Chemical Oceanography – Flashcards

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Radionuclides
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Used to measure rates of processes in the ocean
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Rates measured by radionuclides
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  • 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
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Types of radionuclides in the environment
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  • Primordial

Present since Earth's formation

Long lived;

  • Cosmogenic;

Formed by cosmic rays in the atmosphere;

  • Anthropogenic

Man made

Nuclear reactors, bombs, etc.;

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Nuclear Decay
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  • Change in neutron/proton ratio

;

  • Results from thermodynamic instability of the nucleaus and is attempt to reach most stable nuclear configuration
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Different modes of nuclear decay
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  • Alpha decay;;
  • Beta decay;;-
  • Electron capture
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Alpha decay
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Larger nuclides

  • Loss of helium nucleaus lowers neutron/proton ratio

Mass and element change

23892U --> 23490Th + 42He

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Beta decay
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Converts neutron to a proton with emission of high energy electron

  • Element change, mass stays the same

146C --> 147N + e-

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Electron capture
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  • 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

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Radiation
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Amount of energy emitted
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Radioactivity
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Measure of nuclear disintergrations per unit time.

Most often in disintegrations per minute.

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Detecting radioactivity
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  • 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
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Curie
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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

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Becquerel
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SI unit of radioactivity

 

1 Bq = 1 dps

 

One curie = 3.7 x 1010 dps

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Specific activity
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Amount of radioactivity per mole of substance

 

i.e. mCi/mmol or dpm/pmol

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238U
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Most abundant radionuclide in seawater

 

Activity - 2.48 dpm/L

 

Can be reduced by microbes becoming insoluble and precipitating

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Activity of a nuclide
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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

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

 

 

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Production rate of daughter nuclide
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dP/dt = λp [P]
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Rate of change of Daughter nuclide
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Determined by daughter production and loss

 

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

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

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Deviations from secular equilibrium
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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.

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 Pathways of 238U
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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

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Any process that adds or removes daughter nuclide will cause deviation from secular equilibrium
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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

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

 

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Application of 234Th scavaging
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234Th is reactive so most is rapidly absorbed during biological activity. This causes a deficit.
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Using Radio-dating to determine sediment accretion rate
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  • Determine age of particular spot in sediment core
  • Divide depth (Δz) by age (Δt)

Sediment accretion rate = Δz/Δt

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How to determine age in deposits
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Use unsupported nuclide activity.

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

 

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Use of nuclides as event markers
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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

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14
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  • 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
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Applications of 14C dating

 

Progress with introduction of accelerator mass spectrometer analysis

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  • 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
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14C aging
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  • 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
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Change in 14C
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Δ14C = (14C/C)sample - (14C/C)std x 1000 - IF

                    _____________________             ↑

 

                             (14C/C)std                 Fractionation 

                                                                 Factor

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1850
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  • 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
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What are stable isotopes used for?
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  • 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
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Light isotopes
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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)

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Isotopes of elements similarities and differences
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Similarities: Same chemistry, reactions, bonds, etc


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


These small differences cause Fractionation

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Isotopic composition of water- SMOW
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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

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Example of Fractionation
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Water containing 18O instead of 16O being 2 mass units heavier per molecule and 12.5% more dense is a tiny bit slower to evaporate or react in a chemical reaction
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Del notation 

(using δ13C as an example, works well for all other isotopes too)

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                δ13C = [{     13C    _      13C      }]

                           [{      ____         ____   }] x 1000

                                [    12Csample      12Cstd  }]

                           [{___________________}]

                           [                                   ]

                           [            13C                   ]

                           [              ___                ]

                           [             12Cstd                 ]

            

                                     OR

δ13C = ll Rsample l  _  1 l  x 1000

          ll _____  l         l

          ll Rstd      l         l

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Positive and negative δ
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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)

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Isotope discrimination
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The instantaneous difference in isotopic composition, usually given in ‰, between the parent substrate undergoing reaction and the product, at any given instant in time
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Discrimination factor
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D (Δ) = δreactant - δproduct

 

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

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Fractionation factor (α)
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  • Expressed in isotope ratios, not del units
  • The realized isotopic composition difference between reactants and products

α = [13C/12C]products/[13C/12C]reactants = Rproducts/Rreactants


α will be close to 1 because isotope differences are small

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Difference between Discrimination and Fractionation
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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

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Heavier isotope prefers ___ and ___ forms because ________
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liquid, mineral, more stable in those molecular configurations
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Factors affecting isotope fractionation
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  • 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 
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Kinetic isotope fractionation
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  • 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. εA-->BεA-->D 

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Oxygen at depth
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Lighter O is used in reaction to create water resulting in heavier O being found by itself at depth.
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Equilibrium isotope effect
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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

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Example of equilibrium isotope effect - 18O during evaporation and precipitation
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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

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Ocean water masses and their isotope compositions
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  • 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

 

 

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Different ocean water masses have different isotope signatures that behave as... 
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conservative tracers aiding distinction of mixing patterns in the ocean
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Rayleigh Distillation
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For a closed or semi-closed system, the isotopic composition of the products and reactants will depend on the extent of the reaction. 
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Foraminifera preserved in sediment determine paleo conditions in the ocean - temp and water volume
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Forams deposit CaCO3 that is in isotopic equilibrium with the seawater. 

