This “marine carbon” comes from the dissolution of ancient carbonate rocks and other geological sources into seawater, and it is depleted in carbon-14 relative to atmospheric carbon because it has been in the ocean system for a long time.

When marine organisms incorporate this carbon into their tissues, and when people eat marine food and incorporate that same carbon into their bones and other tissues, the result is a radiocarbon date that appears older than the actual date of death.

The size of this marine reservoir effect varies by location — different parts of the ocean have different amounts of old dissolved carbon, depending on local ocean circulation patterns and the specific geological history of the area.

The Pacific Ocean off the Northwest Coast has a marine reservoir effect that has been measured for specific locations, and corrections can be applied to the radiocarbon dates of individuals from the region.