Ocean Water Chemistry

Water sets Earth apart from other planets

On Earth, 71% of surface is water (hydrosphere); only 29% is land 97% of earth’s water is oceanic (salty) 2% is ice (cryosphere) in valley &

continental glaciers .5% is ground water <.1% is all else (atmosphere, lakes,

rivers, soil moisture)

How did all that water get here?

• 2 major sources

Volcanic outgassing

water from within the Earth

Comets

Water from outside the Earth

The Water Molecule and Water

• Exists as gas, liquid, & solid on Earth’s surface• Remarkably high heat capacity (specific heat)

makes it slow to heat and slow to cool• Very high solvent power (dissolves everything!)• Molecule is “dipolar” (hydrogen[+], oxygen [-])• Expands when freezes (ice floats, role in

erosion)• Water density increases as temperature

decreases (inverse relationship like air)

Salinity in Steady State Equilibrium

• Even though salts are continually washed into the sea, the average salinity has remained the same for the last billion yrs.

• Several mechanisms exist to remove salt:– Salt spray on land and into air– Salts precipitating along coastlines and

marginal ocean basins– Marine organisms use salt ions to build shells,

then die and are incorporated into sediments

Salinity• Average in oceans is 3.5% or 35 ppt (written as

35 0/00)• 1-20 ppt “brackish” (hyposaline) water -

common in estuaries.• 20-30 ppt “transitional marine” – coastal areas

near fluvial runoff point source• 40-50 ppt “restricted lagoon” – water enclosed,

strong sunlight• >50 ppt “hypersaline” high evaporation – rare Because brackish water is so common & hypersaline

water is so rare, most organisms adapt to the former and die in the latter.

Salinity

• 5 different ways to measure salinity

• Three ways to examine salinity in the ocean– Surface salinity (isohalines)– At one site from surface to ocean floor

(halocline)– Looking at an ocean in profile N-S ………... And fifty ways to leave your lover (Paul Simon)

Salinity Measurement

• Simple evaporation (gasses are lost)

• Hydrometer (direct ratio) density

• Salinometer (direct ratio) electrical conductivity of sea water

• Refractometer (direct ratio) refraction (bending) of light

• Chlorinity (titration) extremely accurate [Forschammer/Ditmar]

Principle of Constant Proportion(principle of constant composition)

• Forschammer/Ditmar– Works only with the major elements Na, Cl,

SO4, Mg, Ca, K

The formula:• 1.80655 x chlorinity 0/00 = salinity 0/00

or: 1.80655 x 18.98* = 34.2 0/00

* average chlorinity of seawater

By using Forschammer’s principle, it is possible to determine salinity very accurately by measuring only one element.

Ocean Salinity• Major elements Na, Cl, SO4, Mg, Ca, K (>99%)• Minor elements HCO3, Br, H3BO3, Sr, F [both major and minor

are 99.99%]• Trace elements Li, I, Mo, Zn, Fe, Al, Cu, Mn, Co, Pb, Hg, Au• Nutrients P, N, Si, Fe, Zn & their compounds (critical for plant

growth)• Organic compounds (lipid, proteins carbohydrates, hormones,

vitamins)• Dissolved gasses (CO2, O2, N2, Ar, H2, Ne, He)• Particulates

– Inorganic-clay, dust, ash– Organic-plants, & animals in the sea

• [Grant Gross refers to seawater as “a dilute, dirty, living organic soup]

CCD and Ocean Temperature• CCD=Calcium Carbonate Compensation Depth• Actually is top of the AABW- below the CCD

water is extremely cold and CaCO3 dissolves easily

• During glacial/interglacial flux the volume of AABW waxes and wanes- So does CCD level– Undergoes “excursions” – rises during glaciation and

lowers during interglacial periods

• During excursions the amount of chalky limestone getting to the sea floor is in flux

• Oceanographers use limestone incidence as an accurate measure of paleotemperature

Isotopes in Oceanography

• All matter is made from atoms• Atoms have two types of particles in nucleus that

have mass (weight) – protons and neutrons• The # of protons defines the element (ex.

Carbon always has 6 protons, oxygen 8)• The # of neutrons can differ• Isotope- like an atom but # protons & neutrons

not equal• Isotopes heavier or lighter than atoms & may be

either stable or unstable

Stable Isotopes

• O18 is stable • O18 1/8 heavier than O16

• During evaporation O18 is left behind and O16 evaporated

• O16 enriched water is locked up on land as glacial ice during an ice age.

• During an ice age, ocean waters are enriched with O18 in animals shells/skeletons – silica (SiO2) or lime (CaCO3) will reflect increased O18

• Can read paleotemperature directly from ratio of O18 in shell/skeleton

Unstable (Radioactive) Isotopes

• Carbon 12 (C12) has an isotope Carbon 14 (C14) made from N in upper atmosphere

• C14 changes back into Nitrogen (N)

• Half-life- it takes 5500 yrs for half of the C14 to change into N

• We measure the ratio of C14 to C12 to determine age

“Old Carbon”expression of how fast an ocean is

recycling

• Pacific – 1000-1600 yrs (large ocean, very slow)

• Atlantic – 500-800 yrs (smaller ocean, more rapid recycling)

• Indian - 700-1200 yrs (“middle child”)

Residence Timerecycling of chemicals in and out of the sea

• Short residence time: Si (8,000 yrs), Mn (700 yrs), Fe (140 yrs), Al (100 yrs)– Chemically reactive (Al), rare (Mn), critical to

biological systems (Fe, Si)• Long residence time Cl (80 m.y.), Ca (1 m.y.), K

(11 m.y.), Na (210 m.y.) Mg (12 m.y.)– Not reactive, extremely abundant or not used

in biology • Water has residence time of ~ 3-4000 yrs