 

Temp is mirror image of 18O content of CaCO3

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Light isotopic signature in otolith of Blue Fin Tuna
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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

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Several isotopes of N have been used with utility in the study of nitrogen cycling
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  • 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
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N isotope
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  • 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 
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Typical del 15N values for marine N pools
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  • 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

                            




 

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Sulfur isotopes
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Seawater sulfate                 +21‰

Sedimentary sulfides (FeS2)   -10 to -40 ‰

Marine Plankton                  +19 ‰

Spartina alterniflora             -8 to +2 ‰

Upland plants                     +4 to 6 ‰    

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Large global burial of "light" sulfur- seawater sulfate pool is heavy (+20‰) compared to the primordial CDT standard
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Dissimilatory sulfate reduction process fractionates sulfur (taking the lighter isotope preferentially) and other sedimentary sulfur cycle processes further fractionate the reduced sulfur such that sulfides preserved in sediments are isotopically light
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Availability of substrate affects fractionation
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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

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Typical values for del 13C
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  • Sea water                 +2‰
  • Atmospheric CO2        -7‰
  • Marine POC                -20 to -22‰
  • Terrestrial plants        -27‰
  • Marsh grasses (C4)     -14‰
  • Benthic algae             -17‰
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Values for biogenic material del 13C are approximate and subject to variation depending on factors such as...
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temperature and availability of substrates (ex. CO2)

 

New data are emerging all the time

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Flux
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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

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Causes of matter and energy to move
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Diffusion

Advection

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Diffusion
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Dispersive movement of mass or energy by random molecular or turbulent motion away from a point of high concentration toward an area of lower concentration
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Things that mix the water column and contribute to turbulent diffusion
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  • Wind stress on the surface
  • Biological mixing - small and large scale
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Flick's first law
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Diffusive flux

 

Fluxdiff = -D ∂C/∂z

 

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

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Why oceanographers focus mainly on flux in vertical dimension in water column or sediment
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While flux occurs in all three directions, the gradients in the vertical direction are often much greater than in the lateral directions
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Flux units
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          mass     =    length2     (mass/length3)

          ____          _______        ________

          length2         time              length

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Advection
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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)

 

 

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Examples of advection
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Particle settling in water column

 

Upwelling of water with high nutrients

 

Sedimentation (burial)

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Total transport flux
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Sum of diffusive and advective flux

 

Fluxdiffusive advective = -D (∂C/∂z) + ωC

 

In one direction

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What controls concentration in one dimension
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ΔC/Δt = Input - Output + Reactions within layer
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Diffusive and advective flux account for both inputs and outputs of dimension (ignoring reactions for now)
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One dimensional diffusion, advection, reaction model
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 dC  =     D  ∂2C  + ω  ∂C    + kC

___             ___         ___

 dt              ∂2 z         ∂z

 

Change in          Diffusion   +   Advection     First 

concentration                                            order

with time                                                  reaction

 

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Types of transport
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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
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Determining how much of a substance (or rate) exists under a unit area of the ocean
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Integrating quantities or rates over depth
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Why integrate quantities over depth
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To compare either standing stock of nutrients or primary productivity between ecosystems
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Depth integration
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ΣCΔz

 

Conc. x depth = mol/m3 x m = mol/m2

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What causes concentration at depth?

Sources of flux

What are the sources of flux?

Diffusion, advection, reactions

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Redox chemistry in the Sea

 

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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) 

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Equilibrium chemistry
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Fe3+ + e- --> Fe2+

 

Keq = {Fe2+}

         _______

      {Fe3+} {e-}

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Oxidation

 

Reduction

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Loss of electrons

 

Gain of electrons

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Chemical losing electrons increases oxidation number

 

Chemical gaining electrons decreases oxidation number

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Oxidation

 

Reduction

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Some elements are Redox Active in the environment and some are not
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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

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Rules for assigning oxidation states
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  • 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
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Thermodynamic equilibrium principles apply to the movement of electrons
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When chemicals have electronic configurations, which are out of equilibrium, relative to another chemical, they will spontaneously react together transferring electrons to attain equilibrium- lowest possible state of free energy
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Two redox active chemicals, at non-equilibrium concentrations, will have an electrical potential between them (i.e. a potential to transfer e-)
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The electrical potential (E) of the system is called Ecell which is the sum of all half reactions (oxidation and reduction are half reactions)
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Electrons cannot exist in a free state, a half reaction cannot occur if there was not something to accept the electrons
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Example of a half reaction

 

