Lecture 15: Ocean chemistry

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Lecture 15 Ocean chemistry

• Questions
• How do the dynamics of the ocean affect the
  chemistry of the ocean, the distribution of
  biological activity, and the type of sediment
  accumulated on the seafloor?
• Tools
• Aquatic chemistry, box modeling, fluid dynamics,
  etc.
• Reading
• White Chapter 15
• Albarède Chapter 6

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The ocean generalities

• The world ocean is very flat this map has 50x
  vertical exaggeration
• The ocean is stratified into a warm, ventilated
  surface ocean and a cold purely advective deep
  ocean the boundary is the thermocline

• Two other significant layered structures
• The photic zone is the depth to which light
  penetrates and where photosynthesis is possible
• The mixed layer is a wind-stirred region in the
  top 100 m where stratification is absent

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Physical properties of seawater

• The important variables are salinity and
  temperature, which together determine the most
  important parameter, density

On this plot, st is density in kg/m3 relative to
pure water at 0C DS,T is specific volume in
cm3/(100 kg) relative to water at 35 psu and 0C
Note seawater, at about 35 salinity, has
monotonically increasing density on an adiabat
and is stably stratified. Freshwater has a
density inversion at 4C and so lakes overturn
completely as surface waters cool down
approaching winter.
Note also, temperature is the dominant source of
density variations in the modern ocean, but as
overall temperatures decline the thermal
expansion nearly disappears, and in glacial
period salinity becomes a more important
dynamical variable.
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Physical properties of seawater

• Salinity, originally defined as weight fraction
  of total solids obtained by drying seawater, is
  now always measured by conductivity (which
  relates to concentration of dissolved ions, of
  course) and given in practical salinity units,
  psu, scaled so that standard seawater at 35 psu
  has 35 total dissolved solids
• Temperature is best given as potential
  temperature, q, the temperature water would have
  if adiabatically expanded to 1 bar pressure
  (although difference between in situ T and q is
  0.1C)
• Both salinity and potential temperature are
  conservative properties of seawaterthey are set
  almost completely by ventilation of water at the
  surface of the ocean
• Diffusion of heat and mass is nearly negligible
  at large scale in the ocean, compared to
  advection, so water masses in the deep ocean
  carry these properties around with little mixing
  or modification
• Because seawater is stably stratified (below the
  mixed layer), flow is almost entirely along
  isopycnal surfaces and water masses are only
  ventilated where isopycnals outcrop at the surface

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Physical properties of seawater

• Away from river inputs, salinity is set at the
  sea surface by the balance between evaporation
  and precipitation
• But temperature is the dominant variable in
  surface ocean density
• Warm low latitude surface waters have high
  evaporation rates
• Hence they have high salinity
• But not high enough to overcome the thermal
  buoyancy
• So the low-latitude surface waters float on
  denser waters below that came from high latitude

dT dst, except here
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Ocean dynamics in a very small nutshell

• The ocean obeys the laws of fluid dynamics in a
  rotating reference frame with some important
  simplifications
• The dynamics of the shallow ocean are forced by
  winds the dynamics of the deep ocean are forced
  by density variations
• Flows are slow enough that momentum is
  negligible therefore the systems obeys
  instantaneous force balance or the geostrophic
  equation in a rotating reference frame
• The Coriolis force is an essential part of the
  dynamics

• Where the wind generates a clockwise surface
  circulation in the northern hemisphere, it drives
  water downwards
• Where the wind generates an anticlockwise surface
  circulation in the northern hemisphere, it pulls
  water upwards
• The resulting dynamic pressure variations can be
  read by satellites that measure sea-surface
  height relative to the geoid

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Ocean dynamics in a very small nutshell

• Ekman pumpingwater moves at right angles to the
  wind stress!
• So a curl in the wind field leads to a divergence
  in sea-surface height, which provides pressure
  gradients to drive vertical flow

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Ocean dynamics in a very small nutshell

• Ekman pumpingwater moves at right angles to the
  wind stress!
• Also, longshore transport can drive coastal
  upwelling or downwelling

You want to go fishing here.
Not here.
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Ocean dynamics in a very small nutshell
Upwelling at high latitudes where the wind stress
is divergent and at eastern edges of oceans where
longshore winds drive offshore shallow water flow
at right angles.
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Ocean dynamics in a very small nutshell

