Chemical Oceanography

Lecture 3: 5/30/2014

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

Inputs Outputs

Weathering: the physical & chemical processes that break down rock

A simplified biogeochemical cycle

Steady State and Equilibrium

• Draw on board

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

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

Seawater Buffering, Alkalinity• More substances can react with [H+]

From Pilson 1998

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

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

mass balance constraintSCO2 = [H2CO3] + [HCO3

-] + [CO3

-2]

RespirationCH2O + O2 CO2 + H2O

DissolutionCaCO3 + H+ Ca+2 + HCO3

-

~1% ~90% ~9%

SCO2

i.e. DIC

Total Dissolved Inorganic Carbon DIC, i.e. SCO2 (mmol/kg)

Total Alkalinity (mmol/kg)

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

Centric diatoms – an alga

Radiolarian – a protozoan

Both make a skeleton based on the element

Si – ‘biogenic silica’or SiO2

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

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)

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

Phosphorus Cycle: global

Ruttenberg, 2001 (Encyclopedia of Ocean Sciences)

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)

Distribution of Dissolved organic phosphorus (DOP) and Soluble Reactive Phosphorus (SRP)

Nitrogen in the marine environment

Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

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

Major Chemical forms/transformations

Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

Global Mean Profiles

Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

Behold … the world’s most awesome elementBIAS ALERT!

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%

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

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)

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

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

Apparent Oxygen Utilization

AOU =[O2]saturated – [O2]measured

Which locations have the highest AOU at depth? Lowest? Why?

• 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

From Broecker et al. 1985

Preformed P (top) & Preformed N (bottom)

From Sarmiento & Gruber 2006

CFC

• 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)

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

-

• 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.

Radiocarbon age – do trends look familiar?