Chemical Oceanography – Flashcards

• Removal of reactive chemical species

Air/sea exchange

Particle scavaging

• Sediment accumulation
• Growth rates of marine organisms
• Sediment mixing by benthic organisms
• Mixing rates in water and water mass tracing
• Aging of organic matter

• Primordial

Present since Earth's formation

Long lived;

• Cosmogenic;

Formed by cosmic rays in the atmosphere;

• Anthropogenic

Man made

Nuclear reactors, bombs, etc.;

• Change in neutron/proton ratio

;

• Results from thermodynamic instability of the nucleaus and is attempt to reach most stable nuclear configuration

• Alpha decay;;
• Beta decay;;-
• Electron capture

Larger nuclides

• Loss of helium nucleaus lowers neutron/proton ratio

Mass and element change

²³⁸₉₂U --> ²³⁴₉₀Th + ⁴₂He

Converts neutron to a proton with emission of high energy electron

• Element change, mass stays the same

¹⁴₆C --> ¹⁴₇N + e-

• Proton captures e- from lowest orbital, creating a neutron.
• Different e- falls to fill empty orbital.
• X-rays emitted
• Same mass, different element

⁴⁰₁₉K --> ⁴⁰₁₈Ar

Ion           Gas

Measure of nuclear disintergrations per unit time.

Most often in disintegrations per minute.

• Ionization detector- nuclide decay emits characteristic energy spectrum and can be distinguished from another

• Fission tracks

• Scintillation counting- uses chemical to absorb radiation energy, leading to chain reactions that produce light. Nuclides can be distinguished based on energy emission

2.22 x 10¹² dpm 

Amount of radioactivity in 1 gram of Radium

Common to use millicuries 2.22 x 10⁹ dpm or microcuries 2.22 x 10⁶ dpm or just plain dpm

SI unit of radioactivity

1 Bq = 1 dps

One curie = 3.7 x 10¹⁰ dps

Amount of radioactivity per mole of substance

i.e. mCi/mmol or dpm/pmol

Most abundant radionuclide in seawater

Activity - 2.48 dpm/L

Can be reduced by microbes becoming insoluble and precipitating

Number of decays per unit time

Activity = dN/dt =λN

λ - decay constant (1/time), fraction of atoms decaying per unit time

N - # of atoms of nuclide present

Most nuclide concentrations are too small to be measured, but their radioactivity can be

Nuclide parent/daughter relationship where daughter/parent activity ratio = 1

Determined by daughter production and loss

dD/dt =                λp[P] -         λ_D [D]

rate of change = Production - Loss (by radioactive decay)

The basis for using the nuclidesz as tracers and chronometers

²³⁴Th activity in the water column is often less than its parent ²³⁸U because of scavaging, which removes the daughter.

²³⁸U in crust-- 

-->Atmosphere --> ²²²Rn ->²¹⁰Pb -----------------------------------------↓

-->Water ----->  ²³⁸U -> ²³⁴Th -> ²³⁴U -> ²³⁰Th      ²²⁶Ra ->²²²Rn -> ²¹⁰Pb

                        ↓               ↓          ↑      ↑     ↓

-->Sediment --> ²³⁸U -> ²³⁴Th -> ²³⁴U -> ²³⁰Th -> ²²⁶Ra-> ²²²Rn -> ²¹⁰Pb -> ²⁰⁶Pb

Red arrows = physical transport

Black arrows = Radioactive decay

d[D]/dt = Production of daughter - loss of daughter

                        λpP[P]       -            {λD[D] + k[D]}

                                                        loss by        Scavaging

                                                                                radioactive       and other

                                                                                   decay           first order                                            

                                                                                                         decay

Must use nuclide with half life close to the rate of process of interest.

• Determine age of particular spot in sediment core
• Divide depth (Δz) by age (Δt)

Sediment accretion rate = Δz/Δt

Use unsupported nuclide activity.

