Andrea Koschinsky General Geo-Astro II Andrea Koschinsky 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 Cold seawater Hot endmember fluid (up to 400°C) Magma chamber Pillow lava Sheetflow lava Conductive cooling White ( °C) and Black (up to 400°C) Smoker Plume Diffuse fluids (<100°C) Hydrothermal habitats Cooling by mixing Mineral precipita- tion

Modified after a model of Halbach et al. (2003) Ca 2+ + SO 4 2- CaSO 4 Mg 2+ + Basalt Mg(OH) x Si y O z + H + SO 4 2- H 2 S Basalt Fe 2+, Mn 2+, Cu 2+, Zn 2+ u.a. Metallionen Fe 2+ + H 2 S FeS + 2H + FeS + H 2 S FeS 2 + H 2 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 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 fold)

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 4.Ratio water/rock 5.Degassing of magma (important for gases CO 2, 3 He) 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

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 Secondary consumers - Carnivores Hydrothermal fluids as sources for material and energy Primary productivity: Microorganisms Chemosynthesis - 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:

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 CO 2 -ice covered polar caps indicate that the C0 2 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

CO 2 as greenhouse gas - global warming Oceans regulate the atmospheric CO 2 concentrations We are in the middle of a global experiment in which several geochemical cycles are being pertubed. The Marine Carbonate System

CO 2 gas is more soluble in cold water than in hot water, and its solubility increases with pressure. CO 2 gas combines with water molecules to produce a weak acid (carbonic acid), which then dissociates to produce hydrogen and bicarbonate ions: CO 2 gas + H 2 O = H 2 CO 3 = H + + HCO 3 - HCO 3 - = H + + CO 3 - A large proportion of bicarbonate comes from river water (weathering of sedimentary rocks) H 2 CO 3 = carbonic acid HCO 3 - = bicarbonate CO 3 - = carbonate H + = proton Total dissoved inorganic carbon = ∑CO 2 River water 63 Carbonate cycle in seawater

Individual components and reactions of the carbonate cycle: 1.CO 2 (g) CO 2 (aq) Air-sea exchange of CO 2 2.CO 2 (aq) + CO HCO 3 - Very fast reaction 3.CO 2 (aq) + H 2 O --> “CH 2 O” + O 2 Photosynthesis, “CH 2 O” = organic material 4.CO 2 (aq) + H 2 O H 2 CO 3 Hydration to carbonic acid 5.H 2 CO 3 H + + HCO 3 - First ionization 6.HCO 3 - H + + CO 3 2- Second ionization Total dissolved inorganic carbon DIC = [HCO 3 - ] + [CO 3 2- ] + [CO 2 ] + [H 2 CO 3 ] At pH around 8, less than 1 % of the DIC exists as [CO 2 ] + [H 2 CO 3 ]. 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: H 2 CO 3 H + + HCO > K 0 pH = pK 0 + log([HCO 3 - ] / H 2 CO 3 ]) 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 6CO 2 + 6H C 6 H 12 O 6 + 6O 2 chlorophyll Due to photosynthesis the upper ocean waters are generally undersaturated in CO 2 When the biological pump is active, and particles sink towards the sea floor, organic tissue and hardshells are destroyed. CO 2 is released again. Carbonate cycle in seawater

As total dissoved inorganic carbon ∑CO 2 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 CaCO 3 (calcium carbonate skeletons) occurs. CaCO 3 + H + ---> Ca 2+ + HCO 3 - ∑CO 2 increases Degradation of organic tissue ∑CO 2 increasespH 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

Summary Carbonate cycle in seawater

Summary Carbonate cycle in seawater