The Scandinavian project Arcanum brings together four artists all well-known to followers of music at ECM: Arve Henriksen, Trygve Seim, Anders Jormin and Markku Ounaskari. They’ve played together in many permutations over the years, but this is their first album as a quartet. Already hailed as a “Nordic supergroup” in some quarters, the designation hardly conveys the thoughtful, reflective quality of the improvising and the sensitivity of the interaction here, whether playing music composed in real time or taking a written theme to new places.