• Deep water circulation very slow driven by deep
  water formation and isopycnal flow all
  southwards in Atlantic Ocean, northwards in
  Indian and Pacific Oceans steered to western
  boundary of each ocean must be compensated by
  return flow in shallow ocean but this is small
  compared to wind-driven motions above
  thermocline. Stommels version

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Ocean dynamics thermohaline circulation

• Broekers version the conveyor belt

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Ocean dynamics thermohaline circulation

• The thermohaline circulation transports large
  amounts of warm water into the North Atlantic and
  keeps Northern Europe warmer than it would
  otherwise be. Here is a forecast of temperature
  changes 30 years after a total shutdown of the
  THC

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Ocean dynamics El Niño

• A notable example of coupled ocean-atmosphere
  dynamics is the El Niño Southern Oscillation
  (ENSO). It is hard to say which is driving the
  system.
• In normal (La Niña) years, the surface winds blow
  strongly from the East. This drives ocean
  upwelling off South America, tilts the
  thermocline towards the West, and piles up a warm
  water pool near Indonesia, with strong convection
  and rainfall above it.

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Ocean dynamics El Niño

• A notable example of coupled ocean-atmosphere
  dynamics is the El Niño Southern Oscillation
  (ENSO).
• The other mode of the oscillation (El Niño) is
  associated with weak trade winds, less tilting of
  thermocline, reversal of upwelling/downwelling
  patterns, displacement of warm water pool to
  central Pacific, drought in Asia, torrential
  rains in America, and failure of Peruvian
  fisheries, among other things.

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Age of water masses

• Oceanographers use age of water masses to refer
  to the time since the water was ventilated at the
  surface of the ocean. This can be traced, e.g.
  with 14C

This plot shows that mixing of deep water in the
Atlantic takes 200 years Generally speaking,
Atlantic deep water is young, Pacific deep water
is very old (no sites of deep water formation in
Pacific, supplied by deep flow from Atlantic)
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Age of water masses

• Problem and opportunity contamination of 14C by
  20th century atmospheric nuclear tests
• Provides tracers of circulation
• Can be corrected out

Diffusion across thermocline
North Atlantic Deep Water formation
3H in N. Atlantic
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Age of water masses
N. Pacific 2200 years old
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Chemistry of seawater

• The chemistry of seawater depends on inputs
  (mostly from rivers) and outputs (mostly to
  sediment) as well as biological pumping within
  the ocean
• The inputs are
• Rivers, Atmospheric deposition, Hydrothermal
  Venting
• The outputs are
• Sedimentation, Evaporation, Hydrothermal
  Alteration
• Remember at steady state the residence time of an
  element is the mass in the ocean divided by the
  mass flux into the ocean
• For water, t 1.37 x 1021 l / 3.6 x 1016 l/yr
  38000 yr
• For K, 10 mM in seawater and 34 mM in rivers, t
  1.1 x 107 yr
• For Pb, 10 pM in seawater and 5 nM in rivers, t
  80 yr

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Chemistry of seawater

• The composition and relative importance of inputs
  varies with time, so seawater composition can
  vary somewhat also.
• Example history of 87Sr/86Sr of seawater
• Controlled by balance between hydrothermal input
  (mantle isotope ratios .702) and continental
  weathering (radiogenic isotope ratios .710)

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Chemistry of seawater

• Elements are divided into categories based on how
  they behave in the ocean
• Conservative elements vary exactly like salinity,
  i.e. only by dilution and concentration. In
  principle there are no sinks. In principle the
  residence time is infinite.
• Nutrient elements are essential for life and are
  stripped efficiently out of shallow waters where
  productivity is high, then regenerated at depth
  by respiration or decay of falling organic
  matter. The residence time in the shallow ocean
  is very short but in the whole ocean is long.
• Scavenged elements are supplied at the surface
  but are readily adsorbed onto particles and
  removed by sedimentation. The residence time is
  short.