Zn(s) <=> Zn2+(aq) + 2e-

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The reaction with the greatest tendency to proceed spontaneously will be the one with the most negative ΔG  value
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In seawater these chemicals are most often O2 and reduced C
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The large amount of "unstable" reduced compounds in nature results mainly from _____
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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

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If respiration with O2 is a balance for photosynthesis, then why is there oxygen in the air?
answer

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

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Anaerobic respieration proceeds in the absence of oxygen, using alternative electron acceptors
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Sequence of electron accepting processes after oxygen reduction is no longer available

 

NO3-       Denitrification

MnO2      Manganese Reduction

NO3-      Nitrate reduction

FeOOH   Iron Reduction

SO42-      Sulfate Reduction

CO2         Methanogenesis

H+           Proton Reduction

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Nitrate reduction
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  • 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
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Metal oxide reduction

 

FeOOH and MnO2

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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
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Dissimilatory Sulfate Reduction
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  • 2 moles of C are oxidized per mole of sulfate reduced
  • No intermediates during sulfate reduction
  • Many intermediates during sulfide oxidation
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Sulfate reduction
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  • Due to high [SO42-] 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
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Methanogenesis
answer

Two pathways for biogenesis of methane

  • Autotrophic methanogenesis  

CO2 + 4H2 --> CH4 + 2H2O

  • Acetate fermintation

CH3COOH --> CH4 + CO2

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Completing the biogeochemical cycles
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Respiration generates oxidized C and reduced inorganic chemicals as end products --> Chemoautotrophy completes the biogeochemical cycle and uses E in reduced chemicals for fixation of inorganic C
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Anaerobic oxidation of ammonia
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  • 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
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Vent communities
answer

Sustained by chemoautotrophic sulfide oxidation

 

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

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Troph metabolic mode guide
answer

Energy source         e- donor        C source


Chemo                     Litho            Autotroph

(inorganic)                                       (fixes CO2)

Chemo                     Organo          Heterotroph

(organic)                                       (C from organic

Photo                                               matter)

(light)

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Troph metabolic modes

 

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  • 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
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Redox environments
answer

Oxygenated

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

Anoxic

  • Suboxic - no O2 and no HS-
  • Sulfidic - no O2 and some sulfide present 
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When sinks exceed sources of O, concentrations decline and areas become anoxic/hypoxic
answer

Hypoxic zones 

Louisiana shelf

Baltic Sea

Arabian Sea

 

Anoxic zones

Black Sea

Cariaco Trench

Certain fjords

Virtually all sediments below upper few cm

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[O2] is controlled by sources and sinks
answer

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

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Why are anoxic systems important?
answer
  • 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
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The Black Sea
answer
  • 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
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What are sediments and what are they made of?
answer

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

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Sediment fluxes
answer

Input from rivers ↓          Net Evaporation ↑


Surface water

 

Downwelling water ↓         Upwelling water ↑

 

Falling particles


Deep ocean↓         Destroyed -


Preserved in sediments

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Why are sediments important?
answer
  • 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
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Pelagic sediments
answer

Calcareous ooze

Pteropod ooze

Diatom ooze

Radiolarian ooze

Pelagic clays

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Sediment environment
answer
  • 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
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Sediment accretion rate (s)
answer

Change in thickness over time

 

Δz/Δt = s

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Rates of sediment vertical accretion in different marine environments
answer

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

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Sedimentation rate
answer

mass flux/ unit area

 

of material to the benthos

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Sediment accumulation rate
answer
vertical accretion rate in length/time i.e. cm/y
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Bioturbation
answer

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

 

High rate of mixing increases sediment diffusion coefficient

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Sediment density
answer
Sediment is composed of particle matter and pore water
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Porosity
answer

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

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What does porosity depend on?
answer

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

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Diagenesis
answer

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

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Diffusive flux in sediment
answer

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

D is whole sediment diffusion coefficient

Ds = D/?2

? is diffusive path length

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Trace metals
answer

<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

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Scavaging
answer
attaching to particles and sinking into sediment and burial
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Biologically active trace metals
answer

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)

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Some trace elements can be taken up because of similarity to other elements

 

Se for S

As for P

 

This can be lethal

answer
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Factors affecting the cycling and fate of metals
answer
  • Complexation
  • Uptake ↓

Bioreduction/oxidation

Methylation

Ligand binding

Surface absorption

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

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

 

May be organic or inorganic

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Ligands
answer

Responsible for keeping some trace metals in euphotic zone

 

Metal2+ + L ↔  [Metal2+ L]

   +

  OH-                                      Euphotic zone


 

   ↑↓

Metal(OH)

(Insoluble

Metal oxide)

 

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Scavaging
answer

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

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Roles of metals in maintaining variability/diversity in the ocean
answer

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

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Biogeochemistry of Hg
answer
  • Rare, but concentrated in ores
  • Most common ore is cinnabar (HgS)

Hg2+ + S2- ↔ 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
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Mobilization of Hg
answer

Mining

Fossil fuel combustion

Industrial uses of Hg

Barite drilling muds

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