THE OCEANS

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THE OCEANS
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ESTUARINE PROCESSES

• Seawater has a much higher ionic strength
  (salinity) than most freshwater.
• Na and Cl-, rather than Ca2 and HCO3- are the
  dominant ions in seawater.
• Mixing occurs in the estuaries, usually with very
  high salinity gradients.
• Colloidal material - solid material present as
  very finely divided particles. Colloids are
  stabilized by repulsion due to surface charge.
• Steep salinity gradients destabilize colloidal
  material causing it to flocculate and sink to the
  bottom.

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SALINITY

• Salinity - weight in grams of inorganic ions
  dissolved in 1 kilogram of H2O. Usually expressed
  as parts per thousand, or per mil ().
• The ratios of major elements in seawater are
  constant throughout the world. But salinity
  varies over a narrow range (32-37 , mostly near
  35 ).
• Salinity can be measured by electrical
  conductance or indices of refraction.
• Mixing in estuaries is usually described as a
  function of salinity. Assumption the salinity at
  any point in an estuary is a function only of
  physical mixing and not chemical changes.

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Schematic plot of trace-element concentration vs.
salinity showing mixing trends between river
water and seawater in an estuary for the case
where the concentration of the trace element in
the river is higher than that in seawater.
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Schematic plot of trace-element concentration vs.
salinity showing mixing trends between river
water and seawater in an estuary for the case
where the concentration of the trace element in
the river is lower than that in seawater.
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HALMYROLYSIS

• Halmyrolysis The process by which terrestrial
  materials adjust to seawater conditions. All
  those processes that affect particles in seawater
  before their incorporation into sediment.
• Clay minerals exchange adsorbed Ca2 for Na, K,
  and Mg2 from seawater.
• Phytoplankton activity is important in
  controlling some element concentrations, i.e.,
  those elements known as nutrients. For example Si
  and P.

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MAJOR ION CHEMISTRY OF SEAWATER

• Three principal features
• High ionic strength (35 g L-1 of salts).
• High abundance of Na and Cl-, followed by lesser
  Mg2 and SO42-.
• Relatively constant ratio of concentrations of
  major ions around the world.

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RESIDENCE TIMES

• Evidence exists that major ion composition of
  seawater has been subject to only relatively
  minor variations over millions of years ?
  long-term geochemical cycling (long residence
  times).
• residence time inventory/input
• The residence times of major components are very
  long
• Na - 78 Ma Cl- - 131 Ma H2O - 3.8x104 a Mg2
  - 14 Ma SO42- - 12 Ma.

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CALCULATING RESIDENCE TIMES

• We must assume
• 1) Dissolved salts in rivers are the dominant
  sources of major ions in seawater.
• 2) Steady state conditions apply.
• For Na
• Input (water flow in rivers) x (river
  concentration)
• (3.6x1016 L a-1) x (0.23x10-3 mol L-1)
• 8.28x1012 mol a-1
• Inventory (water content of oceans) x (ocean
  concentration)
• (1.37x1021 L) x (470x10-3 mol L-1)
• 644x1018 mol

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• Residence time (644x1018 mol)/(8.28x1012 mol
  a-1)
• 78 Ma
• Long residence times mean that there is plenty of
  opportunity for ocean mixing, which is why ratios
  of ion concentrations are reasonably constant
  from place to place.
• Long residence times result from high solubility
  of ions, which is in turn controlled by Z/r
  ratios.
• For trace elements, atmospheric and mid-ocean
  ridge hydrothermal processes can be significant
  sources in addition to river water.

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• Chloride is concentrated in seawater, but has a
  low crustal abundance. Why? Most of the chloride
  in the Earth was degassed from the mantle as
  HCl(g) early in Earth history. The Cl- has since
  been recycled in a hydrosphere-evaporite cycle.
  Most rock-forming minerals exclude Cl-.
• The relative ratios of elements show that ocean
  water is not just river water that has been
  concentrated by evaporation. Rates of removal
  differ for various ions, showing that processes
  other than evaporation are operating.

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How did we think we knew that major-ion chemistry
of seawater has been relatively constant on a
geologic time scale?

• Evaporites Salts that have precipitated
  naturally from evaporating seawater in basins
  isolated from the open ocean.
• Over the last 900 Ma, marine evaporites have
  produced the same general sequence of salts as
  evaporation proceeds
• anhydrite-gypsum ? halite ? bittern salts (KCl,
  Mg and Br salts, etc.)
• The sequence sets limits, because beyond these
  limits, the sequence will change. Calculations
  suggest that the concentrations of the individual
  major ions could not have changed by more than a
  factor of about 2 over 900 Ma.

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ADDITION-REMOVAL MECHANISMS

• sea-to-air fluxes
• evaporite formation
• cation exchange on clays
• carbonate precipitation
• opaline silica formation
• sulfide formation
• hydrothermal reactions

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SEA-TO-AIR FLUXES

• Caused by bubble bursting and breaking waves.
• Most salts immediately fall back into sea.
• Some salts travel long distances and contribute
  to salts in river water.
• Important sink only for Na and Cl-.

