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General Geo-Astro II Andrea Koschinsky

General Geo-Astro II Andrea Koschinsky. Chemical Oceanography: Hydrothermalism The Carbonate System. Mid-ocean ridge systems with volcanic and tectonic activity. Global occurrence of hydrothermal systems and biogeographic provinces.

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General Geo-Astro II Andrea Koschinsky

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  1. General Geo-Astro IIAndreaKoschinsky Chemical Oceanography: • Hydrothermalism • The Carbonate System

  2. Mid-ocean ridge systems with volcanic and tectonic activity

  3. Global occurrence of hydrothermal systems and biogeographic provinces • Known hydrothermal vents along the spreading axes of the Earth • Six different biogeographic provinces

  4. Hydrothermal habitats Plume Diffuse fluids (<100°C) White (200-300°C) and Black (up to 400°C) Smoker Conductive cooling Cooling by mixing Mineral precipita-tion Hot endmember fluid (up to 400°C) Cold seawater Principle of a hydrothermal circulation cell Pillow lava Sheetflow lava Magma chamber

  5. Fe2+ + H2S FeS + 2H+ FeS + H2S FeS2 + H2 Ca2+ + SO42- CaSO4 SO42- H2S Mg2+ + Basalt Mg(OH)xSiyOz + H+ Basalt Fe2+, Mn2+,Cu2+, Zn2+ u.a. Metallionen Sinks and sources of elements in a hydrothermal cell Modified after a model of Halbach et al. (2003)

  6. Export into the oceanic water column Physical and chemical properties of hydrothermal fluids Precipitation of minerals (sulfides, sulfates, oxides, ...) Development of hydrothermal ecosystems Hydrothermal fluids as media for the transport of material and energy Geological setting

  7. Composition and characteristics of hydrothermal fluids - Temperature: up to 400°C - Pressure: depends on water depth (mostly 100-300 bar) - pH value: mostly acidic (pH 2-6) - Redox potential: reducing - Salinity: 1/10 to >2-fold seawater salinity (--> boiling) - Gas content: high concentrations of methane, hydrogen sulfide, carbon dioxide, hydrogen, helium - Ion content: some ions are depleted compared to seawater (such as Mg, sulfate, partly alkali metals) most metals are strongly enriched (Mn, Fe up to 106-fold)

  8. Composition and characteristics of hydrothermal fluids • Variables for chemical control of hydrothermals fluids: • 1. p-T conditions • Important: p and T in the subseafloor reaction cell and at the seaflorr • 2. Boiling and Phase separation: Separation of gases and salts + metals, and phase segregation (spatial separation of vapor and brine) • 3. Chemical composition and mineralogy of the rock, alteration state • Ratio water/rock • Degassing of magma (important for gases CO2, 3He) • 6. Time; largely unknown, how long fluids remain on the respective T-p paths

  9. Fluxes into the hydrosphere: the plume

  10. Chemical signals and processes in hydrothermal plumes Lupton, 1995 (Seafloor Hydrothermal Systems)

  11. Chemical signals and processes in hydrothermal plumes

  12. Temporal variability of hydrothermal fluxes

  13. Hydrothermal element fluxes

  14. Fluxes into the hydrosphere: the plume

  15. Hydrothermal sulfide deposits Picture: S. Petersen

  16. Hydrothermal Mn-Fe oxides

  17. Hydrothermal sediments

  18. Primary productivity: Microorganisms Chemosynthesis - Free-living cloud- and mat-forming organisms - Symbiotic bacteria Hydrothermal ecosystems; the trophic levels Secondary consumers - Carnivores Primary consumers - Filterer and particle grazer - Symbiotic hosts Hydrothermal fluids as sources for material and energy

  19. Hydrothermal ecosystems Mussels covered with bacteria, and with symbiotic bacteria in their gills Vent fish Tube worms with crab

  20. Picture: http://people.cornellcollege.edu/d-waite1/geo105/chemosynthesis.htm Interactions between fluids and organisms: Chemosynthesis Chemosynthesis produces the same nutrients as photosynthesis, but it does by means of using chemical energy from hydrogen sulfide, hydrogen, methane and other compounds instead of energy from the sun.

  21. Hydrothermal origin of life? It is assumed that the biology and ecology of hydrothermal organisms may provide clues to the origins of life on Earth and, possibly, on other worlds. Conditions in our planet’s primordial seas may have been similar to those surrounding hydrothermal vents, favoring the birth and evolution of extremophilic organisms.

