It's too soon for me to even guess at an ID, but I would bet that it is something that you are growing that went to seed (or in a neighbor's yard, etc.). I wouldn't bother with Round-Up, just get in there with your hand cultivator and stiir up the mulch to uproot them. Let them dry in the sun for a day or so then put down your new mulch.

With most weeds, RU is overkill, and you run the risk of harming the good plants. Uprooting the weeds in time to prevent them going to seed works MUCH better and is less dangerous to many things, including you.

This is what scuffle hoes were invented for.

I have the exact same look in some of my beds (I'm also in SE PA), but in my case I know a lot of them are cleome seedlings, which I'll thin as they get bigger.

The second picture shows Cirsium arvense seedlings. The first is something different. But the solution is the same. Hoe on a dry day and the seedlings will die on the surface. Keep the hoe sharp and use it as soon as you spot a seedling. Don't wait.