Chemical Oceanography – Flashcards
Unlock all answers in this set
Unlock answersRadionuclides |
Used to measure rates of processes in the ocean |
Rates measured by radionuclides |
Air/sea exchange Particle scavaging
|
Types of radionuclides in the environment |
Present since Earth's formation Long lived;
Formed by cosmic rays in the atmosphere;
Man made Nuclear reactors, bombs, etc.; |
Nuclear Decay |
;
|
Different modes of nuclear decay |
|
Alpha decay |
Larger nuclides
Mass and element change 23892U --> 23490Th + 42He |
Beta decay |
Converts neutron to a proton with emission of high energy electron
146C --> 147N + e- |
Electron capture |
4019K --> 4018Ar Ion Gas |
Radiation |
Amount of energy emitted |
Radioactivity |
Measure of nuclear disintergrations per unit time. Most often in disintegrations per minute. |
Detecting radioactivity |
|
Curie |
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 |
Becquerel |
SI unit of radioactivity
1 Bq = 1 dps
One curie = 3.7 x 1010 dps |
Specific activity |
Amount of radioactivity per mole of substance
i.e. mCi/mmol or dpm/pmol |
238U |
Most abundant radionuclide in seawater
Activity - 2.48 dpm/L
Can be reduced by microbes becoming insoluble and precipitating |
Activity of a nuclide |
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 |
Secular equilibrium |
Nuclide parent/daughter relationship where daughter/parent activity ratio = 1
|
Production rate of daughter nuclide |
dP/dt = λp [P] |
Rate of change of Daughter nuclide |
Determined by daughter production and loss
dD/dt = λp[P] - λD [D] rate of change = Production - Loss (by radioactive decay) |
Deviations from secular equilibrium |
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. |
Pathways of 238U |
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 |
Any process that adds or removes daughter nuclide will cause deviation from secular equilibrium |
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 |
Which nuclide to use? |
Must use nuclide with half life close to the rate of process of interest.
|
Application of 234Th scavaging |
234Th is reactive so most is rapidly absorbed during biological activity. This causes a deficit. |
Using Radio-dating to determine sediment accretion rate |
Sediment accretion rate = Δz/Δt |
How to determine age in deposits |
Use unsupported nuclide activity.
|
Use of nuclides as event markers |
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 |
14C |
|
Applications of 14C dating
Progress with introduction of accelerator mass spectrometer analysis |
|
14C aging |
|
Change in 14C |
Δ14C = (14C/C)sample - (14C/C)std x 1000 - IF _____________________ ↑
(14C/C)std Fractionation Factor |
1850 |
|
What are stable isotopes used for? |
|
Light isotopes |
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) |
Isotopes of elements similarities and differences |
Similarities: Same chemistry, reactions, bonds, etc Differences: Different bond energies, free energy, rate constants, equilibrium constants These small differences cause Fractionation |
Isotopic composition of water- SMOW |
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 |
Example of Fractionation |
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 |
Del notation (using δ13C as an example, works well for all other isotopes too) |
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 |
Positive and negative δ |
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) |
Isotope discrimination |
The instantaneous difference in isotopic composition, usually given in ‰, between the parent substrate undergoing reaction and the product, at any given instant in time |
Discrimination factor |
D (Δ) = δreactant - δproduct
D is positive when light isotope reacts faster. Expressed in ‰ |
Fractionation factor (α) |
α = [13C/12C]products/[13C/12C]reactants = Rproducts/Rreactants α will be close to 1 because isotope differences are small |
Difference between Discrimination and Fractionation |
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 |
Heavier isotope prefers ___ and ___ forms because ________ |
liquid, mineral, more stable in those molecular configurations |
Factors affecting isotope fractionation |
Fractionation decreases with increasing temperature. Thermal energy increases and fractional differences between light and heavy bond energies become less significant
|
Kinetic isotope fractionation |
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-->B = εA-->D |
Oxygen at depth |
Lighter O is used in reaction to create water resulting in heavier O being found by itself at depth. |
Equilibrium isotope effect |
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 |
Example of equilibrium isotope effect - 18O during evaporation and precipitation |
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 |
Ocean water masses and their isotope compositions |
|
Different ocean water masses have different isotope signatures that behave as... |
conservative tracers aiding distinction of mixing patterns in the ocean |
Rayleigh Distillation |
For a closed or semi-closed system, the isotopic composition of the products and reactants will depend on the extent of the reaction. |
Foraminifera preserved in sediment determine paleo conditions in the ocean - temp and water volume |
Forams deposit CaCO3 that is in isotopic equilibrium with the seawater.
