both result in heavier δ 18 O of foraminifera tests Low δ 18 O Warmer, less ice Climate variability High δ 18 O Colder, more ice Present (Holocene): Warmer, less ice- so more 16 O in ocean Holocene Last Glacial Maximum: Colder, more iceso more 16 O in ice sheets 450,000 years ago
The last 500,000 years High δ 18 O Colder, more ice Low δ 18 O Warmer, less ice Changes in earth climate over last 70 million years from δ 18 O of benthic foraminifera 70 million years ago We are in a brief warm period (Holocene) of a relatively cold period in the earth s history (Pleistocene glaciation), but its warming back up and the ice is melting
So, sediments tell about evolution of the earth, climate, and biology of many taxa