Silicon (Si)

Atomic number: 14
Atomic weight: 28.0855
Average concentration in ocean: 100 µmol/L
Residence time: 20,000 years

Distribution in ocean
Dissolved Si has a nutrient type profile.  Concentrations are low at the surface (data) due to biological uptake.  Si is an essential nutrient for diatoms, a type of phytoplankton that makes a shell (called a test) of SiO2, and they can completely strip Si from surface waters.  The Si concentrations increase with depth as the siliceous tests of dead diatoms sink into the deep sea  and the Si particles redissolve.  Not all of the Si redissolves and the sediments in some areas of the ocean are composed primarily of these tests (called diatom oozes).

Silica and the ocean conveyor
Changes in the dissolved Si concentrations of deep waters in the ocean have been used to determine the pattern of water flow in the deep-sea.  Si concentrations are extremely low in deep waters of the North Atlantic.  The salty and silicon depleted surface waters become dense enough during winter to sink into the deep sea near Greenland.  These deep waters then flow south, through the Atlantic, around Antarctica and north into the Pacific.  Concentrations of Si increase along this pathway as diatoms sink into the ocean depths and dissolve.  Silicon acts like a clock - its concentrations increase with time.  Older waters have more Si. The water in the conveyor eventually surfaces along the flow path and returns at the surface to the North Atlantic, carrying large amounts of heat that warm northern Europe.  This pattern of flow is called the ocean conveyor. 

Speciation
Dissolved Si is present in seawater primarily as ionized orthosilicic acid H2SiO42-.    

Residence time
The silicon residence time is from Tracers in the Sea (Broecker, W. S. and Peng, T.-H., 1982.  Eldigo Press, Palisades, NY).  It was calculated from the global average river input.

Analysis
Orthosilicic acid is determined colorimetrically by forming a reduced silico-molybdate dye.

References & Notes