Temp is mirror image of 18O content of CaCO3 |
Light isotopic signature in otolith of Blue Fin Tuna |
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 |
Several isotopes of N have been used with utility in the study of nitrogen cycling |
|
N isotope |
|
Typical del 15N values for marine N pools |
|
Sulfur isotopes |
Seawater sulfate +21‰ Sedimentary sulfides (FeS2) -10 to -40 ‰ Marine Plankton +19 ‰ Spartina alterniflora -8 to +2 ‰ Upland plants +4 to 6 ‰ |
Large global burial of "light" sulfur- seawater sulfate pool is heavy (+20‰) compared to the primordial CDT standard |
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 |
Availability of substrate affects fractionation |
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 |
Typical values for del 13C |
|
Values for biogenic material del 13C are approximate and subject to variation depending on factors such as... |
temperature and availability of substrates (ex. CO2)
New data are emerging all the time |
Flux |
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 |
Causes of matter and energy to move |
Diffusion Advection |
Diffusion |
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 |
Things that mix the water column and contribute to turbulent diffusion |
|
Flick's first law |
Diffusive flux
Fluxdiff = -D ∂C/∂z
Direction of flux is opposite to concentration increase, hence the negative sign |
Why oceanographers focus mainly on flux in vertical dimension in water column or sediment |
While flux occurs in all three directions, the gradients in the vertical direction are often much greater than in the lateral directions |
Flux units |
mass = length2 (mass/length3) ____ _______ ________ length2 time length |
Advection |
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)
|
Examples of advection |
Particle settling in water column
Upwelling of water with high nutrients
Sedimentation (burial) |
Total transport flux |
Sum of diffusive and advective flux
Fluxdiffusive advective = -D (∂C/∂z) + ωC
In one direction |
What controls concentration in one dimension |
ΔC/Δt = Input - Output + Reactions within layer |
Diffusive and advective flux account for both inputs and outputs of dimension (ignoring reactions for now) |
One dimensional diffusion, advection, reaction model |
dC = D ∂2C + ω ∂C + kC ___ ___ ___ dt ∂2 z ∂z
Change in Diffusion + Advection First concentration order with time reaction
|
Types of transport |
Advective Convective (heat, density driven) Diffusive
Reactions (producing or destroying chemicals in a system)
|
Determining how much of a substance (or rate) exists under a unit area of the ocean |
Integrating quantities or rates over depth |
Why integrate quantities over depth |
To compare either standing stock of nutrients or primary productivity between ecosystems |
Depth integration |
ΣCΔz
Conc. x depth = mol/m3 x m = mol/m2 |
What causes concentration at depth? ↓ Sources of flux ↓ What are the sources of flux? ↓ Diffusion, advection, reactions |
Redox chemistry in the Sea
|
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) |
Equilibrium chemistry |
Fe3+ + e- --> Fe2+
Keq = {Fe2+} _______ {Fe3+} {e-} |
Oxidation
Reduction |
Loss of electrons
Gain of electrons |
Chemical losing electrons increases oxidation number
Chemical gaining electrons decreases oxidation number |
Oxidation
Reduction |
Some elements are Redox Active in the environment and some are not |
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 |
Rules for assigning oxidation states |
|
Thermodynamic equilibrium principles apply to the movement of electrons |
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 |
Two redox active chemicals, at non-equilibrium concentrations, will have an electrical potential between them (i.e. a potential to transfer e-) |
The electrical potential (E) of the system is called Ecell which is the sum of all half reactions (oxidation and reduction are half reactions) |
Electrons cannot exist in a free state, a half reaction cannot occur if there was not something to accept the electrons |
Example of a half reaction
Zn(s) <=> Zn2+(aq) + 2e- |
The reaction with the greatest tendency to proceed spontaneously will be the one with the most negative ΔG value |
In seawater these chemicals are most often O2 and reduced C |
The large amount of "unstable" reduced compounds in nature results mainly from _____ |
