If its untreated, and most of it is, I'd just cover it up with soil and call it added organic matter.

I see that they're now selling painted, or stained, bark mulch, thats supposed to stay the 'right' color. I'd hesitate to bury that stuff, but thats not based on anything scientific.

thanks for the speedy reply, David! that's just what i was hoping to hear, and it's what i'll do. this bark mulch was here when we bought the house 12 years ago, and was likely put down at least 8 years before that, so i assume it was untreated (it's just plain light brown now).

thanks again!

Bark is a good and one of the expensive mulches. I would rake it up and use it.

Make sure there is no plastic sheet weed barrier under the mulch before you cover it up.

Zeuspaul

hmmm, weed barrier -- good idea to check this as the previous (original) owners did use it elsewhere. horrible stuff! thanks, Zeuspaul!

yep you can simply cover it.

len

Here is a link that might be useful: lens bale garden

You could bury that bark mulch, a kind of hugelkultur thing that len was refering to, but if that bark mulch has been there as long as you say that should indicate that it was treated with something. I have never had bark mulch that lasted that long, 5 years at most, before the soil bacteria digested it.

I've seen large chunk pine bark last 5 years as loose mulch. But at the 20-year life you estimate I would expect that there is little real mulch left. If you dig down in the mulched area is it really loose bark mulch, or is it highly decomposed mulch with a lot of dark humate-like stuff underneath the surface? How deep is that 20-year old mulch?

Why not just pull the bark aside and plant in the soil beneath? It's not always possible, but it will work for some plants.

Large is a relative term.
How "large" of an area are you talking about.

I would rake it up and use it as mulch on top of the new bed or somewhere else. I can never have too much mulch on hand.