Carbon (C)

Atomic number: 6
Atomic weight: 12.011
Average concentration in ocean:

DIC  2248 µmol/kg
DOC    42 µmol/kg
POC    0.4 µmol/kg
PIC
Residence time: DIC   83,000 years
DOC   
POC           7 days
PIC

Distribution in ocean
Dissolved inorganic carbon  has a nutrient-like vertical profile due to uptake by phytoplankton.  Concentrations are lowest in the Atlantic, where deep water sinks into the ocean depth,  and higher in the deep waters of the Pacific as POC sinks into the deep-sea and is remineralized to DIC (data).  The titration alkalinity values increase as deep water flows along the conveyor.  The increase in TAlk is driven, primarily, by dissolution of calcium carbonate shells sinking through the water column.  The changes in TCO2 and TAlk drive large changes in the concentrations of  (CO2), (HCO3-) and (CO32-), which are all chemical species of inorganic carbon.

Particulate organic carbon (POC) concentrations are high within the euphotic zone where the POC is produced by photosynthesis and the POC concentrations decrease with depth.  Dissolved organic carbon (DOC) concentrations accumulate to high concentrations near the surface, as well.  There is a small decrease in DOC as deep water flows from the Atlantic to the Pacific, which implies the material is fairly non-reactive.

Speciation
Carbon exists in seawater as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate organic carbon (POC) and particulate inorganic carbon (PIC).  The DIC consists of carbon dioxide (CO2), bicarbonate (HCO3-) and carbonate (CO32-), which can interchange rapidly with each other to maintain chemical equilibrium (Johnson, 1982).  The total amount of inorganic carbon is called TCO2.  Seawater is slightly alkaline due to bases such as bicarbonate and carbonate.  A measure of the basicity of seawater is the Titration Alkalinity (TAlk) = (HCO3-) + 2x(CO32-) + (B(OH)4-) + (OH-) - (H+).  If TCO2 and TAlk are measured, then the concentration of the individual components can be calculated. 

Phytoplankton consume DIC, or TCO2, during photosynthesis to produce POC and O2.  Some of the DIC that is fixed into organic carbon is transformed into DOC during photosynthesis and remineralization. 

Residence time
The residence time of DIC was estimated from the river input (Berner and Berner, 1996).  The residence time of POC was estimated from the standing stock of POC in the surface ocean and estimates of the global primary production rate (Falkowski et al., 1998).

Analysis
TCO2 can be measured by a coulometric titration of the CO2 extracted from acidified seawater.  Titration alkalinity is determined by a potentiometric titration of seawater directly with dilute acid.  The partial pressure of CO2 can be measured by equilibrating a small volume of air with seawater and measuring its carbon dioxide content with an infra-red detector.  DOC is measured by a high temperature, catalytic system that converts DOC to CO2.  POC is determined by oxidation and measurement of CO2.

References