The problem is caused by what is known as the “reservoir effect”. This is briefly described below and requires some explanation of the radiocarbon dating method. The method is based on the radioactive isotope ¹⁴C, which is produced in the atmosphere by cosmic radiation. After forming CO₂, it becomes part of the biosphere, including humans via the food chain; see Fig. 1. Upon death, there is no longer intake of food; from then on, the ¹⁴C-content of the remains (e.g., the buried bones) decreases due to radioactive decay. Thus, by measuring the remaining ¹⁴C-content of a human bone, its age can be determined^8,9.

This sounds simple, but there are complications such as isotopic effects (named fractionation) and calibration of the ¹⁴C measurement to calendar dates. For technical details, we refer to the extensive literature^8,10,11. An additional significant complication is the reservoir effect mentioned above, which applies to organisms whose dietary intake is not based on terrestrial reservoirs, but on aquatic reservoirs: ocean/sea, rivers, and lakes/ponds. In these reservoirs, the natural ¹⁴C-content is significantly lower than that of the terrestrial environment.

Typical numbers are shown in Fig. 1. These numbers are ballpark numbers varying from case to case. For the upper layer of the sea, the ¹⁴C content is 95% of that of the atmosphere. This is caused by upwelling of dissolved CO₂ from deep ocean water in certain regions, which is “old”. For rivers, the ¹⁴C content is about 15% less; this is caused by dissolved geologically derived carbonate, which does not contain ¹⁴C (see Mook 2006¹² and references therein). For lakes/ponds with streaming water, the effect is also approximately 15%. When the pond is closed off (like fishing ponds), the CO₂ between water and atmosphere reaches equilibrium after some time, causing the reservoir effect to disappear.

The depletion of ¹⁴C in aquatic reservoirs also applies to organisms living therein, such as fish, shells, and marine mammals. From the formula describing the radioactive decay, one can calculate that a depletion of 5% in ¹⁴C concentration corresponds to 400 years; for 15%, this is 1300 years. Hence, marine organisms that are contemporaneous with terrestrial organisms show ¹⁴C dates that are approximately 400 years older; for rivers, e.g., the river Rhine, it can be up to 1300 years older. This effect also applies to terrestrial organisms consuming aquatic food sources. For example, bones of polar bears and prehistoric Inuit people with a 100% marine diet show ¹⁴C dates which require a correction of 400 years^13,14.

The reservoir effect caused by freshwater fish consumption is much more complicated to correct for. What is missing here is the uniform 400 years, as is the case for a 100% diet based on marine fish. Freshwater reservoir offsets can vary extremely, from zero to very high numbers – numbers up to 1000 years have been observed^15,16. We note that the magnitude of the marine reservoir effect differs at various regions of the global oceans¹⁷. However, for the North Sea basin, the number is well established as 400 years^11,12,18.