• Unsupported = excess daughter nuclide over secular equilibrium
• Supported = Nuclide activity from Parent decay

Man-made nuclides won't be found in sediment because they aren't naturally made. 

i.e ¹³⁷Ca- doesn't appear in sediment before 1953

• Cosmogenic nuclide - produced by spallation of ¹⁴N
• Becomes ¹⁴CO₂ in atmosphere and is taken up by plants and dissolved in the ocean
• Manmade ¹⁴C was produced by weapons testing in the 60s which increased atmospheric ¹⁴C

• Observation of atmospheric CO₂ entering into the ocean
• DIC of ocean water can be aged- gives estimate of deep residence time
• POC and DOC have been aged (DOC is old)
• Bacteria on surface use new and old combo

• During carbon fixation, ¹⁴C is incorporated into organic matter based on the amount of ¹⁴C in the atmosphere or seawater
• Once an organism dies, no more ¹⁴C is incorporated into the organism. There is only decay, telling us the age of the organism
• Since the decay rate of ¹⁴C is 1.209 x 10^-4, the deficit of ¹⁴C activity tells us how much time has passed since the organic matter was alive

Δ¹⁴C = (¹⁴C/C)_sample - (¹⁴C/C)_std x 1000 - IF

                    _____________________             ↑

                             (14C/C)std                 Fractionation 

                                                                 Factor

• A zero value for Δ¹⁴C represents the ¹⁴C content before this year

• This year was chosen because it was before the industrial revolution and bomb testing

• Trace sources and sinks of material in the environment

• Determine extent and type of biogeochemical processes which have acted on materials

• Privide info on paleooceanographic conditions

• Trace specific elements using stable isotopes i.e. ¹⁵N

More abundant than heavy isotopes

Element                   Standard material

Hydrogen                 SMOW (Standard Mean Ocean Water)

Carbon                     PDB CaCO₃

Nitrogen                   Air

Oxygen                    SMOW

Sulfur                      Canyon Diablo triolite (Meteorite material)

Similarities: Same chemistry, reactions, bonds, etc

Differences: Different bond energies, free energy, rate constants, equilibrium constants

These small differences cause Fractionation

SMOW is the reference material for isotopic analysis of δD (del-deutritium) and δ¹⁸O

Isotope      H₂¹⁶O   H₂¹⁸O    DH¹⁶O    D2¹⁶O    DH¹⁸O

Mass             18       20          19         20        21

                     ↑                                   ↑

                  Most                              Very

               Abundant                           Rare

                δ¹³C = [{     ¹³C    _      ¹³C      }]

                           [{      ____         ____   }] x 1000

                                [{     ¹²C_sample      ¹²C_std  }]

                           [{___________________}]

                           [                                   ]

                           [            ¹³C                   ]

                           [              ___                ]

                           [             ¹²C_std                 ]

                                     OR

δ¹³C = ll R_sample l  _  1 l  x 1000

          ll _____  l         l

          ll R_std      l         l

Positive δ value indicates the substance is enriched in the heavy isotope (relative to the standard)

Negative δ value indicates the substance is depleted in the heavy isotope (relative to the standard)

D (Δ) = δ_reactant - δ_product

D is positive when light isotope reacts faster. Expressed in ‰

• Expressed in isotope ratios, not del units
• The realized isotopic composition difference between reactants and products

α = [¹³C/¹²C]_products/[¹³C/¹²C]_reactants = R_products/R_reactants

α will be close to 1 because isotope differences are small

Chemical reactions/processes (i.e. photosynthesis) has associated discrimination, which would be constant if all other things were constant.

In the real world, conditions are variable and discrimination will change over time, producing net isotope Fractionation

• Temperature- affects kinetic isotope fractionation

Fractionation decreases with increasing temperature. Thermal energy increases and fractional differences between light and heavy bond energies become less significant

• Kinetics- Heavier isotopes less likely to react and therefore react slower (affected by temp)
• Equilibrium processes- phase changes reactions
• Diffusion- light isotopes diffuse slightly faster 

• Depends on differential rate of reaction for light vs. heavy isotopes

Ex. Reaction sequence of 4 different compounds containing C

A-->B-->C-->D

If all of A is converted to D = no fractionation

If some of A is converted to B and A is replentished = fractionation likely

Even if all of B is converted to C and C to D, fractionation will still be evident. ε_A-->B = ε_A-->D 

Caused by preferential enrichment of one isotope in a crystal lattice site (or mineral phase) relative to another, based on thermodynamic stability

Molecules containing the heavy isotope are more stable and have higher bond dissociation energies

Heavier isotopes preferentially partition into solid phases or larger complexes

This type of equilibrium fractionation is strongly affected by temperature

Water evaporates leaving heavier ¹⁸O behind in liquid form and having lighter O in water vapor

As water vapor moves through the atmosphere, precipitation removes even more ¹⁸O and the water vapor becomes lighter still

The initial liquid will have a more positive δ¹⁸O

• Water, slightly depleted in ¹⁸O evaporates from warm sub-tropical waters
• Heavy ¹⁸O-rich water condenses over mid-latitudes
• Near the poles, atmospheric water vapor is increasingly depleted in ¹⁸O
• Snow in the interior of Antarctica has 5% less ¹⁸O than ocean water
• Meltwater from glacial ice is depleted in ¹⁸O

Forams deposit CaCO₃ that is in isotopic equilibrium with the seawater. 