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 |
If respiration with O2 is a balance for photosynthesis, then why is there oxygen in the air? |
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 |
Anaerobic respieration proceeds in the absence of oxygen, using alternative electron acceptors |
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 |
Nitrate reduction |
|
Metal oxide reduction
FeOOH and MnO2 |
At seawater pH and in the presence of oxygen, Fe and Mn form insoluble oxides
|
Dissimilatory Sulfate Reduction |
|
Sulfate reduction |
|
Methanogenesis |
Two pathways for biogenesis of methane
CO2 + 4H2 --> CH4 + 2H2O
CH3COOH --> CH4 + CO2 |
Completing the biogeochemical cycles |
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 |
Anaerobic oxidation of ammonia |
|
Vent communities |
Sustained by chemoautotrophic sulfide oxidation
Hemoglobin of tube worms carry both H2S and O2 to bacterial symbionts that oxidize the sulfide with O2 |
Troph metabolic mode guide |
Energy source e- donor C source Chemo Litho Autotroph (inorganic) (fixes CO2) Chemo Organo Heterotroph (organic) (C from organic Photo matter) (light) |
Troph metabolic modes
|
|
Redox environments |
Oxygenated
Anoxic
|
When sinks exceed sources of O, concentrations decline and areas become anoxic/hypoxic |
Hypoxic zones Louisiana shelf Baltic Sea Arabian Sea
Anoxic zones Black Sea Cariaco Trench Certain fjords Virtually all sediments below upper few cm |
[O2] is controlled by sources and sinks |
Sources of O2 - photosynthesis and atmosphere exchange Physical mixing
Sinks for O2 - biological respiration and chemical oxidation, small ventilation to atmosphere when O is supersaturated |
Why are anoxic systems important? |
|
The Black Sea |
|
What are sediments and what are they made of? |
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 |
Sediment fluxes |
Input from rivers ↓ Net Evaporation ↑ Surface water
Downwelling water ↓ Upwelling water ↑
Falling particles Deep ocean↓ Destroyed - Preserved in sediments |
Why are sediments important? |
|
Pelagic sediments |
Calcareous ooze Pteropod ooze Diatom ooze Radiolarian ooze Pelagic clays |
Sediment environment |
|
Sediment accretion rate (s) |
Change in thickness over time
Δz/Δt = s |
Rates of sediment vertical accretion in different marine environments |
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 |
Sedimentation rate |
mass flux/ unit area
of material to the benthos |
Sediment accumulation rate |
vertical accretion rate in length/time i.e. cm/y |
Bioturbation |
Enhanced mixing of particles and solutes by action of benthic animals.
High rate of mixing increases sediment diffusion coefficient |
Sediment density |
Sediment is composed of particle matter and pore water |
Porosity |
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 |
What does porosity depend on? |
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 |
Diagenesis |
Alteration of matter upon reaching the sediments
Ex. cementation of unconsolidated sediments into solid rock |
Diffusive flux in sediment |
F = -?Ds (?C/?z) D is whole sediment diffusion coefficient Ds = D/?2 ? is diffusive path length |
Trace metals |
<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 |
Scavaging |
attaching to particles and sinking into sediment and burial |
Biologically active trace metals |
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) |
Some trace elements can be taken up because of similarity to other elements
Se for S As for P
This can be lethal |
Factors affecting the cycling and fate of metals |
Bioreduction/oxidation Methylation Ligand binding Surface absorption
|
Ligands |
electron donors molecules capable of forming relatively stable complexes with cations including metals
May be organic or inorganic |
Ligands |
Responsible for keeping some trace metals in euphotic zone
Metal2+ + L ↔ [Metal2+ L] + OH- Euphotic zone
↑↓ Metal(OH) (Insoluble Metal oxide)
|
Scavaging |
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 |
Roles of metals in maintaining variability/diversity in the ocean |
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 |
Biogeochemistry of Hg |
Hg2+ + S2- ↔ HgS (mercury in Hg(II) form)
|
Mobilization of Hg |
Mining Fossil fuel combustion Industrial uses of Hg Barite drilling muds |