Chemical Oceanography
description
Transcript of Chemical Oceanography
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Chemical Oceanography
Lecture 3: 5/30/2014
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Salinity
• Definition: weight of inorganic salts in one kg of seawater
• There are many ions and salts in seawater, but they are never the dominant mass
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Inputs Outputs
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Weathering: the physical & chemical processes that break down rock
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A simplified biogeochemical cycle
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Steady State and Equilibrium
• Draw on board
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Acidity
• pH = -log[H+]– Dissociated water moleculeH2O = H+ + OH-
In 1L of water (55.6 moles) 10-7 moles dissociated; therefore, 10-7 moles/L of both H+ and OH-
(i.e. pH = 7, pOH = 7)
• pH < 7 = acidic, pH > 7 alkaline
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Seawater Buffering, Alkalinity
• Alkalinity = measure of the amount of ions present that can react with, or neutralize, H+
– Higher alkalinity of a solution more difficult to produce a pH change by adding acid
– Alkalinity measures acid buffering capacity• Simple measure of Alkalinity (A)
A = [HCO3-] + 2[CO3
-] + [OH]- - [H+]Assumes bicarbonate, carbonate, hydroxyl ions dominate seawater alkalinity
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Seawater Buffering, Alkalinity• More substances can react with [H+]
From Pilson 1998
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Two important carbon reactions pertain to primary production:CO2 + H2O CH2O + O2 (consumes acid)Ca+2 + HCO3
- CaCO3 + H+ (produces acid)
CO2 (g) H2CO3 (aq) HCO3
- CO3-2
Corg CaCO3
Air
Sea – photic zone
Sea – aphotic zone
‘export’
Ecology influences the net effect of biology on the air-sea transfer!
Seawater Carbonate Buffer System
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Thermodynamic ConstantsKH = pCO2/{H2CO3}K1 = {H+}{HCO3
-}/{H2CO3} K2 = {H+}{CO3
-2}/{HCO3-}
‘Apparent’ ConstantsK1’ = K1 H2CO3/HCO3- = {H+}[HCO3
-]/[H2CO3] 10-6.0 (@25oC, I=0.7)
K2’ = K2 HCO3-/CO3-2 = {H+}[CO3
-2]/[HCO3-]
10-9.1 (@25oC, I=0.7)
3 Equationsbut, 5 unknowns!
How can system be defined uniquely?• pCO2 (open system)• pH (≡ -log aH+)• SCO2 (mass balance)• Alkalinity (acid-neutralizing capacity)
H2CO3 – a diprotic weak acid
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mass balance constraintSCO2 = [H2CO3] + [HCO3
-] + [CO3
-2]
RespirationCH2O + O2 CO2 + H2O
DissolutionCaCO3 + H+ Ca+2 + HCO3
-
~1% ~90% ~9%
SCO2
i.e. DIC
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Total Dissolved Inorganic Carbon DIC, i.e. SCO2 (mmol/kg)
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Total Alkalinity (mmol/kg)
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Emiliania huxleyi, a coccolithorophorid
Discospaera sp., another coccolithophorid
planktonic foraminifera
pteropods
These organisms all make skeletal material from
calcium carbonate – calcite in some cases, aragonite in
others
Both CaCO3
bryozoa stalks sponge spicules
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Centric diatoms – an alga
Radiolarian – a protozoan
Both make a skeleton based on the element
Si – ‘biogenic silica’or SiO2
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CaCO3 (s) Ca+2 (aq) + CO3-2 (aq)
Ksp* = [Ca+2]saturated + [CO3
-2]saturated
Ksp*
calcite (e.g., foraminifera, coccolithophorids): 3.3 x 10-9
aragonite (e.g., coral, pteropods): 4.6 x 10-9
Biogenic Silica (e.g. diatoms, radiolarian): 2.0 x 10-3
Q: What is more soluble – CaCO3 or SiO2?Q: Which form of calcium carbonate is more soluble?
