What causes white marks on teeth?

The white lines are most likely resulting from a Zinc deficiency. It is the same as the white lines you get on your finger nails. Too much calcium in your diet can cause improper absorption of zinc and iron. Vitamin D also plays a role in this so make sure you're getting enough of that. If this is a serious problem for a good amount of your teeth/nails I suggest contacting a dietitian or your doctor.