Temp is mirror image of ¹⁸O content of CaCO₃

Otoliths show depletion of ¹³C in response to change in Earth's atmospheric δ¹³C

Atmospheric δ¹³CO₂ is going down due to input of fossil carbon with light isotopic signature

• 14N is the most abundant stable form of N

• 15N is stable and has a natural abundance of 0.365 atom%

• 13N is radioactive with a half life of 10 minutes- not very useful, but has been used in some studies

• Atmospherics N₂ is the reference for δ¹⁵N (i.e. δ¹⁵Natoms = 0)

• Fractionation of N occurs through each level of the food chain, with each trophic level becoming isotopically heavier (higher δ¹⁵N)

• Phytoplankton fractionate N (take lighter isotope preferentially) when N is available. When N is limiting, fractionation decreases. Thus δ¹⁵N values can tell us something about nutrient status. Useful for paleo-reconstructions 

• Deep ocean nitrate   +5 (up to +12‰ in denitrification zones)
• Atmospheric N          0‰
• Phytoplankton          -4 to +8 ‰
• N fixer biomass         0‰ (they draw on atmospheric N₂)
• Consumers               Variable - trophic enrichment of ¹⁵N     along food chain - about 3‰ per trophic level

Seawater sulfate                 +21‰

Sedimentary sulfides (FeS₂₎   -10 to -40 ‰

Marine Plankton                  +19 ‰

Spartina alterniflora             -8 to +2 ‰

Upland plants                     +4 to 6 ‰    

If a substrate is non-limiting, maximum fractionation will take place

If a substrate is limiting, fractionation will be low

Ex. CO₂ limitation of phytoplankton affects δ¹³C

Nitrate availability affects phytoplankton δ¹⁵N

• Sea water                 +2‰
• Atmospheric CO2        -7‰
• Marine POC                -20 to -22‰
• Terrestrial plants        -27‰
• Marsh grasses (C4)     -14‰
• Benthic algae             -17‰

temperature and availability of substrates (ex. CO₂)

New data are emerging all the time

The rate of transport of matter or energy from one location to another

Flux of mass in one direction are the amount of mass passing a unit of area per unit time [mass/(area*time)]

Fluxes can occur in all three directions

Diffusion

Advection

• Wind stress on the surface
• Biological mixing - small and large scale

Diffusive flux

Flux_diff = -D ∂C/∂z

Direction of flux is opposite to concentration increase, hence the negative sign

          mass     =    length²     (mass/length³)

          ____          _______        ________

          length²         time              length

Movement of mass or energy withing a flow, typically in air or water where v= velocity of flow (cm/s) along z dimension and C is concentration of substance (mole/cm³)

Particle settling in water column

Upwelling of water with high nutrients

Sedimentation (burial)

Sum of diffusive and advective flux

Flux_{diffusive advective} = -D (∂C/∂z) + ωC

In one direction

 dC  =     D  ∂²C  + ω  ∂C    + kC

___             ___         ___

 dt              ∂² z         ∂z

Change in          Diffusion   +   Advection     First 

concentration                                            order

with time                                                  reaction

Advective

Convective (heat, density driven)

Diffusive

• Molecular
• turbulent

Reactions (producing or destroying chemicals in a system)

• Chemical euilibria b/t dissolved and solid phases- dissolution and precipitation
• Biochemical reaction
• Radioactive decay
• Photochemical reactions

ΣCΔz

Conc. x depth = mol/m³ x m = mol/m²

Major driver of biogeochemical cycles

Chemical reactions that involve transfer of electrons 

RedOx -  reduction-oxidation

Redox active chemicals spontaneously transfer electrons in order to achieve thermodynamic equilibrium (lowest free energy state) 

Fe³⁺ + e- --> Fe²⁺

K_eq = {Fe²⁺}

         _______

      {Fe³⁺} {e-}

Loss of electrons

Gain of electrons

Oxidation

Reduction

Ex. elements without appreciable redox chemistry in the environment

Cl^-, Na^-, K⁺, Mg²⁺, Ca²⁺

These elements are already oxidized relative to their native metallic form

• Any element in its native state will have an oxidation number of zero
• In most other cases, the element O is assigned the oxidation state of -2 and H = +1. 
• The sum of the oxidation numbers in a molecule must equal the charge on the molecule

