This has always seemed to been glossed over in any history texts I’ve read. It goes from large slave populations in the ancient world, to not hearing much about slavery, and then an explosion of African slaves during the mercantile system.
Hello, I wrote an answer about slavery in early medieval England here.
Beyond England I should point out that a prominent part of Europe concerning slavery would be the Iberian peninsula. Clashes between Islamic polities (Umayyad and later Córdoba, Seville and Zaragoza) against Christian kingdoms such as (Portugal, Castile, Aragon, Asturias ect) left opportunity for enslavement seeing as while both Islamic and Church law forbid the enslavement of their own religion neither forbid the enslavement of others. So in the on and off conflicts between various factions people who couldn't pay a ransom were often enslaved.
I think you hit what I was getting at when you mentioned no direct link between Atlantic slave trade and medieval slave trade. My European history is limited, but it makes sense to view debt-beholden people as slaves; I hadn’t thought to view it like that. Also, I think you’re again seeing where my thoughts were in your last paragraph about the romanticism around knights and the medieval period. I appreciate your answer.