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EVAPORITE FORMATION

• 47 water volume evaporated ? CaCO3
• 75 water volume evaporated ? gypsum/anhydrite
• 90 water volume evaporated ? halite
• This is probably not a major removal mechanism
  because of the volume of H2O that must be removed.

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ION-EXCHANGE ON CLAYS

• Removes 26 of river flux of Na to oceans.
• Similar percentage of K and Mg2 removed.
• A significant source of Ca2 to seawater, adding
  an additional 8 to the river flux.
• Dependent on glaciation because this affects the
  rate of supply of suspended solids.

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CARBONATE PRECIPITATION

• It is difficult to calculate the saturation index
  of seawater with respect to CaCO3(s).
• CaCO3(s) ? Ca2 CO32-
• We must compare the IAP with the KSP for this
  reaction. However, seawater is a highly
  concentrated solution, so we must use activities,
  not concentrations.

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• Ca2 0.01 mol L-1 CO32- 0.00029 mol L-1
• IAP (0.26x0.01)(0.20x0.00029) 1.5x10-7 mol2
  L-2
• KSP 4.5x10-9 mol2 L-2
• ? (1.5x10-7 mol2 L-2)/(4.5x10-9 mol2 L-2)
  33.3
• So calcite is supersaturated and should
  precipitate.
• The above calculation does not take into account
  the formation of ion pairs or complexes, e.g.,
• Ca2 SO42- ? CaSO40
• Ca2 HCO3- ? CaHCO3

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• Because
• ?Ca 0.01 mol L-1 Ca2 CaHCO3 CaCl
  CaSO40 CaCO30
• it is clear that complex formation lowers Ca2
  and therefore IAP. Another way of saying this is
  that complex formation increases the solubility
  of calcite.
• Only 90 of ?Ca is present as Ca2 and 10 of
  ?CO32- is present as CO32-. Thus,
• IAP (0.26x0.01x0.90)(0.20x0.00029x0.10)
  1.35x10-8 mol2 L-2
• ? (1.35x10-8 mol2 L-2)/(4.5x10-9 mol2 L-2) 3

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• Thus, surface seawater is supersaturated with
  respect to calcite and we would expect it to
  precipitate from seawater.
• However, abiotic CaCO3 precipitation is limited
  to unusual conditions. This is probably a kinetic
  problem, with CaCO3 precipitation inhibited by
  the presence of other ions such as Mg2 and
  PO43-.
• Removal of CaCO3 into skeletons of organisms is
  much more important.
• Important reaction
• CaCO3 CO2(g) H2O(l) ? Ca2 2HCO3-

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• CaCO3 precipitates as skeletons of organisms in
  shallow water. As particles of CaCO3 sink into
  the deep ocean, they tend to dissolve because
• 1) Increased pCO2 owing to decomposition of
  organic matter.
• 2) Decreased temperature (retrograde solubility
  - solubility of CaCO3 increases with decreasing
  temperature).
• 3) Increased pressure.
• Formation of carbonate sediments on the seafloor
  reflects conditions where the rate of supply of
  biogenic CaCO3 is greater than the dissolution
  rate.

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CALCITE COMPENSATION DEPTH

• Calcite Compensation Depth (CCD) the depth at
  which the rate of supply and dissolution of CaCO3
  are just in balance. Below the CCD, calcite
  cannot accumulate in sediments.
• The CCD is variable from place to place. In the
  Atlantic Ocean it occurs at 4.5 km.
• The presence of carbonate sediments throughout
  geologic time suggests that seawater has always
  been roughly in equilibrium with calcite.

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Schematic diagram showing the relationship
between degree of saturation and depth. The
lysocline marks the depth at which seawater is
just saturated with respect to calcite. Below the
lysocline the rate of calcite dissolution
increases markedly. Calcite can survive if buried
rapidly. Below the CCD, all calcite dissolves.
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OPALINE SILICA

• Biologically produced as skeletons of
  phytoplankton (diatoms, etc.).
• Formula is SiO2nH2O.
• Opaline silica-rich deposits cover 1/3 of the
  seabed, in areas of high sedimentation and
  upwelling of nutrients.
• Seawater is undersaturated with respect to
  silica, and 95 of solid silica dissolves as it
  sinks. Rapid burial is required for its
  preservation.

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SULFIDE FORMATION

• Oxidation of organic matter in sediments by
  sulfate produces sulfides
• 2CH2O(s) SO42- ? 2HCO3- HS- H
• This process is most important in continental
  margin sediments where the greatest accumulation
  of organic matter occurs.
• Some HS- diffuses upward out of sediments where
  it is reoxidized to sulfate. 10 reacts with
  soluble Fe(II).
• Fe2 HS- ? FeS(s) H

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• With time, FeS(s) converts to pyrite
• FeS(s) S2O32- ? FeS2(s) SO32-
• S2O32- is thiosulfate - an important sulfur
  species in pyrite formation its oxidation state
  (2) is intermediate between that of sulfate and
  sulfide.
• SO32- is sulfite its oxidation state (4) is
  also intermediate between that of sulfate and
  sulfide.
• Sedimentary pyrite is a major sink for seawater
  sulfate. Many trace metals are also locked up in
  pyrite and other sulfides.
• The HCO3- formed during organic matter oxidation
  accounts for 7 of the total flux to seawater.

