138K? Sounds like it is about time for a timing belt anyhow, right? To answer your question, yes it is run off of the timing belt. While in there you may want to replace the tensioner, tensioner pulley and idler pulley. Some people replace all of it while they are in there, some do it on a case by case basis. You are looking at list prices of approx:

Idler pulley 100.00
tensioner 150.00
tensioner pulley 100.00
timing belt 50.00
water pump 120.00

You can find the parts cheaper online but unless you are going to do the work yourself I would recommend that you let the shop that does the work provide the parts. If there is anykind of problem you know what kind of warranty you are going to get with parts that you provide. The potential to destroy your motor is definitely there, I would hate to see you have something go wrong and then be stuck with it because you tried to save a few bucks and provide your own parts.
If you are going to do this yourself then shop the prices online and buy the good stuff. Some of those bearings and pulleys can be purchased at a signifigant savings by getting the OE manufacturer instead of Volvo. In this case most of that stuff is made by Febi, the tensioner I believe is Aisin. Shop it around and make up your mind but because of the nature of the job it is only going to get so cheap in end.

Mark