  22. Photos from the CO2-ice covered polar caps indicate that the C02 ice erodes, adding carbon dioxide to the Martian atmosphere. This greenhouse effect would eventually warm the whole planet enough for water to return to the Martian surface. In the past, Mars had a thicker atmosphere.  Geothermal areas may have been conducive to life. Mars was once awash with great basins of water, but the water is thought to have disappeared or become subsurface ice as the planet cooled. Extraterrestrial hydrothermal systems?

  23. Photo: NASA Europa’s surface is completely covered with ice. Under the 100 km thick ice sheet the existence of a large ocean is assumed. Europa's surface is -145°C cold. However, it is possible that hydrothermal vents, are spewing energy and chemicals into Europa's ocean. Io is the volcanically most active body of our solar system - a possible source of energy for life. However, it seems to lack water. Photo: NASA Extraterrestrial hydrothermal systems?

  24. The Marine Carbonate System CO2 as greenhouse gas - global warming Oceans regulate the atmospheric CO2 concentrations We are in the middle of a global experiment in which several geochemical cycles are being pertubed.

  25. Carbonate cycle in seawater CO2 gas is more soluble in cold water than in hot water, and its solubility increases with pressure. CO2 gas combines with water molecules to produce a weak acid (carbonic acid), which then dissociates to produce hydrogen and bicarbonate ions: CO2 gas + H2O = H2CO3 = H+ + HCO3- HCO3- = H+ + CO3- A large proportion of bicarbonate comes from river water (weathering of sedimentary rocks) H2CO3 = carbonic acid HCO3- = bicarbonate CO3- = carbonate H+ = proton Total dissoved inorganic carbon = ∑CO2 River water 63

  26. Carbonate cycle in seawater • Individual components and reactions of the carbonate cycle: • CO2 (g) <--> CO2 (aq)Air-sea exchange of CO2 • CO2 (aq) + CO32- <--> 2 HCO3- Very fast reaction • CO2 (aq) + H2O --> “CH2O” + O2 Photosynthesis, “CH2O” = organic material 4. CO2 (aq) + H2O <--> H2CO3 Hydration to carbonic acid • H2CO3<--> H+ + HCO3- First ionization • HCO3- <--> H+ + CO32- Second ionization Total dissolved inorganic carbon DIC = [HCO3-] + [CO32-] + [CO2] + [H2CO3] At pH around 8, less than 1 % of the DIC exists as [CO2] + [H2CO3].

  27. Carbonate cycle in seawater Distribution of inorganic carbonate species in seawater in relation to pH: Nearly all carbon dioxide in seawater is in the form of bicarbonate and carbonate Buffering capacity of sea water: pH of sea water = 8 ± 0.5 Dissociation of carbonic acid (weak acid - conjugate base equilibrium) forms a buffering system: H2CO3<--> H+ + HCO3- --> K0 pH = pK0 + log([HCO3-] / H2CO3])

  28. Carbonate cycle in seawater

  29. Carbonate cycle in seawater

  30. Carbonate cycle in seawater The Biological pump Rapid descent through the water column is only the first step towards the conversion of calcarous skeletal material into carbonate sediments at the sea bed. The chemistry of the deep ocean determines whether or not this conversion occurs. The basic equation that describes photosynthesis can be written as follows: light energy 6CO2 + 6H20 ------------------ C6H12O6 + 6O2 chlorophyll Due to photosynthesis the upper ocean waters are generally undersaturated in CO2 When the biological pump is active, and particles sink towards the sea floor, organic tissue and hardshells are destroyed. CO2 is released again.

  31. Carbonate cycle in seawater As total dissoved inorganic carbon ∑CO2 increases, the ratio of bicarbionate and carbonate increases and so does H+, i.e. there are more hydrogen ions and the water becomes more acid (pH decreases) Then dissolution of CaCO3 (calcium carbonate skeletons) occurs. CaCO3 + H+ ---> Ca 2+ + HCO3 - ∑CO2 increases Degradation of organic tissue ∑CO2 increases pH decreases

  32. Carbonate cycle in seawater The Lysocline and Carbonate compensation depth The depth at which dissolution of carbonate skeletons begins is called Lysocline. The depth at which the proportion of carbonate skeleton material in sediments falls below 20 % is called carbonate compensation depth (CCD).

  33. Carbonate cycle in seawater The surface waters are supersaturated and the deep waters understaturated with respect to carbonate.Aragonite becomes undersaturated at a shallower level than calcite, i.e., calcite is the stable phase at these temperatures and pressures. The oceanic distributions of carbonate ion concentration can be represented relative to the value at saturation at that same temperature and pressure.

  34. Carbonate sedimentation

  35. Carbonate sedimentation

  36. Carbonate cycle in seawater Summary

  37. Carbonate cycle in seawater Summary

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