Since a suprafix can be the change of stress somewhere in the word (or other suprasegmental elements), and since accentuation plays a role in differentiating the noun arithmetic from the adjective arithmetic, I suspect that a derivational suprafix is at play.
The noun: [uh-RITH-muh-ik]
The adjective: [ar-ith-MET-ik]
The pseudo-phoneticization is taken from Dictionary.com.
Now, the only difference isn't just the accentuation. There is a change in phonemes:
The noun: /əˈɹɪθmətɪk/
The adjective: /æɹɪθˈmɛtɪk/
The phonetic transcription taken from Wiktionary.com.
Now, assuming the noun to be the original lexeme, the adjective would be affixed with a simulfix located at the first and sixth phoneme, replacing the two əs in the noun with an æ and an ɛ respectively.
So, if in fact there is both a simfulfix and suprafix deriving the adjectival form of the noun arithmetic, how would one write the morphological structure then? I've tried to represent simulfixes before (in a comment responding to a comment by @Draconis), but I have no idea how the format is supposed to look. Less so when it comes to suprafixes.