Chemical Oceanography

The ecology of elements and molecules

Understanding the distribution, fluxes and

transformations of chemicals in the sea

http://www.youtube.com/watch?v=rSAaiYKF0cs

Fundamental questions of Chemical Oceanography

1. What is seawater? What chemicals are dissolved in seawater and where did they come from? Why are some substances present at high concentrations while others are at low concentrations? What are the consequences of these differences?

2. How does the chemistry of the ocean affect life and how does life affect the chemistry of seawater? How does Man (a form of life) affect the ocean chemistry?

3. How is the ocean system connected to, and influenced by the geosphere (the Earth’s crust) and the atmosphere?

Adapted from Pinet, An invitation to oceanography

ChemistryBiology

Physics Geology

Oceanography

biochemistry

geochemistry

geophysics

biophysics

Interdisciplinary nature of oceanography

Mathematics encompasses all

The Crustal-Ocean-Atmosphere Factory (Libes, Chap 1)

“the ocean acts as a giant stirred flow-through reactor in which solutes and solids are added, transformed, and removed” - Libes

A conceptual box model

Libes, Fig. 1.2

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/bdgt.rxml

97.5%

2.4%

0.001%

83% of the land-based water is in ice!

The Hydrological Cycle

Percentages indicate fraction of Earth’s surface water

Boxes represent reservoirs

Arrows represent fluxes

A quantitative box model

Residence time of water in the ocean is ~3800 years

Residence time of water in the atmosphere is only 11.2 days!

The World Ocean - covers 72% of the Earth’s surface

Major ocean basinsPacific, Atlantic, Indian, Southern (Antarctic), Arctic

Gulfs, marginal seas, fjords, estuaries etc.

The ocean is a thin skin of water on the planet. Vertical depth averages 4 km, but the horizontal dimension of ocean basins is 5000-10000 km.

The average depth of the ocean is ~3,700 meters; the maximum depth is about 11,000 m

Horizontal zones

Nearshore (estuarine, coastal, littoral)

Neritic (Shelf environments shallower than 200 m)

Pelagic (Open ocean -off the shelf break >200 m isobath)

These zones refer to both the water column and the underlying sediments. For example - Nearshore sediments, pelagic sediments etc.

Epi-pelagic

(0-200m)

Meso-pelagic

(200-1500 m)

Abysso-pelagic

(>1500 m)

Vertical zonations in the water column

Euphotic zone

Aphotic zone

Depth ranges are approximate

050

500

1000

Meters

1000

4000

Temperature

Mixed Layer

Thermocline

Well mixed deep ocean

Salinity Density

Pycnocline restricted vertical mixing

The ocean is a layered system- controlled by density, which is a function of salinity and temperature

Complex pattern of salinity with depth, depending on water mass

Mixed Layer

Relatively well mixed vertically

Horizontal mixing occurs along isopycnals (lines of constant density)

Salinity is broadly defined as the salt content of the water in grams per kilogram or parts per thousand (ppt). (A more precise modern definition will be given later).

Most ocean water falls in a narrow salinity range, with an average salinity of 34.72 o/oo.

Most ocean water is very cold! The average temperature is 3.5 oC.

From Libes

Sources: Open University: Seawater: its composition, properties and behavior

Another view

Evaporation minus precipitation

De

gre

es

latit

ude Where E-P (evaporation

– precipitation) is > zero, salinity tends to be high.

Surface salinity distribution in the oceans is partially driven by Net Evaporation (Evap-Precip).

What causes Variations in salinity -• coastal run-off, groundwater input • evaporation/precipitation.

Latitudinal differences- • high latitudes; high precip, low evap --> low salinity• low latitudes; high evap, high precip --> med salinity•Mid latitudes; high evap, low precip --> high salinity

The density of seawater is the primary factor governing movement and distribution of a water mass. = mass/volume

Density is a function of temperature, salinity and pressure. Seawater at 1 atm pressure with a salinity of 35 and a temperature of 15 oC has a density of 1025.9728 kg m-3 solution (=g cm-3). This many significant figures are typically necessary, but are tedious.

Oceanographers use a shorthand notation called Sigma (): = ( -1) x 1000Where is the density (kg m-3)

Sigma-T (t) refers to density at 1 atm of pressure. So for water with salinity of 35, at 15 oC and 1 atm

t = 25.9728

The density of seawater can be calculated from Salinity & Temperature and Pressure using the International Equation of State for Seawater(I have and Excel spreadsheet set up with this equation)

S= Salinity, t = temperature(oC), P = pressure (bars), v = specific volume (m3 kg-1), K = fitting factor

There is a new formulation called TEOS-10 based on thermodynamic principles

Water is relatively incompressible, but, with hydrostatic pressures of up to 1000 atmospheres the effects of compression on temperature and density cannot be ignored in all cases.

Because of compressive heating, oceanographers use potential temperature (), which is the temperature a piece of water would have if brought to the surface without internal heat exchange (adiabatically). Potential density is the density the water would have if brought to the surface adiabatically (without heat loss or gain.

Hydrostatic pressure is the pressure exerted by the column of water above. Pressure increases approx. 1 atmosphere (=~1 bar) for every 10 m of depth

Hydrostatic pressure is the pressure exerted by the column of water above. Pressure increases approx. 1 atmosphere (=~1 bar) for every 10 m of depth

Pressure heating of water becomes quite significant at extreme ocean depths.

The potential temperature is relatively uniform below 4000 m

Figure from Millero.

Cross section of potential temperature (θ) in the Atlantic Ocean (Millero)

The structure of the ocean water column is controlled by the density of seawater.

Dense water sinks below less dense water. The sinking of dense water will force lighter water upward (upwelling). Freshwater plumes float over saltier (denser) water.

Water masses - Major parcels of ocean water have unique set of properties which set them apart from other water masses. Characteristics include, salinity, temperature, (density), oxygen, nutrients, alkalinity, pH.

Oceanographers rely heavily on use of conservative tracers to delineate water masses. Salinity is one of these tracers, along with stable and radioactive isotopes, non-reactive gases, anthropogenic molecules (freons).

Mississippi River Plume near Southwest Pass

Mississippi River Plume near Southwest Pass

View from Dauphin Island bridge showing multiple water masses and fronts

Te

mp

era

ture

(oC

)

Salinity

T-S diagrams can distinguish different water massesT-S diagrams can distinguish different water massesLines of constant density

Atlantic Salinity section from WOCE 94 data set

North Atlantic Deep Water

Antarctic bottom water

Antarctic intermediate water

Water masses are defined by density and other characteristics

Equator NorthSouth

Ocean Conveyor (Meridional Overturning) circulation

End

Surface mixed layer - Wind driven mixing homogenizes the density in the surface layer to some depth.

Deep ocean - Weak gradients (relatively well mixed vertically)

Thermocline (stratified with temperature), or pycnocline (stratified with density). Cline = strong gradient or rapidly changing property. Vertical density stratification restricts vertical mixing of water, chemicals, or microplankton.

Horizontal mixing occurs along lines of constant density (isopycnals)