Lecture 4 What Controls the Composition of Seawater

Transcription

1 Lecture 4 What Controls the Composition of Seawater Seawater is salty! Why? What controls the composition of seawater? Do Chemical Equilibrium reactions control the composition of the Ocean? What is meant by Mass Balance Control? Sources - Rivers, Mid-Ocean Ridges (MOR) Sinks Sediments, MOR

2 Observed Mean Ocean Concentrations large range Logarithmetic: log 10 c = x c = 10 x

3 Could seawater originate by evaporation of river water?

4 River Water Sea Water Mainly Ca 2+ /HCO 3 - Mainly Na + /Cl - SiO 2 = H 4 SiO 4 º Both composition and key ratios are different

5 What Controls the Composition of Rivers?

6 Weathering Weathering of CaCO 3 is considered a congruent reaction (all solid dissolves) CaCO 3 (s) + CO 2 (g) + H 2 O = Ca HCO Weathering of alumino-silicate minerals to clay minerals are examples of incongruent reactions (solid partially dissolves) silicate minerals + CO 2 (g) + H 2 O == clay minerals + HCO H 4 SiO 4 + cation A specific reaction written in terms of CO 2 (g) KAlSi 3 O 8 (s) + CO 2 (g) + 1 1/2H 2 O (orthoclase = 1/2 Al 2 Si 2 O 5 (OH) 4 (s) + K + + HCO H 4 SiO 4 feldspar) (kaolinite) * With these reactions you could calculate how much CO 2 (g) is consumed by weathering

7 Variability in Erosion Among Continents Europe, North America and Asia are more calcareous continents. Most of variability due to Ca 2+ and HCO 3- which come from weathering of carbonate rock SO 4 2- and Cl - come from aerosols and weathering of evaporite rocks (e.g. Salt or NaCl). Na +, K +, Mg 2+, SiO 2 come from weathering silicate rocks

8 Evaporation of River water Makes a Na, HCO 3, CO 3 brine. ph is very basic. ph = -log (H + ) Examples: Mono Lake, CA Soap Lake, WA

9 Mono Lake, California Tufa Towers

10 Equilibrium approaches Some History Goldschmidt (1933) igneous rock (0.6kg) + volatiles (1kg) === seawater (1 L) + sediments (0.6kg) + air (3 L) Sillen (1959, 1961) Sources - Weathering reactions Sinks - Reverse weathering reactions Organizational framework: Gibbs Phase Rule f = c + 2 p f = degrees of freedom (variables like T,P, concentrations, e.g. Na +, Cl -, Ca 2+, SO 4 2- ) c = components (ingredients, e.g., HCl, NaOH, MgO)) p = phases at equilibrium (domains of uniform composition, e.g. gas, liquid, pure solids)

11 Sillen: Nine component model (C = 9) Acids: HCl, H 2 O, CO 2 Bases: KOH, CaO, SiO 2, NaOH, MgO, Al(OH) 3 The ocean chemistry results from a giant acid-base titration. Acids from the volcanoes and bases from the rocks. Sillen suggested that the following phases were at equilibrium. Kaolinite, illite, chlorite, montmorillonite and phillipsite are types of clay minerals If these phases at equilibrium at constant T and Cl, then the SW composition is fixed and it could only change if temperature or Cl - changed.

12 Most clays are detrital chlorite in deep-sea sediments detrital = particles of rock derived from pre-existing rock by weathering and erosion

13 illite in deep-sea sediments

14 So, an equilibrium approach doesn t work. The composition of seawater has changed in the past and The phases suggested do not appear to be at equilibrium

15 Mass Balance Approach Kinetic Model of Seawater What is the origin of seawater s composition? Sources Rivers?? Mid-Ocean Ridges?? Other?? Aerosols Sinks Sediments?? Mid-Ocean Ridges?? Other?? Aerosols

