Second, there are local errors of varying magnitude introduced by dissolved bicarbonate from calcareous rock formations. Third, there is the problem of upwelling. During the early years of the radiocarbon method, discrepancies were noted between samples of charcoal and shell which were excavated from the same stratigraphic layers of the same site and were therefore assumed to be the same age. Deep ocean water is depleted in radiocarbon so it reduces the activity of surface waters. This is termed the upwelling effect .
Radiocarbon spends a comparatively short time in the atmosphere.
On the other hand, residence times of C14 in the ocean may be up to 1000's of years. The hard water effect refers also to the dilution of radioactivity in marine reservoirs but this time the diluting factor is dissolved bicarbonate originating from geological deposits of infinite age. Usually, the rocks are carbonaceous sedimentary rocks such as limestone, sandstone or mudstone. Shell material may undergo a post-depositional recrystallisation which may affect its true radiocarbon age. This is described in the pretreatment section of these pages.