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Nutrients and Biomineralization
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• Both plants and animals make mineral hard parts,
  either of silica or CaCO3 (a) Coccolithophorids
  (plants, CaCO3), (b) Foraminifera (animals,
  CaCO3), (c) Diatoms (plants, SiO2), (d)
  Radiolaria (animals, SiO2)

Exceptions Acantharia, a class of protozoans,
make SrSO4 shells vertebrates make Ca5(PO4)3(OH)
bones
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Nutrient Elements
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• Nitrate, Phosphate, Silica, and Iron are
  essential nutrients and are almost totally
  consumed in surface waters by photosynthesis of
  organic matter. As falling organic matter is
  respired (or remineralized), the nutrients are
  regenerated
• Oxygen has the opposite behavior. Cartoon
  photosynthesis/respiration reaction
• CO2 H2O CH2O O2

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Oxygen, the anti-nutrient element

• Oxygen increases with depth below the oxygen
  minimum because it is supplied by relatively
  young ventilated water below.
• However, when productivity is very high or where
  supply from below is cut off, deep waters may
  become anoxic
• The Black Sea is permanently anoxic
• The Gulf of Mexico has a seasonal dead zone
  caused by fertilizer-rich Mississippi River
  runoff
• Occasional global anoxic events associated with
  mass extinction events leave widespread black
  shale deposits full of unoxidized organic matter

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Nutrient Elements
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• Oceanic organic matter contains the element C, N,
  and P in nearly constant ratios, the Redfield
  Ratios C106N16P1
• Seawater contains N and P in exactly the same
  ratio! This implies two seemingly contradictory
  things

• All P and N in the shallow ocean comes from
  remineralization of organic matter at depth, so
  it is supplied to the cycle with the Redfield
  ratio
• Life has evolved to optimally utilize available
  nutrients, leaving neither in significant excess
• Very slight excess in PO4 (which comes only from
  weathering input) implies NO3 is limiting, but N2
  can be fixed to NO3 if Fe is available, hence
  ideas about Fe fertilization of ocean and CO2
  sequestration...

(anoxic)
(with Fe)
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Nutrient Elements

• So PO4 concentrations, e.g., indicate where
  upwelling is providing nutrients and hence where
  primary productivity (i.e. photosynthesis) occurs
  in the ocean
• These are also the locations where diatoms are
  making SiO2 shells that rain out to form
  siliceous sediments

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Dynamics, Nutrients, Productivity, Silicate
sediment
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• We have completed one logic chain Wind stresses
  via the Coriolis force drive upwelling of deep
  waters at high latitudes and west coasts, which
  brings remineralized nutrients into the photic
  zone, which allows plankton to mineralize opal,
  which falls onto the seafloor and accumulates at
  such locations

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Carbonate Chemistry
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• Another critical aspect of ocean chemistry is the
  coupled behaviors of CO2, HCO3, and CO32-
• These buffer the pH of seawater, provide the
  carbon to be reduced by photosynthesis and the
  carbonate to make shells, and constitute one of
  the Earths principal reservoirs of Carbon (60x
  more than the atmosphere!).

• The key reactions are
• CO2 H2O HCO3 H
• HCO3 CO32 H
• Given total CO3 and pH these speciation
  equilibria are determined at the pH of seawater
  bicarbonate is the dominant ion
• Note high sensitivity of CO32 concentration to
  pH at neutral pH

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Carbonate Chemistry Chicken and egg
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• So primary production increases seawater pH
• HCO3 H -gt CO2 H2O -gt CH2O O2 , consumes
  H
• This drives up CO3 concentrations in surface
  waters, although biomineralization of CaCO3
  moderates this effect a little
• Ca2 HCO3 CaCO3 H , produces H
• These mechanisms keep shallow ocean
  supersaturated with respect to calcite

But carbonate solubility increases with pressure,
and there is a crossover at the Carbonate
Compensation Depth
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Water depth, Solubility, Calcareous sediment
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• The shallow oceans are supersaturated with
  respect to calcite, the deep ocean is
  undersaturated. Hence Calcareous sediment only
  accumulates (either by reef building or rain of
  planktonic shells) in water shallower than the
  CCD, and these locations are restricted to young
  seafloor, continental margins, and oceanic
  plateaux

Areas with gt70 CaCO3 sediment
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Oceanic sediment patterns, explained

• So we have explained the areas of the seafloor
  dominated by calcareous (shallow spots) and
  siliceous (high productivity spots) biofossil
  ooze. The remaining deep seafloor accumulates
  (very slowly) only pelagic clay. Finally, areas
  near continents may be dominated by terrigenous
  or periglacial clastic sediments.

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