| Atomic number: | 90 |
| Atomic weight: | 232.0281 |
| Average concentration in ocean: | 90 fmol/kg |
| Residence time: | 45 years |
The major isotope is 232Th, which has a primordial origin in the "big bang." The primary 232Th source is deposition of dust at the sea surface. The concentration profile of the dissolved isotope is relatively uniform with a near bottom increase (data).
234Thorium is produced by decay of 238U, which has a conservative distribution in seawater. In the deep sea, where there are few particles to sorb dissolved Th, the major loss of 234Th is radioactive decay. The isotope is in equilibrium with its 238U parent (data). However, near the surface, where particulate organic carbon concentrations are high, chemical scavenging occurs at a much higher rate. The activity of dissolved 234Th is much lower than its parent activity, which indicates rapid removal of the thorium to the sediments.
The concentration profile of dissolved plus
particulate 230Th shows a linear increase with depth and
concentrations are higher in the Pacific than in the Atlantic (data).
The linear increase is believed to result from reversible exchange of thorium
from particles to solution, and a linear increase in particulate 230Th
that is created by the homogenous distribution of its parent isotope 234U.
Speciation
The major form of thorium is Th(OH)40.
Residence Time
The residence was calculated from the 230Th average
concentration in the water column and the production rate from 234U
(Roy-Barman et al., 1996).
Analysis
232Th is determined by mass spectrometry. 230Th
and can be determined by mass spectrometry or a-counting
after chemical separation. 234Th is determined by b-counting
after chemical separation.