It was historically spoken by the Karkin people,who lived in the Carquinez Strait region in the northeast portion of the San Francisco Bay estuary. Its only documentation is a single vocabulary obtained by linguist-missionary Felipe Arroyo de la Cuesta at Mission Dolores in 1821. Although meager, the records of Karkin show that it constituted a distinct branch of Costanoan, strikingly different from the neighboring Chochenyo Ohlone language and other Ohlone languages spoken farther south. Karkin has probably not been spoken since the 19th century.
All Costanoan languages went extinct, but some are being studied and revived.
Beeler, Madison S. 1961. "Northern Costanoan." International Journal of American Linguistics 27: 191–197.
Callaghan, Catherine A. 1997. "Evidence for Yok-Utian." International Journal of American Linguistics 63:18–64.
Golla, Victor. 2007. "Linguistic Prehistory." California Prehistory: Colonization, Culture, and Complexity. Terry L. Jones and Kathryn A. Klar, eds., pp. 71–82. New York: Altamira Press. ISBN 978-0-7591-0872-1.
Milliken, Randall T. 1995. A Time of Little Choice: The Disintegration of Tribal Culture in the San Francisco Bay Region, 1769–1810. Menlo Park, CA: Ballena Press.
Milliken, Randall T. 2008. Native Americans at Mission San Jose. Banning, CA: Malki-Ballena Press. ISBN 978-0-87919-147-4.