Geography 104 - “Physical Geography of the World’s Oceans”
Properties of Seawater
Last time talked about properties of water (Table 7.2)
- dissolves solids and gases readily (“universal solvent”)
Addition of dissolved solids over geologic time (billions of years) has
converted pure water to “salt water” or seawater.
Dissolved solids change water properties significantly:
- density (very important for ocean circulation)
- freezing point
A salt, in chemistry, is an ionic compound composed of cations
(positively charged ions) and anions (negative ions) so that the product
is electrically neutral (without a net charge).
Salts are formed by a chemical reaction between:
- a base and an acid, e.g. NH3 + HCl → NH4Cl
- a metal and an acid, e.g. Mg + H2SO4 → MgSO4 + H2
- a base and an acid anhydride, e.g. 2 NaOH + Cl2O → 2 NaClO + H2O
- an acid and a basic anhydride, e.g. 2 HNO3 + Na2O → 2 NaNO3 + H2O
-salts can also form if solutions of different salts are mixed, their ions recombine, and the new
salt is insoluble and precipitates (see: solubility equilibrium), for example:
Pb(NO3)2(aq) + Na2SO4(aq) → PbSO4(s) + NaNO3(aq)
sources of salt ions
sources
sinks
hydrogen bonds
electrical attraction
between water molecules
water as a solvent
salt has alternating ions; water molecules work their way between salt ions
water as a solvent
ocean
dissolved or “hydrated” salt
ions have a charged attraction
to water molecules and thus
move exactly with the
individual water molecule to
which they are attached
solid salts would sink under gravity
dissolved solids in seawater
99.9% of dissolved solids
99.3%
< 1%
dissolved solids in seawater
principle of constant proportions –
ratio between major ions is nearly
constant by weight and independent
of salinity concentration, thus need
to measure only a single ion
this is true for “open ocean” waters
ratios not exact for “coastal” waters
First salinity measurements:
S = 1.80655 x Cl (ppt)
salinity
salt content of seawater quantified by salinity (S)
salinity – mass of salts (dissolved solids) per kg seawater
salinity units – parts per thousand (ppt or o/oo)
1g salts / 1kg seawater = 1 ppt
typical open ocean salinity S = 35gsalts / 1 kgsw
dissolved solids in seawater
Pure
seawater by mass
~96.5% water
~3.5% “salts” (salinity)
35 ppt or o/oo on average
33 -> 38 ppt in open ocean
minor constituents < 1 ppm essential to
marine organisms, and also used as “tracers” in
physical oceanography
atmospheric deposition of iron
nutrients for biological oceanography
NO3 – nitrate
NO2 – nitrite
NH4 – ammonium
Si: SIO4 – silicate
P: PO4 – phosphate
N:
nutrients: Atlantic vs. Pacific
phosphate
nitrate
silicate
Necessary for primary production, depleted in surface due to PP uptake,
nutrients accumulate over time in deeper water time (decomposition of
particles). Differences in curves show “age” of water.
residence time of water in the atmosphere
precipitation and evaporation cause variations in salinity
residence time of water in the atmosphere
reservoir amount: Q = (4.5+11)x1015 kg = 15.5x1015 kg
residence time of water in the atmosphere
outflow: Qout = (107+398)x1015 kg yr-1 = 505x1015 kg yr-1
inflow: Qin = (71+434)x1015 kg yr-1 = 505x1015 kg yr-1
inflow = outflow => steady state
residence time of water in the atmosphere
residence time = reservoir amount / flow amount
15.5x1015 kg
= .03 yr = ~ 1.5 weeks
505x1015 kg yr-1
reservoirs of dissolved solids
measurement of salinity, or “collection of salinity profiles”
CTD or Conductivity Temperature Depth sensor is the workhorse
of physical oceanographic measurements
CTD measures conductivity, temperature, and pressure, to give
ocean water salinity, temperature, as a function of depth.
measuring salinity with electrical conductivity
measurement standard and units
developed in 1978 gave rise to:
practical salinity units (psu)
practical salinity scale (pss)
oceanographers are very loose
with salinity units: ppt, psu, pss,
no units.
measuring salinity with electrical conductivity
1. measure temperature
(and pressure)
2. measure
conductivity
3. derive salinity
horizontal interleaving of water masses
Fig. 7.15.c
World Ocean Circulation Experiment (WOCE)
Section P16 – salinity along 150° W
salinity fronts
Readings for next time (more properties of seawater, gases):
- should have read all of Chapter 6, and reader article on salinity (pgs 13 – 22).
- read reader articles on Fate of Carbon Dioxide (pgs 23 - 24)
- read reader article on hypoxia (pgs 25 – 29)
Get a start on first half of HW problems!
My Seawater Summary
Water has hydrogen bonds, electrical attraction between molecules. Salts have
alternating ions. This combination of chemical compositions allows water molecules
to dissolve salts. There are 11 major salt constituents. They exist in constant
proportion, which allows total salts to be determined from the measurement of the
mass of a single ion. Six (6) are generally considered because the combination of the
5 with the least concentrations comprise < 1% of the total weight of salts.
Salinity (S) is defined as the mass of salts per kg seawater. S is measured in ppt
(o/oo). S = 1 ppt = 1 g salts / 1 kg seawater. Open ocean S values average ~35 ppt
and range from ~33 to 38 ppt. Values can be higher in small enclosed water bodies
and lower in coastal areas. The ocean also has minor constituents that have
negligible contribution to S, but are important for primary production and for tracing
the “age” of water.
Evaporation and precipitation are responsible for variations in S. The hydrologic cycle
was used to demonstrate the idea of residence time for a steady state process.
Residence times of salt ions are very long,1000’s of years and greater.
CTD’s are used to measure conductivity, temperature and pressure profiles. Salinity
is determined by conductivity in psu, or pss units. But, any all or no units are now
used. There is structure with depth related to density that we’ll learn about next
week.
S at sea surface shows spatial structure. Atlantic saltier than Pacific due to
evaporation over Atlantic trades blowing and precipitation in Pacific. Fresher in polar
regions due to snow melt, fresher in coastal regions due to river runoff.