Assume the surface mixed layer of North Atlantic is ~50m deep and the average upwelling velocity (the rate at which deeper water is mixed into the surface layer) is 2 m/day. Given that the volume of the surface waters does not change over time, write the equation to determine the residence time of water in the mixed layer (Ignore the complication of river inflow to surface waters - your only input of water to the surface is from below).

As with all elements in the ocean, Silicon (Si) is chemically weathered from rocks in terrestrial environments and transported via riverine flow to the coastal oceans, where over time it is mixed throughout the oceans. Dissolved Silicon is removed from seawater mainly through uptake and growth of some phytoplankton (diatoms), and it leaves the ocean via burial of these microscopic skeletons. The average ocean concentration is 3 grams Si/m^3 and the average river concentration is 2 grams Si/m^3. Given that the global annual riverine discharge to the oceans is on the order of 2.6 x 10^14 m^3/yr, and volume of the global ocean is on the order of 1.37 x 10^18 m^3, calculate the residence time of the average Si molecule before it is removed again from the ocean.

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