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![Page 1: Andrea Koschinsky General Geo-Astro II Andrea Koschinsky Chemical Oceanography: Hydrothermalism The Carbonate System.](https://reader036.fdocuments.in/reader036/viewer/2022081519/56649f315503460f94c4d592/html5/thumbnails/1.jpg)
General Geo-Astro II
AndreaAndrea KoschinskyKoschinsky
Chemical Oceanography:Chemical Oceanography: • Hydrothermalism
• The Carbonate System
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Mid-ocean ridge systems with volcanic and tectonic activity
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• Known hydrothermal vents along the spreading axes of the Earth• Six different biogeographic provinces
Global occurrence of hydrothermal systems and biogeographic provinces
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Principle of a hydrothermal circulation cell
a
mixing
Cold seawater
Hot endmember fluid (up to 400°C)
Magma chamber
Pillow lava
Sheetflow lava
Conductive cooling
White (200-300°C) and Black (up to 400°C) Smoker
Plume
Diffuse fluids (<100°C)
Hydrothermal habitats
Cooling by mixing
Mineral precipita-tion
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aa
WärmequelleBrine
a
BasaltischeKruste
Modified after a model of Halbach et al. (2003)
a
ÜberkritischePhasenseparationEintragmagmatischervolatilerPhasenHochtemperatur-Reaktionszone > 400°C
aa
Schornsteine ausFe-, Cu- und Zn-Sulfiden u.a.Fluid: heiß, sauer,reduzierend,angereichert anMetallen, Gasen u.a.StoffenCa2+ + SO4
2- CaSO4
Mg2+ + Basalt Mg(OH)xSiyOz + H+
SO42- H2S
Basalt Fe2+, Mn2+, Cu2+, Zn2+ u.a. Metallionen
a
Meerwasser ca. 2°CAufladezone
aa
AufstiegszoneUnterkritischePhasenseparation möglichFe2+ + H2S FeS + 2H+
FeS + H2S FeS2 + H2
Sinks and sources of elements in a hydrothermal cell
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Hydrothermal fluids as media for the transport of material and energy
Geological setting
Development of hydrothermal ecosystems
Physical and chemical
properties of hydrothermal
fluids
Export into the oceanic water
column
Precipitation of minerals (sulfides, sulfates, oxides, ...)
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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)
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Composition and characteristics of hydrothermal fluids
Variables for chemical control of hydrothermals fluids:
1. p-T conditionsImportant: 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
4. 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
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Fluxes into the hydrosphere: the plume
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Lupton, 1995 (Seafloor Hydrothermal Systems)
Chemical signals and processes in hydrothermal plumes
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Chemical signals and processes in hydrothermal plumes
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Temporal variability of hydrothermal
fluxes
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Hydrothermal element fluxes
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Fluxes into the hydrosphere: the plume
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Hydrothermal sulfide deposits
Picture: S. Petersen
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Hydrothermal Mn-Fe oxides
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Hydrothermal sediments
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Hydrothermal ecosystems; the trophic levels
Primary consumers- Filterer and particle grazer
- Symbiotic hosts
Secondaryconsumers- Carnivores
Hydrothermal fluids as sources for material and energy
Primary productivity: MicroorganismsChemosynthesis
- Free-living cloud- and mat-forming organisms- Symbiotic bacteria
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Hydrothermal ecosystems
Mussels covered with bacteria, and with symbiotic bacteria in their gills
Vent fish
Tube worms with crab
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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.
Picture: http://people.cornellcollege.edu/d-waite1/geo105/chemosynthesis.htm
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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.
Hydrothermal origin of life?
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Extraterrestrial hydrothermal systems?
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.
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.
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Io is the volcanically most active body of our solar system - a possible source of energy for life. However, it seems to lack water.
Extraterrestrial hydrothermal systems?
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.
Photo: NASA
Photo: NASA
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CO2 as greenhouse gas - global warmingOceans regulate the atmospheric CO2 concentrations
We are in the middle of a global experiment in which several geochemical cycles are being pertubed.
The Marine Carbonate System
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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 acidHCO3
- = bicarbonateCO3
- = carbonateH+ = proton
Total dissoved inorganic carbon = ∑CO2
River water
63
Carbonate cycle in seawater
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Individual components and reactions of the carbonate cycle:
1. CO2 (g) <--> CO2 (aq) Air-sea exchange of CO2
2. CO2 (aq) + CO32- <--> 2 HCO3
- Very fast reaction
3. CO2 (aq) + H2O --> “CH2O” + O2 Photosynthesis, “CH2O” = organic material
4. CO2 (aq) + H2O <--> H2CO3 Hydration to carbonic acid
1. H2CO3<--> H+ + HCO3- First ionization
2. HCO3- <--> H+ + CO3
2- 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].
Carbonate cycle in seawater
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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])
Carbonate cycle in seawater
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Carbonate cycle in seawater
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Carbonate cycle in seawater
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The Biological pumpRapid 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 energy6CO2 + 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.
Carbonate cycle in seawater
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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
Carbonate cycle in seawater
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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).
Carbonate cycle in seawater
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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.
Carbonate cycle in seawater
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Carbonate sedimentation
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Carbonate sedimentation
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Summary
Carbonate cycle in seawater
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Summary
Carbonate cycle in seawater