Travertine on the terrace - What to do against ugly changes?

We have had travertine installed on the terrace for a few years. Slowly, ugly darkenings have appeared. The stone is cleaned twice a year with neutral soap and water – but it seems to me that dirt has settled in fine pores or depressions. In general, it seems to me that the stones here are different – but see for yourself in the pictures.

I understand that you must not use aggressive agents on travertine, which is a limestone. But what options are there? Or should the stone be completely sanded down – does some kind of impregnation or sealing make sense outdoors at all?

Never seen it like this before. I would spray it off with a pressure washer and then seal it.

Ask Jonastone. They really know their stuff about Travertine and maybe they can help.

I would not use a pressure washer because it almost always causes damage regardless of the type of paving or surface.

I would be interested to know if a so-called "Schmutzradierer" helps.. or a steam cleaner?