Which of the following has the longest residence time?

A. water in the oceans

B. carbon in limestone

C. carbon in the atmosphere

D. water in the oceans

E. silica in plants

F. all are approximately the same

Relevance
• 2 months ago

Average residence times are probably longest for carbon in limestone. Heck, I can take you to limestone that formed 3 billion years ago, and show you lots of limestone of ages well beyond many tens of millions of years.  My basic point is that once in the limestone, that carbon isn't going anywhere, and there is lots of limestone that is millions ot billions of years old.

The carbon in the air, and the water in the oceans, is recycling constantly.   We see the proof in that the amount remains roughly stable over time yet we can see measurable inputs at any given time.  Since there is no constant secular (time-dependent) increase yet constant input, then the stuff must be leaving at basically the same rate as it is coming in.  You can calculate it fairly easily, and the calculations will give you values on the order of a few tens of millions of years, at most. Less for C in air than for water in oceans.

Clearly, silica in plants would be limited by the lifetime of the plants, and that is generally short even for long-lived things, in terms of geological time.

• Anonymous
2 months ago

C.

A and D are approximately the same..