Example of a half reaction

Zn_(s) <=> Zn²⁺_(aq) + 2e-

Photosynthesis, which takes advantage of light E to drive otherwise thermodynamically unfavorable reactions

Positive ΔG means not a spontaneous reaction. E has to be put in to drive the reaction

E can come from the sun or chemical oxidation of other matter

Preservation of organic carbon allows excess O₂ to accumulate

Oxidation of all organic matter in the biosphere would only lower atmospheric O by only 1%

Reducing equibalents are buried- peat, CH₄ hydrates, reduced sulfur

Sequence of electron accepting processes after oxygen reduction is no longer available

NO₃^-       Denitrification

MnO₂      Manganese Reduction

NO₃^-      Nitrate reduction

FeOOH   Iron Reduction

SO₄^2-      Sulfate Reduction

CO₂         Methanogenesis

H⁺           Proton Reduction

• Most energetically favorable e- acceptor after O₂
• Nitrate reduced to N₂
• Removes biologically-available nitrogen from ecosystem
• Occurs in water column oxygen minimum zones, possible microzones
• In estuarine sediments can remove 50% of N input to estuaries
• Global denitrification may control ocean pp over long time scales i.e. glacial/interglacial

At seawater pH and in the presence of oxygen, Fe and Mn form insoluble oxides

• Used as e- acceptors by bacteria, also chemically labile
• Reduced end-products are highly soluble and diffusible. Subject to oxidation when they reach zones where O₂ is around
• Reduction/oxidation of metals influence chemistry of ther trace metals

• 2 moles of C are oxidized per mole of sulfate reduced
• No intermediates during sulfate reduction
• Many intermediates during sulfide oxidation

• Due to high [SO₄^2-] in seawater, sulfate is important biogeochemical process responsible for oxidation of organic matter
• Responsible for ~50% of C oxidation in coastal marine sediments
• Generates highly reactive sulfide and contributes to alkalinity
• Reacts w/ important metals, forming insoluble metal sulfides, greatly affecting metal chemistry
• Domicates natural sulfur cycle in terms of mass flux in aquatic systems. Exchange of S w/ atmosphere is primarily via organic S

Two pathways for biogenesis of methane

• Autotrophic methanogenesis  

CO₂ + 4H₂ --> CH₄ + 2H₂O

• Acetate fermintation

CH₃COOH --> CH₄ + CO₂

• Anammox - recently discovered reaction in the N cycle (form of denitrification)
• Nitrate comes from the denitrification pathway
• Discovered mid 90s
• Carried out by bacteria
• Major role in ocean N cycle- 15-30% of N₂ production
• Occurs in sediments and anoxic water columns i.e. Black Sea

Sustained by chemoautotrophic sulfide oxidation

Hemoglobin of tube worms carry both H₂S and O₂ to bacterial symbionts that oxidize the sulfide with O₂

Energy source         e- donor        C source

Chemo                     Litho            Autotroph

(inorganic)                                       (fixes CO₂)

Chemo                     Organo          Heterotroph

(organic)                                       (C from organic

Photo                                               matter)

(light)

• Prokaryotes fit each of these models and some can carry out mixed mode metabolisms
• Eukaryotes are generally chemo-organo-heterotrophs
• Sulfide, ammonium, and methane oxidizers are all chemo-litho-autotrophs

Oxygenated

• Oxic - >10-30% O₂ saturation 
• Hypoxic - <10-30% O₂ saturation

Anoxic

• Suboxic - no O₂ and no HS-
• Sulfidic - no O₂ and some sulfide present 

Hypoxic zones 

Louisiana shelf

Baltic Sea

Arabian Sea

Anoxic zones

Black Sea

Cariaco Trench

Certain fjords

Virtually all sediments below upper few cm

Sources of O₂ - photosynthesis and atmosphere exchange

Physical mixing

• Water column- advection and turbulent eddy diffusion 
• Sediments/microscale- molecular diffusion, currents, bioturbation

Sinks for O₂ - biological respiration and chemical oxidation, small ventilation to atmosphere when O is supersaturated

• Sites of intensive organic matter decomposition 
• Sites of interesting redox reactions affecting chemistry of the system and its surroundings
• Metabolic adaptations- organisms which harness E from sulfide oxidation must compete w/ relatively rapid abiotic autooxidation
• Because of lack of bioturbation, sediments are laminated and holding valuable records of the past

• World's largest anoxic basin
• Freshwater input exceeds evap. causing complete stratification of surface water from deep water
• Anoxic from 50-150 m below surface to bottom, 1000-1800 m below

Particles that fall or accumulate on the benthos in aquatic systems or on the soils suface in wetland habitats

Sediment material generally consists of inorganic and organic materials, as well as live and dead material. 

