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Homework Help: Carbon 14, radioactive decay of ocean waters

  1. Nov 17, 2008 #1
    The youngest bottom waters in the North Atlantic had a [delta 14 C] value of approx -70o/oo (o/oo being parts per thousand). Bottom waters in the South Pacific have a [delta 14 C] value of -170o/oo, while bottom waters in the North Pacific had a [delta 14 C] value of -230o/oo.
    a)In the [delta 14 C] notation, the specific sample activity is approximately given by 1+([delta 14 C]/1000). Using this fromula, convert these [delta 14 C] values to specific carbon 14 activities. For this i got north atlantic-> 1+ (-70/1000)=0.93, south pacific->0.83, north pacific->0.77.
    b)Now, assume that a water mass sinks out of the north atlantic at t=0 with the specific activity based on a [delta 14 C] value of -70o/oo. Using the Radioactive Decay equation N=N0e^-(0.693t/(T1/2)), calculate the "age" of the waters in the south pacific and north pacific. recall that T1/2 is half-life and the half-life of carbon is 5,730 years.
    c)The average replacement or residence time for the entire ocean is ~500-1000 years. Please discuss the ages you calculated for north pacific and south pacific waters in the context of this average replacement time.

    Now, i'm stuck on part B i'm not sure whether to put the specific activity values in for N or N0. If i put it in for N and then put 0 for N0 as the initial value it doesn't work obviously. i don't know any guidance or assistance would be much appreciated. thanks
  2. jcsd
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