Tulip fields for the Dutch explaines the gold bonus for that tile. But it would likely be an unique replacer of farms (+1 gold besides current benefits of farms), and not something special only for Marshes as suggested earlier.
I know there are tullips, but if you look closely you see it's surrounded by a dike and even some sort of drainage system (windmill I guess). This means it is certainly a Polder, I'm very sure of this. I think we can confirm the Polder as the Dutch UI. Now I hope it is buildable not only on marshes but also on lakes!