Its not mainly Marines. It's equal parts all four branches, and is ran by the Army's 3rd IR (aka Old Guard). And it is mainly enlisted personnel, but no rank is worn at the Tomb of the Unknown Soldier.
As a Marine, I remember volunteering for this duty about 6 times in 8 years, but never made it past the application process. These men (and now women I believe) take this duty very seriously, and regardless of whether or not this photo was taken in September or not, 21 steps were taken at all sentry changes, on any day, regardless of conditions.