Transcript Lecture 6: Igneous classification, mid

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
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• The mixed layer is a windstirred region in the top
~100 m where stratification
is absent
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 0°C
DS,T is specific volume in
cm3/(100 kg) relative to
water at 35 psu and 0°C
Note: seawater, at about 35‰
salinity, has monotonically
increasing density on an
adiabat and is stably stratified.
Freshwater has a density
inversion at 4°C 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
≤0.1°C)
• Both salinity and potential temperature are conservative
properties of seawater…they 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
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dT ~ –dst,
except here
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
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V  2sin   
 P

– 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 seasurface height relative to the geoid
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Ocean dynamics in a very small nutshell
• Ekman pumping…water 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 pumping…water 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
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where longshore winds drive offshore shallow water flow at right angles.
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. Stommel’s version:
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Ocean dynamics: thermohaline circulation
• Broeker’s 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
Nutrient Elements
• 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
• 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)
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
• 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
• 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 Earth’s 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+ > CO2 + H2O -> 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
Water depth, Solubility, Calcareous sediment
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
>70% 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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