Protactinium (Pa)
| Atomic number: |
91 |
| Atomic weight: |
231.0359 |
| Average concentration in ocean: |
18 amol/kg |
| Residence time: |
200 years |
Distribution in ocean
231Protactinium (1/2 life 32,500 years) is a naturally occurring
daughter product of 235U. The vertical profile
of total 231Pa (most of the isotope is sorbed onto particles) is
similar to that of 230Th, with a nearly linear increase in
concentration, or activity, with depth (data).
The parent isotope has a near conservative distribution and as daughter isotope
is produced, it is rapidly sorbed onto sinking particles. The
concentration at any depth is approximately equal to the integrated production
in the overlying water column. 231Pa differs from 230Th
because there also appears to be remineralization of Pa from sinking particles
at a greater rate than the loss due to scavenging.
Speciation
Residence time
The protactinium residence time was estimated with a box model (Nozaki and
Nakanishi, 1985).
Analysis
231Protactinium is determined by a-spectrometry
using silicon surface barrier detectors after chemical separation from seawater
and concentration.
References & Notes