No way to know. English is wildly variable in how various vowels and vowel combinations are pronounced. The "i" after the "z" could be pronounced "ih" or "ee" and somewhere in between. If more towards the latter, many immigrants tended to used phonetic spellings, i.e., spelling that corresponded to the sounds. The vowel combination "ie" can easily sound like "ee" as in "believe". The combination "ei" also can sound the same as in "receive". Likewise the letter "y" in English often is pronounced "ee" as in "fully".
So I could see the first part of the name ending up being spelled Ziel-, Zyl-, Zeil-, Zeel-, or just plain Zil-. My maternal grandfather's surname was Kuckailis, pronounced KOOTS-kai-lis (because the "c" has no birdie over it). In America, he changed it by dropping the ending and spelling the rest phonetically: KOOTS-kel. But wouldn't you know it, one of his children changed that to KOOT-sel, dropping second "k" sound altogether. So now I have relatives named both Kutskel and Kutsel. Can't explain how human beings deal with their names sometimes.