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

1

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

2

Physical properties of seawater• The important variables are salinity and temperature, which

together determine the most important parameter, density

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.

On this plot, t is density in kg/m3 relative to pure water at 0°CS,T is specific volume in cm3/(100 kg) relative to water at 35 psu and 0°C

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. 3

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, , the temperature water would have if adiabatically expanded to 1 bar pressure (although difference between in situ T and 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

4

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

5

dT –dt, 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

– The Coriolis force is an essential part of the dynamics:

V ⋅2Ωsinφ=−1ρ∇ ⋅P

• 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

6

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

7

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

8

You want to go fishing here. Not here.

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. 9

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:

10

Ocean dynamics: thermohaline circulation• Broeker’s version: the ‘conveyor belt’

11

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:

12

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.

13

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.

14

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)

15

Age of water masses• Problem and opportunity:

contamination of 14C by 20th century atmospheric nuclear tests

• Provides tracers of circulation • Can be corrected out

North Atlantic Deep Water formation

Diffusion across

thermocline

3H in N. Atlantic

16

Age of water masses

N. Pacific ~2200 years old

17

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, = 1.37 x 1021 l / 3.6 x 1016 l/yr = 38000 yr• For K, 10 mM in seawater and 34 M in rivers, = 1.1 x 107 yr• For Pb, 10 pM in seawater and 5 nM in rivers, = 80 yr

18

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)

19

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.

20

Nutrients and Biomineralization• 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

21

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

22

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

23

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...

(with Fe)(anoxic)

24

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

25

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

26

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– = CO3

2– + 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

27

Carbonate Chemistry: Chicken and egg• 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

28

Water depth, Solubility, Calcareous sedimentThe 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

29

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.

30