In the New Testament, "kurios" (Lord) replaces "Yahweh" — this is standard Greek translation convention, reflected in English by small-cap LORD

When Mark quotes the OT ("prepare the way of the Lord/Yahweh"), the OT Hebrew has the divine name Yahweh. In the first century, Jewish scribes stopped writing God's name in Greek texts and used "kurios" (Lord) instead. This convention is preserved in English Bibles: whenever LORD or GOD appears in all caps (or small caps), the underlying Hebrew is Yahweh. The Greek NT follows the same convention. So "kurios" in Mark is accurately representing "Yahweh" from the OT source.