General Geo-Astro II

AndreaAndrea KoschinskyKoschinsky

Chemical Oceanography:Chemical Oceanography: • Hydrothermalism

• The Carbonate System

Mid-ocean ridge systems with volcanic and tectonic activity

• Known hydrothermal vents along the spreading axes of the Earth• Six different biogeographic provinces

Global occurrence of hydrothermal systems and biogeographic provinces

Principle of a hydrothermal circulation cell

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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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WärmequelleBrine

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BasaltischeKruste

Modified after a model of Halbach et al. (2003)

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ÜberkritischePhasenseparationEintragmagmatischervolatilerPhasenHochtemperatur-Reaktionszone > 400°C

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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

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Meerwasser ca. 2°CAufladezone

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AufstiegszoneUnterkritischePhasenseparation möglichFe2+ + H2S FeS + 2H+

FeS + H2S FeS2 + H2

Sinks and sources of elements in a hydrothermal cell

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, ...)

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)

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

Fluxes into the hydrosphere: the plume

Lupton, 1995 (Seafloor Hydrothermal Systems)

Chemical signals and processes in hydrothermal plumes

Chemical signals and processes in hydrothermal plumes

Temporal variability of hydrothermal

fluxes

Hydrothermal element fluxes

Fluxes into the hydrosphere: the plume

Hydrothermal sulfide deposits

Picture: S. Petersen

Hydrothermal Mn-Fe oxides

Hydrothermal sediments

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

Hydrothermal ecosystems

Mussels covered with bacteria, and with symbiotic bacteria in their gills

Vent fish

Tube worms with crab

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

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?

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.

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

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

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

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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

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Carbonate cycle in seawater

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

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

Carbonate cycle in seawater

Carbonate cycle in seawater

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

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

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

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

Carbonate sedimentation

Carbonate sedimentation

Summary

Carbonate cycle in seawater

Summary

Carbonate cycle in seawater