16 Chemical Weathering and the Geological Carbon Cycle 1. CO 2 is removed by weathering of silicate and carbonate rocks on land. 2. The weathering products are transported to the ocean by rivers where they are removed to the sediments. 3. When these sediments are subducted and metamorphosed at high T and P, 4. CO 2 is returned to the atmosphere. For more detail see Berner (2004) The Phanerozoic Carbon Cycle: CO 2 and O 2. Oxford Press, 150pp. Ittekkot (2003) Science 301, 56

17 Mass Balance approaches Mackenzie and Garrels 1966 proposed that the input from rivers was balanced by removal to sediments but they had to invoke a reverse weathering hypothesis for which there was (and still is) little evidence. The river inputs are given below (total amount for 10 8 y). For a steady state ocean, these have to be removed. Mackenzie and Garrels (1966) American Journal of Science, 264,

18 Mackenzie and Garrels (1966) A Chemical Mass Balance for Seawater Still need to remove: 15% of Na 90% of Mg 100% of K 90% of SiO2 42% of HCO3

19 Specific reactions proposed to remove excess ions. Newly formed clays would equal 7% of sedimentary mass.

20 Mass Balance Model Modern Version. Includes ridge crest processes.

21 How about mid-ocean ridges?? 350ºC vents have no Mg 2+, SO 4 2- or alkalinity (HCO 3- ). What s left is Cl -, Na +, Ca 2+, K +, Fe 2+

22 Hydrothermal Vent Compositions German and Von Damm (2004) Treatise on Geochemistry, Vol. 6, The Oceans and Marine Geochemistry, Elsevier

23 Kinetic model of seawater mass balance model Main input and removal fluxes for major ions in seawater (from McDuff and Morel, 1980) Note: Vr = 4.55 x 1016 L y-1 Volume of ocean = 1.37 x 1021 L

24 Group Ia Cl - short term cycle = aerosols and rivers main sink over geological time = evaporites = controlled by tectonics, geometry of marginal seas residence time is so long (~100 My) that changes are hard to see. Group Ib Mg, SO 4, probably K input from rivers ; main sink through ocean crust Thus control is mass balance: V r C r = V hydro (C sw C exit fluid ) for Mg 2+, C exit fluid = 0 thus: C sw = ( V r / V hydro ) C r = 300 C r The dominant control is V hydro, thus tectonics.

25 Group II (e.g. Ca, Na) (e.g. the remaining cations with long residence times) Consider the charge balance for seawater: 2[Ca 2+ ] + [Na + ] + 2[Mg 2+ ] + [K + ] = [HCO 3- ] + [Cl - ] + 2[SO 4 2- ] or rearranged: 2[Ca 2+ ] + [Na + ] - [HCO 3- ] = [Cl - ] + 2[SO 4 2- ] - 2[Mg 2+ ] - [K + ] This side is controlled by tectonics Therefore this sum is also controlled by tectonics The controls on the relative proportions of elements on the left hand side are complicated but include: a) Ca/Na ion exchange in estuaries b) Ca/HCO 3 regulation by calcium carbonate equilibria

26 Group III (e.g. nutrients (Si, P, C, N) and trace metals Internal cycling can be described by the simple 2-box ocean model The main balance is input from rivers and removal as biological debris to sediments V r C r = f B where f is the fraction of biogenic flux that is buried (escapes remineralization)

27 Summary Salinity of seawater is determined by the major elements. Early ideas were that the major composition was controlled by equilibrium chemistry. Modern view is of a kinetic ocean controlled by sources and sinks. River water is main source composition from weathering reactions. Evaporation of river water does not make seawater. Reverse weathering was proposed but the evidence is weak. Sediments are a major sink. Hydrothermal reactions are a major sink.

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29 Weathering Susceptibilities Minerals Weather at Different Rates

30 East Pacific Rise, from Von Damm et al., (1985) Mg Alk

31 East Pacific Rise, continued SO4 350 C vents have no Mg, SO 4 or HCO 3. What s left is Cl, Na, Ca, K, Fe

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