yahelco11 12 ocean compost ion - accumulation hypothesis minimized

Chemical Oceanography - 10

What control the (mean) chemical composition of the ocean (I)

Gitai Yahel The School of Marine Sciences

Ruppin Academic Center

[email protected].(09)830 4110Skype gitaiyahelWeb http://Moodle.Ruppin.ac.il

http://www.si.umich.edu/Space/browser/Space/1007/10074383.gif

http://moodle.ruppin.ac.il/

Tuesday, April 11, 2023 Chemical Oceanography, [email protected] 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, [email protected] (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


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


• 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?


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


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()(


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

)(


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


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


Testing the accumulation hypothesis (II) Some more examples of Coc, C river and residence times


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


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)


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

yahelco11 12 ocean compost ion - accumulation hypothesis minimized

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