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Eh-pH diagram for the system S-O-H, showing the
solubility of native sulfur. Note that S2O32- and
SO32- are never predominant species, so they do
not appear on an Eh-pH diagram. They are present
in highest concentrations near the
sulfate/sulfide boundary.
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HYDROTHERMAL PROCESSES
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A SEAFLOOR HYDROTHERMAL VENT
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A BLACK SMOKER FROM THE EAST PACIFIC RISE
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HYDROTHERMAL PROCESSES

• A major problem in geochemistry is quantifying
  the fluxes from this source.
• Major-ion sinks
• Mg2 is fixed in low-temperature reactions of
  seawater with basalts on the flanks of the ridge
• 11Fe2SiO4(fayalite) 2H2O(l) 2Mg2 2SO42- ?
  Mg2Si3O6(OH)6(sepiolite) 7Fe3O4(magnetite)
  FeS2(pyrite) 8SiO2(aq)
• This is probably the most important sink for Mg2!

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• Minor-ion sinks
• Hydrothermal fluids are enriched in rare earth
  elements (REE) compared to seawater.
• These same fluids are also enriched in reduced
  Fe2 and Mn2. These ions are oxidized when the
  hydrothermal fluids debauch onto the seafloor and
  mix with seawater.
• Fe-Mn oxyhydroxides are formed.
• These scavenge REE from seawater by adsorption,
  resulting in a net removal of REE owing to
  hydrothermal processes.

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• Major-ion sources
• Some ions are removed from basalt and added to
  seawater.
• Ca2, silica are added at 25 of river flux.
• The major-ion budget for seawater is relatively
  well-balanced, except for K. This means that the
  inputs approximately equal the outputs.
• For K, rivers input 1.1x1012 mol a-1. Cation
  exchange reactions on clays may remove 0.1x1012
  mol a-1. In hotter parts of hydrothermal systems,
  K is probably leached (0.8 mol a-1). This
  leaves 1.8x1012 mol a-1 K that must be balanced
  by sinks.

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• Potential sinks for K
• 1) 17 K is removed by low-temperature
  reactions between basalt and seawater.
• 2) Reverse weathering
• kaolinite K HCO3- H4SiO40
• ? illite CO2(g) H2O(l)
• 3) Permanent burial in sediment pore water (lt2
  of river input).

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MINOR CHEMICAL COMPONENTS IN SEAWATER

• These are much more susceptible to biologic or
  human activities.
• They have complex cycling processes.
• Three classes of minor element behavior
• conservative
• nutrient
• scavenged

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CONSERVATIVE BEHAVIOR

• Elements have essentially constant concentrations
  with depth.
• Elements behave like major ions, having long
  residence times and being well-mixed.
• Not major components of seawater owing to very
  low crustal abundances.
• Elements form simple anions or cations with low
  Z/r, e.g., Cs, Br-, or complex oxyanions, e.g.,
  WO42-, MoO42-.
• Little involvement in biological cycles.

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EXAMPLES OF CONSERVATIVE BEHAVIOR
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NUTRIENT BEHAVIOR

• Production of biological material removes
  nutrient elements from surface seawater.
• On the death of organisms, this biological
  material sinks through the water column,
  decomposing and releasing the nutrients.
• Involvement in biological cycling involves
  chemical transformations.
• Elements showing nutrient behavior tend to have
  long residence times.

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EXAMPLES OF NUTRIENT BEHAVIOR
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• Iodine IO3- is the thermodynamically stable form
  in seawater, but biological cycling results in
  the formation of I- because its production rate
  is greater than its oxidation rate.
• Nitrate Phytoplankton absorb NO3- and reduce it
  to N(-3) to use in proteins. Upon their death,
  NH4 is released.
• Some elements, e.g., Cd, may have nutrient-like
  profiles even though they are not actual
  nutrients. Probably because they are
  inadvertently taken up owing to similarities to
  actual nutrient elements, e.g., Zn.

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COMPARISON OF Zn AND Cd
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SCAVENGED BEHAVIOR

• Elements that are highly particle-reactive,
  characterized by intermediate Z/r.
• They have a minimum concentration at an
  intermediate depth and a maximum concentration at
  the surface.
• Initial decreasing concentration with depth is a
  result of adsorption and cation exchange on
  particle surfaces.
• These elements have short residence times the
  river inputs are largely removed by estuarine
  processes.
• The atmosphere provides the main input of these
  elements to the open ocean (e.g., input of Pb
  from gasoline emissions can be traced in corals).
• The element concentration may increase again
  after a certain depth if particles become
  unstable with depth, e.g., reduction of Fe-Mn
  oxyhydroxides or decomposition of organic matter.

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AN EXAMPLE OF SCAVENGED BEHAVIOR