He found, arranged, and published a number of collections of such music from his travels and research.
[1] In 1805, Zarnack moved to Beeskow to be the town church's second preacher, and also became teacher at a girls' school.
In 1815, after the end of the Napoleonic Wars and after a major expansion of Brandenburg-Prussia, Zarnack was appointed director of education at the Royal Military Orphanage in Potsdam, originally intended for the children of slain soldiers.
In 1822, Zarnack was involved in a scandal after accusations by an orphan girl under his care, and was temporarily suspended.
Ernst Anschütz would write the most famous and prominent version of O Tannenbaum in 1824 that is still sung today.