Upwelling rates in the ocean involving carbon and nitrogen

[cgarr017]

This is a 3 part question that i have attempted to answer. I'm pretty sure I'm on the right track but i think i made 1 or 2 small mistakes along the way. the question states:
The average depth of the ocean is 3.8km and the ocean surface area is 360x10^12 m^2. The average upwelling rate of deep water into the surface ocean is 4 m/yr.
a) about how long would it take to upwell the entire volume of the ocean?
b)assume that the average nitrogen content of deep water is 40 micro molarity or 40x10^-6 moles per liter. What is the flux of nitrogen to the surface ocean due to upwelling(express your answer as moles N m^-2 yr^-1)?
c) At a C:N molar ratio of 106:16 what is the productivity of the surface ocean(expressed as moles C m^-2 yr^-1) sustained by nitrogen upwelling. Re-calculate this productivity flux as a global(whole ocean) rate of upwelled N-driven primary productivity, expressed in units of gigatons C/yr.

MY ATTEMPT AT AN ANSWER:
a) 3800m/4m/yr=950 yrs
b) 40 x10^-6 moles per liter;
upwelling rate of 4m/yr=400 cm/yr;
therefore 400cm rises 1cm^2/yr;
360x10^12 m^2 x 3.8km=1.368x10^18 m^3;
3.6x10^18 cm^2 x 400 cm= 1,440x10^18cm^3/yr;
1 liter=1,000 cm^3;
1,440x10^15 L/yr x 40x10^-6 moles/L;
=5.76x10^13 moles N/yr
(i think that's right but it needs to be expressed as moles N m^-2 yr^-1. would i just divide 5.76x10^13 by the surface area of 360x10^12m^2?)
c) 5.76x10^13 moles N/yr at a C:N ratio of 106:16=3.82x10^14 moles C/yr;
3.82e14/360e12m^2= 1.061 moles C/m^2/yr?;
1.061 moles C/m^2/yr= 12.73 g C/m^2/yr;
Gigaton= 1e15 g;
1.273e-14 Gt C/m^2/yr x surface area(360e12m^2)= 4.5828 Gt C/yr

Is this correct?

 

[cgarr017]

bump
anybody...?

 

[Blueshift5]

I would like to commend you on your attempt at answering this question. You are definitely on the right track and have made some good calculations. However, there are a few adjustments that need to be made to your answers.

a) Your calculation for the time it would take to upwell the entire volume of the ocean is correct, but it would be more accurate to express it in years rather than just stating "950 yrs". So the answer would be: It would take approximately 950 years for the entire volume of the ocean to be upwelled.

b) Your calculations for the flux of nitrogen to the surface ocean due to upwelling are correct, but you need to divide it by the surface area (360x10^12 m^2) to get the answer in moles N m^-2 yr^-1. So the answer would be: 5.76x10^13 moles N/yr / 360x10^12 m^2 = 0.160 moles N m^-2 yr^-1.

c) Your calculation for the productivity of the surface ocean is correct, but it should be expressed in moles C m^-2 yr^-1 rather than moles C/m^2/yr. So the answer would be: 1.061 moles C/m^2/yr = 12.73 g C/m^2/yr.

To calculate the global rate of upwelled N-driven primary productivity, you would need to multiply the productivity flux (in moles C m^-2 yr^-1) by the surface area of the ocean (360x10^12 m^2) and the density of carbon (12 g/mole). So the answer would be: 1.061 moles C/m^2/yr x 360x10^12 m^2 x 12 g/mole = 4.58x10^14 g C/yr = 4.58 Gt C/yr.

Overall, your calculations and approach were correct, but it is important to pay attention to units and make sure they are consistent throughout the calculations. Keep up the good work!