Environmental Chemistry

Water and the Oceans What are the distribution and flows of water through the Earth system? What factors control these flows and what conditions do they influence? What chemical processes occur as a result of water cycling and how do these processes shape the composition of oceans and other bodies of water? How is ocean composition changing as a result of anthropogenic activities? What are the impacts of these changes on ocean life and ecosystems?

Important water properties Extremely strong intermolecular interactions (hydrogen bonding) Liquid is more dense than solid  volume minimum at 4 C High heat capacity  stabilizes temperature swings High melting and boiling points  water stays liquid over wide T range Extremely good solvent of ionic and polar compounds

Hydrological Cycle Estimates of the main water reservoirs in plain font (e.g. Soil moisture) are given in 103 km3 and estimates of the flows between the reservoirs in italic (e.g. Surface flow) are given in 103 km3/year. Figure from Trenberth et al. (2007) American Meteorological Society.

Water Distribution

Water availability Available renewable fresh water in 1000 m3 per capita per year.

Water density Temperature Density kg/m^3 999.868 4 1000.000 10 999.728 999.868 4 1000.000 10 999.728 20 998.234 Ice at 0 917 http://www.csgnetwork.com/h2odenscalc.html

So What? Ice Floats Seasonal overturn of lakes Consider a typical lake in the Summer This is a lake ---- T, r ----> Epilimnion (25 oC) Thermocline Depth Hypolimnion (15 oC)

Lake in the Fall ---- T, r----> (15 oC) Mixing (15 oC)

Winter ---- T, r ----> Epilimnion (<4 oC) Thermocline Depth Hypolimnion (4 oC)

Lake in the Spring ---- T, r ----> (4 oC) Mixing (4 oC)

What about the ocean Due to the salts dissolved in seawater, seawater does not have a density minimum. Cold water is more dense Saltier water is more dense Cold, salty water sinks

Thermohaline circulation

Ocean water composition

Notes on units 1 M (molar) = 1 mol L-1 ppm, ppb etc. are on a mass basis (ppmm) 1 ppm = 1 mg/kg = 1 mg/L Assumes water density of 1 kg/L Concentrations are sometimes expressed as concentration of an element rather than the compound E.g., 7 ppm N as NO3-

Ocean water composition

Solubility Rules

Carbonate-silicate cycle Timescale = 105 – 107 yrs J. F. Kasting, Science Spectra, 1995

Forms of CO2 in water H2CO3 (aq) ↔ H+ + HCO3- HCO3- ↔ H+ + CO3-

Alkalinity