yahelco11 12 ocean compost ion - accumulation hypothesis minimized
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Chemical Oceanography - 10
What control the (mean) chemical composition of the ocean (I)
Gitai Yahel The School of Marine Sciences
Ruppin Academic Center
Yahel@ruppin.ac.ilTel.(09)830 4110Skype gitaiyahelWeb http://Moodle.Ruppin.ac.il
http://www.si.umich.edu/Space/browser/Space/1007/10074383.gif
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@Ruppin.ac.il 2
Quantitative models a reminder from week 1
• Explanation Working hypothesis (qualitative)• Model formalized process description
o Set of equation and constraintso Predictionso Comparison to the data
• Quantitative treatments are possibleo Stoichiometrico Mass balanceo Predictions of reaction rates and extentso Age estimateso Paleotemperatures
• Chemical changes reflect controlling processes and the integrated net effect of multiple previous events. o most of the ocean is
inaccessible to direct observation• Past environmental records are preserved in marine
sediments.
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (3)
What controls the composition of the ocean?(Or why is the sea that salty?)• Constant Input
(rivers, wind dust, volcanoes, hydrothermal activity)• Three hypotheses:
1. Accumulation oceanic concentrations represent the accumulated inflow (mostly from rivers) since the ocean came into existence. i.e., seawater are concentrated river water
2. Equilibrium oceanic composition is controlled by chemical equilibria between seawater the solid and gas phase it reacts with (Sillén, 1961)1. Water2. The atmosphere3. The solid particles of continental origin that sink though the water
column, the sediments, and the oceanic crust that underlies the ocean
3. Dynamic flux ocean composition is a results from a balance between the input to the ocean from external sources and the rate of removal (Kinetic model, Broecker, 1971)
Alexander river flooding into the sea
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (4)
1. Accumulation hypothesisComparison of the composition of rain, river, and sea water
Rain
River
Ocean
* Note the scale difference!
Mono lake, CA, USAAn evaporative basin of river water.Alkaline (pH 10) Soda lakes (Bicarbonate)
Van lake, southeast Turkey
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (5)
• Most of world’s rivers are rich in (bi)carbonates (and fall close to the
HCO3- = 2Ca2
+ line)
• Rain water are acidic (pH<6)
o Carbonic acid
o Sulfuric acid
• Both igneous and sedimentary rock are reach in calcium (Ca)
• Acidic rain water are weathering carbonate minerals in rocks and soils
2. Accumulation hypothesis What control river water composition?
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (6)
Testing the accumulation hypothesisA simple box model of the ocean (I)
• Let us look at the changes in the concentration of element A
o denoted as [A] or Coc
o units of mmol m-3
• Total amount of A in the ocean:
Ocean volume (m3) ● Ocean mean concentration (mmol m-3)
• The rate of accumulation (or change) of A in the ocean is:
Rate of Accumulation
Rate of Concentration change
LossesInputs
t
VC
t
M OCOCAOC
ococAoc VCM
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (7)
A simple box model of the ocean (II)• For the accumulation hypothesis
o We assumes no loses of Ao We assumes that rivers are the only
major source of Ao Therefore:
River input =River flow (m3 s-1) * River
concentration
riverCriver• And since the ocean volume is relatively
constant
The solution of this differential equation is for the change in concentration in the time interval Δt between t and t=0.
riversriversOC
OC Ct
CV
tV
CCC
oc
riverriverococ tt
)0()(
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (8)
A simple box model of the ocean (III)
• If we assume that the ocean came into being at t=0 as essentially fresh water
o Coc(t=0)=0
• Than:
• Thus our prediction iso Coc Criver
i.e., today’s mean ocean concentration of any element A is directly proportional to its river concentration Criver.
o Hence the ratio of various elements in the ocean should be equal to the ratio of these elements in rivers.
tV
CCC
oc
riverriverococ tt
)0()(
tV
CC
oc
riverriveroc t
)(
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (9)
A simple box model of the ocean (IV) - more predictions:
• Lets define for each element an accumulation age:a=Δt
• a is the time (Δt) it took for the buildup of current oceanic inventory of element A a = the age of the ocean
• PredictionsIf the accumulation hypothesis is correct:
o a ~ the age of the oceano a should be ~equal for all elements
• Thus
Ocean Volume: Voc=1.291018 m3
Rivers flow: river=3.71013 m3 yr-1
river
oca
oc
riverriveroc C
Ctt
V
CC
river
ocV
t
)(
river
oc
river
oca C
Cyr
C
Cyrmm
500,341313
318
107.3
109.2
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (10)
Testing the accumulation hypothesis (I)
River
Ocean
* Note the scale difference!
Let us look at few examples (mmol m-3):
Sodium (Na):
Iron (Fe):
Calcium (Ca):
Chlorine (Cl):
river
oca C
Cyr 500,34
yr 52,600,000315
670,480500,34 yrNa
yr 35700
7.0500,34 yrFe
yr 1,000,000364
600,10500,34 yrCa
yr ,900,00083230
520,559500,34 yrCa
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (11)
Testing the accumulation hypothesis (II) Some more examples of Coc, C river and residence times
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (12)
Testing the accumulation hypothesis (III)
• Our Prediction for the accumulation hypothesis were:1. a ~ the age of the ocean
2. a should ~equal for all elements
3. Coc Criver
• Looking at the former exampleso All of the accumulation times are at least a factor of 30
less than what we think the age of the ocean is (3.85 billion years).
o The accumulation times for different elements vary by almost eight orders of magnitude
o Clearly Coc Criver
Conclusion reject the accumulation hypothesis
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (13)
2. Thermodynamic equilibria hypothesis
• Goldschmidt (1933) – Ocean and atmosphere formation:
Igneous rock (0.6kg) + Volatiles (1kg) Seawater (1 L) + sediments (0.6kg) + air (3 L)
• Sillén (1959, 1961)o Ocean concentrations are determined by thermodynamic
equilibria between seawater and mineral phaseo Nine component model (C = 9)
Acids: HCl, H2O, CO2
Bases: KOH, CaO, SiO2, NaOH, MgO, Al(OH)3
o The ocean chemistry results from a giant acid-base titration
o If these phases are at equilibrium and you fixed the temperature and Cl then the seawater composition is fixed
Atmosphere (gas)
Seawater (solution)
Sediment (solid)
Tuesday, April 11, 2023 Chemical Oceanography, Yahel@ruppin.ac.il (14)
2. Thermodynamic equilibria hypothesis• Sillén (1961) - Ocean concentrations are determined by
thermodynamic equilibria between seawater and mineral phase
• Observations that are in disagreement with model predictions:o Concentration vary with changes of inputs (e.g., during
glaciations) o seawater is usually under-saturated with respect to minerals
in deep-sea sedimentso Some of the postulated mineral phases do not exist
Element % in continental crust
Conc. in seawater (g L-1)
g rock required (assuming 75% efficiency)
Na 2.4 10.76 597
K 2.1 0.387 25
Ca 4.1 0.413 42
Mg 2.3 1.294 75
Sr 0.038 0.008 28
Conclusion reject the equilibrium hypothesiso The concentrations of some elements may be controlled by a
hybrid of the kinetic control and equilibrium mechanisms
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