Sediments, Sedimentation, and Paleoceanography. -Types of sediments -Distribution of ocean sediments and Processes of sedimentation -Paleoceanography

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1 Sediments, Sedimentation, and Paleoceanography -Types of sediments -Distribution of ocean sediments and Processes of sedimentation -Paleoceanography

2 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.)

3 Particle Size Poorly sorted many different particle sizes Well sorted nearly uniform particle size

4 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

5 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

6 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.

7 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

8 Map of major consituents of marine sediments

9 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

10 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.

11 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

12 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

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14 Siliceous Oozeooze of siliceous organisms (>30% siliceous) Diatomaceous ooze: tests of diatoms (phytoplankton) Radiolarian ooze- tests of radiolarians (zooplankton)

15 Diatoms

16 Radiolarians (These are rapidly evolving organisms that have been around for a long time and are often used for chronostratigraphies)

17 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

18 Diatomaceous ooze Radiolarian ooze Radiolarian ooze Diatomaceous ooze

19 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)

20 Coccolithophores

21 Foraminifera

22 Planktonic foraminifera

23 Pteropods Limacina Cavolinia

24 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

25 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

26 Stokes Law Vertical velocity of particles are related to: V g( ρ ρ ) = negative buoyancy V r V 1/ μ The size, shape, and density of particles are the most variable

27 Horizontal Displacement Depends on strength of current and particle size: Even small particles appear to be deposited close to source, suggesting clumping.

28 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

29 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

30 From zooplankton

31 Understanding sedimentation and paleoceanography requires understanding marine biology and marine biologists

32 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

33 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 ~ m depending on location.

34 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.

35 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)

36 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. ~ m thick

37 Summary of Sediments

38 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

39 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

40 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)

41 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

42 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

43 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).

44 Piston coring is easier and less expensive than ocean drilling and can recover many meters of sediment

45 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

46 Oxygen isotopes in planktonic and benthic foraminifera Oxygen isotopes: -Most oxygen atoms have 8 protons and 8 neutrons (oxygen %) -Some oxygen atoms have 8 protons and 9 neutrons (oxygen %) -Or 8 protons and 10 neutrons (oxygen %) 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

47 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

48 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

49 So, sediments tell about evolution of the earth, climate, and biology of many taxa