Chemical Oceanography

1. Chemical Oceanography Understanding the distribution, fluxes and transformations of chemicals in the sea The ecology of elements and molecules http://www.youtube.com/watch?v=rSAaiYKF0cs

2. 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?

3. Interdisciplinary nature of oceanography biochemistry Biology Chemistry Oceanography geochemistry biophysics geophysics Physics Geology Mathematics encompasses all Adapted from Pinet, An invitation to oceanography

4. 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

5. Libes, Fig. 1.2 A conceptual box model

6. Residence time of water in the atmosphere is only 11.2 days! The Hydrological Cycle A quantitative box model 0.001% Arrows represent fluxes 83% of the land-based water is in ice! 2.4% Boxes represent reservoirs Percentages indicate fraction of Earth’s surface water 97.5% Residence time of water in the ocean is ~3800 years http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/bdgt.rxml

7. The World Ocean - covers 72% of the Earth’s surface Major ocean basins Pacific, Atlantic, Indian, Southern (Antarctic), Arctic Gulfs, marginal seas, fjords, estuaries etc. The average depth of the ocean is ~3,700 meters; the maximum depth is about 11,000 m 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.

8. 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.

9. Vertical zonations in the water column Epi-pelagic (0-200m) Euphotic zone Meso-pelagic (200-1500 m) Abysso-pelagic (>1500 m) Aphotic zone Depth ranges are approximate

10. The ocean is a layered system- controlled by density, which is a function of salinity and temperature Temperature Density Salinity 0 Mixed Layer Mixed Layer 50 Complex pattern of salinity with depth, depending on water mass Pycnocline restricted vertical mixing Thermocline 500 Meters Horizontal mixing occurs along isopycnals (lines of constant density) 1000 1000 Relatively well mixed vertically Well mixed deep ocean 4000

12. 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.

13. From Libes

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

15. Another view

16. Evaporation minus precipitation Surface salinity distribution in the oceans is partially driven by Net Evaporation (Evap-Precip). Degrees latitude Where E-P (evaporation – precipitation) is > zero, salinity tends to be high.

17. 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

20. 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 1000 Where  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

21. 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) There is a new formulation called TEOS-10 based on thermodynamic principles S= Salinity, t = temperature(oC), P = pressure (bars), v = specific volume (m3 kg-1), K = fitting factor

22. 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

23. Pressure heating of water becomes quite significant at extreme ocean depths. The potential temperature is relatively uniform below 4000 m Figure from Millero.

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

25. 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.

26. 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).

27. Mississippi River Plume near Southwest Pass

28. Mississippi River Plume near Southwest Pass

29. View from Dauphin Island bridge showing multiple water masses and fronts

30. Temperature (oC) Salinity T-S diagrams can distinguish different water masses Lines of constant density

31. Water masses are defined by density and other characteristics Atlantic Salinity section from WOCE 94 data set Antarctic intermediate water North Atlantic Deep Water Antarctic bottom water South Equator North

32. Ocean Conveyor (Meridional Overturning) circulation

33. End

34. Surface mixed layer - Wind driven mixing homogenizes the density in the surface layer to some depth. Thermocline(stratified with temperature), orpycnocline (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) Deep ocean- Weak gradients (relatively well mixed vertically)