I am often asked about the sowing depths for seeds. The rule of thumb is no deeper that 2-3 times the distance across the widest part of the seed. That means very shallow for carrots so it’s best to sow onto the surface and cover with a very light amount of fine soil or bagged compost. If you soil is not fine enough then add compost and sow into it to ensure the seed has good contact with the growing medium. Water before you sow.
Revisiting some old research from a few years back I found these root depth and spread charts. Notice how deep the central or tap root can go with beetroot down to 10 feet and carrots to 8 feet. That is why it is best to make deep fertile soil by using lots of compost and mulch.