Dead organic material is referred to as detritus

Input from rivers ↓          Net Evaporation ↑

Surface water

Downwelling water ↓         Upwelling water ↑

Falling particles

Deep ocean↓         Destroyed -

Preserved in sediments

• Sites of intensive biogeochemical processes (fueled by rich organic matter) and chemical processes (dissolution/precipitation reactions)
• Repositories for large quantities of reduced C,sulfur, metals, Ca carbonate, etc (important in global geochemical and biogeochemical fluxes)
• Significant source of nutrients and other chemicals to the water column
• Form geological time record of materials and processes

Calcareous ooze

Pteropod ooze

Diatom ooze

Radiolarian ooze

Pelagic clays

• Changes with time as conditions change or location moves with tectonic plates
• Record of sedimenting material will be preserved
• Fine bands (varves) generally only found when bioturbation is low or absent
• Anoxic basins are good places to find banded sediments - no macrobenthos

Change in thickness over time

Δz/Δt = s

Area                               Accretion rate

Marshes                               1-5 cm/y

Estuaries                             1-20 cm/y

Coastal Shelf                       0.1-1 cm/y

Continental Slope                 0.05-0.5 cm/y

Abyssal plain                       0.0001-0.001 cm/y

Ranges are approximate - rates vary greatly from place to place and time to time

mass flux/ unit area

of material to the benthos

Enhanced mixing of particles and solutes by action of benthic animals.

High rate of mixing increases sediment diffusion coefficient

φ = interconnected pore volume/total sediment volume

Closely approximated by volume of water/volume total sediment

In practice: Measure total volume of sediement, dry, and measure weight loss 

Grams water loss =~cm³

Size, shape, and chemical structure of particles

Degree of compaction

Degree of inundation/desiccation (in intertidal sediments)

Clay/mud sediment have higher porosity than quartz

Alteration of matter upon reaching the sediments 

• Early diagenesis- alterations taking place in zone of active biogeochecimal activity. Usually in upper 0.2-2 m of sediment
• Later diagenesis- alterations taking place in deeper sediment column. Often driven by increased pressure and temperature                                   

Ex. cementation of unconsolidated sediments into solid rock

F = -?D_s (?C/?z)

D is whole sediment diffusion coefficient

D_s = D/?²

? is diffusive path length

<50nM availability

Sources

Rivers- particluate clay mostly, some dissolved

Atmosphere- wet and dry deposition, Usually well away from land masses

Hydrothermal vent- major source of metals, but many are immediately precipitated

Sinks

Sediment- precipitation of metal as insoluble oxide --> adsorption of trace metal to particulate (clays) --> sedimentation

Actively taken up by biological systems for use as cofactors in enzymes

Fe, Zn, V, Cr, Mn, Ni, Co, Cu, Mo

Certain metals can be nutrients and limiting, or toxicants and inhibit biological processes (pp)

• Complexation
• Uptake ↓

Bioreduction/oxidation

Methylation

Ligand binding

Surface absorption

• Advective transport (moving with water flow)
• Remineralization
• Scavaging from water column leading to sediment burial

electron donors molecules capable of forming relatively stable complexes with cations including metals

May be organic or inorganic

Responsible for keeping some trace metals in euphotic zone

Metal²⁺ + L ↔  [Metal²⁺ L]

   +

  OH-                                      Euphotic zone

   ↑↓

Metal(OH)

(Insoluble

Metal oxide)

The stability constants of metals with surfaces of clays, metal oxides, opal and organic coatings. Often sufficiently high to allow "adsorption" and scavaging of the trace metal from solution

Scavenging loss rates from water column to depth can be estimated by looking at distribution of a particle reactive radionuclide such as ²³⁴Th

Trace metals have short residence times and input is dependant on atm sources, upwelling, etc - result is changeable conditions for organisms that might be starved for or inhibited by those metals

Might explain random occurance of blooms

• Rare, but concentrated in ores
• Most common ore is cinnabar (HgS)

Hg²⁺ + S^2- ↔ HgS (mercury in Hg(II) form)

• Heating of ore causes reduction resulting in liquid Hg
• Hg is in coal and introduced to the atm when coal is burned

Mining

Fossil fuel combustion

Industrial uses of Hg

Barite drilling muds