Solubility of Calcite versus Aragonite
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Dissolution of biogenic particles• Solubility also is a function of
temperature and pressure• In the deep ocean, CaCO3
becomes very soluble– Carbonate Compensation Depth
(CCD)• Below CCD calcium carbonate
is under-saturated (like SiO2)– Decrease in pH also can
increase calcium carbonate solubility
– CCD is a dynamic depth (NOT fixed)
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Nutrients
• In oceanography, “nutrient” refers to important and commonly measured element needed for growth of plants
• Includes the major nutrients (i.e. macronutrients):– Phosphorus– Nitrogen– Silicon
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Phosphorus Cycle: global
Ruttenberg, 2001 (Encyclopedia of Ocean Sciences)
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Phosphorus• Forms of occurrence in seawater
– Inorganic phosphate (i.e. orthophosphate)• No major redox state differences• Nearly all dissolved phosphorus present in deep sea
– Organic phosphorus• Phospho- … -lipids, -proteins, -carbohydrates• Nucleic acids & nucleotides• Phosphonic acid derivatives
– Polyphosphates• Wide variety of straight-chain, branched and cyclic polymeric forms
• Sorption affects bioavailability– Fe oxy-hydroxides, Carbonate-mineral sorption
• Redox sensitivity– Low Dissolved oxygen induces phosphate release from sediments (VERY
IMPORTANT IN Gulf of Mexico and adjacent estuaries)
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Distribution of Dissolved organic phosphorus (DOP) and Soluble Reactive Phosphorus (SRP)
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Nitrogen in the marine environment
Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)
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Nitrogen acquisition• Chemical forms of nitrogen and their major characteristics
Chemical Form
Nitrate (NO3
-)Nitrite (NO2
-)Nitrous oxide (N2O)
Nitrogen gas (N2)
Ammonia (NH4
+)Amines (-NH2)
Oxidation State
+5 +3 +2 0 -3 -3
Used by plants
Yes Yes No Yes Yes Yes
Oxidized Reduced
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Major Chemical forms/transformations
Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)
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Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)
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Global Mean Profiles
Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)
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Behold … the world’s most awesome elementBIAS ALERT!
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Silicon
• Second most abundant element in earth’s crust– 25.5% of crust by weight (Oxygen is 49%)– Si-O chemical bond one of most abundant
• In seawater Si is relatively scarce ~0.0003 atom%
• In diatoms (a phytoplankton group beloved by your instructor) = 5.0 atom %
• Some vertebrates = 0.001 atom%
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Current view of the marine Si cycle
Tréguer and De La Rocha Annu. Rev. Mar. Sci. 2013
NOTE:
• No major gas phase
• No major organic Si pool
• UNITS: Tmols Si year-1
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Dissolved silicate
• At seawater pH– >97% Si(OH)4 (orthosilicic acid)
• Dominant form transported by diatom (Del Amo and Brzezinski 1999, Journal of Phycology)
• pH 8.7-8.9 – 14-23% ionic (Si(OH)3
-
• May be transported across the membrane but typically much lower rates (Reidel et al. 1984 Journal of Phycology)
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Ocean Chemical Tracers• Tracer conservation equations establish the relationship
between the time rate of change of tracer concentration at a given point and the processes that can change that concentration (Sarmiento and Gruber 2006)– Processes include:
• Physical transport (advection, mixing)• Sources and sinks (biological and chemical transformation)
• Examples: chemical ocean tracers– AOU = apparent oxygen utilization– Chlorofluorocarbons (CFC)– Carbon 14
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AOU
• Apparent Oxygen Utilization– AOU = [O2]saturated – [O2]measured
• Difference between measured oxygen and what equilibrium saturation (as a function of the physical/chemical characteristics)– From biological activity– Oxygen increased by primary production– Oxygen used by respiration
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Apparent Oxygen Utilization
AOU =[O2]saturated – [O2]measured
Which locations have the highest AOU at depth? Lowest? Why?
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• Preformed nutrients: those initially present at the time of downwelling= total nutrient – regenerated nutrient- Calculated using AOU
• Characteristic of waters originating from different regions– Hence use as tracer
AOU and Preformed Nutrients
‘Preformed’NutrientAO
U
Phosphate
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From Broecker et al. 1985
Preformed P (top) & Preformed N (bottom)
From Sarmiento & Gruber 2006
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CFC
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• Manmade compounds (where are highest values?)• High radiative forcing (relative to CO2)
• 12,400x higher for CFC-11• 15,800x higher for CFC-12
• Useful as ocean tracers (i.e. only manmade source is from atmosphere)
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Natural vs Anthropogenic 14C Production
Industrial RevolutionBurning 14C-dead Coal!
“Suess Effect”Tree Ring Records
Coral RecordsNuclear Weapons Testing!Test Ban Treaty – 1963!14C now decreasing
-
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• surface waters (-50‰) contain more 14C than deep waters• deep waters in the Atlantic contain more 14C than those in the Pacific while those in the Indian Ocean and Antarctic have intermediate values.
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Radiocarbon age – do trends look familiar?