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### Topic: Dissolved and Particulate Th-234/U-238 Problem...don't even know how to start.  (Read 1185 times)

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#### pvreppin

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##### Dissolved and Particulate Th-234/U-238 Problem...don't even know how to start.
« on: April 29, 2015, 03:45:58 PM »
Hi guys,

I'm working on a homework problem and completely hit a wall. I don't even know how to start. I was wondering if someone could help me out or at least point me in the right direction. The problem is:

A SeaWiffs image of coastal ocean color reveals a mosaic of chlorophyll (primary production) distributed in complex patterns along our coast. Ground truth measurements by ships deployed in this area allow the calibration of such images. During such an expedition, a swarm of salps were observed filtering large voluments of surface water (mixed layer depth = 30m), you measured the disequilibria between Th-234 and U-238 in the surface waters of the green portions of this figure and found the following information:

Activity of Uranium-238 = 2.4 dpm L-1
Half-life of U-238 = 4.45 billion years
Activity of Dissolved Thorium-234 = 0.5 dpm L-1
Activity of Particulate Thorium-234 = 0.25 dpm L-1
The half-life of Thorium-234 = 24.1 days
The ratio of Carbon:Thorium-234 = 20 micromole:dpm

Calculate:
a) The flux of dissolved Th-234 onto particles (JTh)
b) The flux of particulate Th-234 out of the mixed layer (PTh)
c) The residence time of dissolved Th-234
d) The residence time of particulate Th-234
e) The flux of carbon from the mixed layer (mmol/m2/day)
f) Using ArcView you integrate the area of the high production and find it covers 10,000 km2. How many tons of carbon were removed over this area during a 30 day period?
g) What was the gross primary production on a square meter per day basis?
h) How much oxygen was produced over this area in 30 days (answer in moles)?
i) How much nitrate was consumed (answer in moles)?

I know parts of what I need to do but overall I'm kind of lost. I'm thinking I need to do a box model. I know for residence time it's just going to be the reservoir divided by flux. And I think that J = APThλTh - P and P = APThλTh - J. But how do I go about calculating the reservoir from the information I'm given? How do I calculate the GPP, O2 produced, and nitrate consumed?

Any help would be GREATLY appreciated.