Parameter definitions

Standard definitions for the parameters shown in this atlas are as follows. Further details can be obtained from the suggested references or from a standard textbook such as Pond and Pickard (1995):

Potential temperature (°reeC)
The potential temperature,
Θ , is defined as the temperature that a sample of seawater would attain if brought adiabatically (without gain or loss of heat to the surroundings) from the pressure appropriate to its depth to the ocean surface (see e.g., Feistel, 1993).

Salinity (PSS78 scale)
The salinity, S, is essentially a measure of the mass of dissolved salts in one kilogram of seawater. Because the major ions in seawater are found in a constant ratio to each other, the salinity of a sample of seawater is now measured in terms of a conductivity ratio relative to a standard solution of potassium chloride. Thus salinity values according to the current definition of the Practical Salinity Scale of 1978 (PSS78) are dimensionless with no units. (See e.g., UNESCO, 1981).

Neutral density (kg/m
Neutral density,
γn , gives a very close approximation to truly neutrally buoyant surfaces over most of the global ocean. γn is a function of salinity, in situ temperature, pressure, longitude, and latitude. (See e.g., Jackett and McDougall, 1997). By convention all densities are quoted as the actual density minus 1000 kg/m3.

Potential density (kg/m3 )
The potential density,
σ , is the density a parcel of water would have if it were moved adiabatically to a standard depth without change in salinity. σ0, σ2 and σ4 are the potential densities of a parcel of seawater brought adiabatically to pressures of 0, 2000 and 4000 decibars, respectively. (See e.g., Pond and Pickard, 1995).

Oxygen (μmol/kg)
The dissolved oxygen content, O2 , can be used to trace certain water masses. Oxygen enters the ocean from the atmosphere, but is also produced in the surface layers by phytoplankton and is consumed during the decomposition of organic material. This leads to relatively large changes in concentration depending on depth, position and initial solubility (which is a function of temperature and salinity). (See e.g., Broecker and Peng, 1982).

Nitrate, Phosphate and Silicate (μmol/kg)
Nitrate, NO
3 , Phosphate, PO4 , and Silicate, Si, are some of the main nutrients utilised by phytoplankton. They are also non-conservative tracers, but vary inversely with oxygen concentration in the upper- and mid-ocean. They are supplied mainly by river runoff and from sediments. (See e.g., Broecker and Peng, 1982).

Chlorofluorocarbons (pmol/kg)
Chlorofluorocarbons, CFCs, are anthropogenically produced chemicals that enter the ocean from the atmosphere. Since they have a time-varying atmospheric history, they can be used to deduce information on mixing rates in the ocean and to follow the movement of water masses forming at the sea surface (see e.g., Weiss et al., 1985).

Total Carbon dioxide (μmol/kg)
The total dissolved inorganic carbon content of seawater is defined as:

2 = [CO2 *] + [HCO3- ] + [CO32- ]

where square brackets represent total concentrations of these constituents in solution (in mol/kg) and [CO
2 *] represents the total concentration of all un-ionised carbon dioxide, whether present as H2 CO3 or as CO2 . (See e.g., DOE, 1994 for further details.)

Alkalinity (μmol/kg)
The total alkalinity of a sample of seawater is defined as the number of moles of hydrogen ion equivalent to the excess of proton acceptors (bases formed from weak acids with a dissociation constant K < 10-4.5 at 25 ° C and zero ionic strength) over proton donors (acids with K > 10-4.5) in one kilogram of sample. Many ions contribute to the total alkalinity in seawater, the main ones being HCO 3-, CO3 2-, B(OH)4 - and OH-. (See e.g., DOE, 1994 for further details.)

Delta Helium-3 (%)
Radioactive tracers such as delta Helium-3, δ3He , can be used to derive quantities such as mean residence times and the apparent ages of certain water masses. Helium isotope variations in seawater are generally expressed as δ3He (%), which is the percentage deviation of the 3He/4 He in the sample from the ratio in air (Clarke et al, 1969). This can be written as:
δ3He (%) = 100 x { (3He/4He) sample
/( 3He/4He) air -1}

Tritium (3H) is produced naturally from cosmic ray interactions with nitrogen and oxygen and as a result of nuclear testing. It is used particularly for examining the structure of and mixing within the oceanic thermocline. If combined with Helium-3 measurements tritium can be used to calculate an apparent age of a water mass. Tritium is reported in Tritium Units, TU, which is the isotopic ratio of 3H/ 1H multiplied by 1018. It is determined mass spectrometrically by the 3H regrowth technique (Clarke et al, 1976) using atmospheric helium as a primary standard. (See e.g., Schlosser, 1992).

Carbon-14 (/mille)
Carbon-14, Δ14C, ratios can be used to infer the rates of mixing in the ocean. These ratios are expressed as the per mil difference from the 14C/C ratio in the atmosphere prior to the onset of the industrial revolution and normalized to a constant 14C/ 12C ratio (see e.g., Broecker and Peng, 1982). The equation used is as follows:

Δ14C = δ14C - 2(δ13C+25)(1+ δ14C/1000)

where δ14C = {(14C/C)sample / (14C/C)standard