Petersen (Norway)

I'm looking for information in regards to my great grandfather's parents.

Anfin Olai Petersen (Peterson) was born in Iowa in 1865. His mother's name was Susan Petersen (Peterson) still not 100% sure of her maiden name. It was listed as Branding on one page and Anfinsen on another record. She was born around 1822 in Norway. Anfin's father is listed in the 1880 census as Peter Peterson and he was born in Norway about 1821. We do know that Peterson was not the original family name when Peter and Susan immigrated to the US but I haven't been able to find any record of that original family name. From what I can gather from birth years of their children, they may have immigrated to the US between 1855 and 1857. Family rumor has it that Peter was a ship's captain, but I haven't been able to locate any records to verify that.

Any help would be greatly appreciated.