Problem Set 6
Ocean 210, Autumn 2008
SOLUTIONS
1. 14C decay in the Ocean
a. Oceanographers use the radioactive decay rate of 14C as a clock to age deep
water. 14C decays at a fixed rate from atmospheric levels, independent of
temperature or pressure. The source of 14C is the atmosphere, so the highest
concentrations of 14C are near the surface of the ocean. As a result, we can
determine the age of the deep water parcel by finding the difference between the
14C measured in the deep water parcel and the 14C that was present at the surface
in the parcel’s formation region.
b. Older water has less 14C.
c. The 14C content of a water parcel would decrease in the deep sea as it moves
from the Atlantic to the Antarctic, then to the Pacific and the Indian Oceans.
d. The age of the water increases because the concentration of 14C decreases.
e. The ages of the water parcels in the Atlantic and Pacific are different because
their 14C concentrations are different. Also, deep water only forms in the Atlantic
Ocean and has to travel through the Atlantic and Antarctic oceans to get to the
Pacific.
f. The range of 14C in the Indian Ocean is -160 per mil to -200 per mil. The
percentage is 80-84%.
!
"160
10 +100 =84%or.84
!
"200
10 +100 =80%or.80
g. You can do this one of two ways, both of which I will accept:
i. We are going to find the age of water since it was at the surface.
The mean concentration of 14C in the Indian Ocean is
approximately -180mil. The percentage of the atmospheric value
is:
!
"180 permil
10 +100 =82%or.83
Now plugging into the half-life formula, using the atmospheric
concentration of 14C at the surface as the initial concentration :
