Burnt food crusts are often dated by radiocarbon (14C), avoiding questionable associations between archaeological pottery and other datable materials that might be found in proximity (Segerberg et al. 1991; Hallgren and Possnert 1997; Hallgren 2008), but several authors have noted that food crust 14C ages may be subject to 14C reservoir effects, if pots are used to cook fish (Persson 1999; Fischer 2002b; Fischer and Heinemeier 2003). Reservoir effects can arise when the base of the food chain is not photosynthesis of atmospheric CO2, but of CO2 dissolved in water (DIC, or dissolved inorganic carbon). In freshwater systems, large reservoir effects usually come from the dissolution of limestone by rainwater (hard-water systems). Some DIC is formed by mineralisation of DOC (dissolved organic carbon), which originates from e.g. the erosion of old soil and peat beds, which can cause reservoir effects in soft-water systems (Olsson 1983; 1996). Aquatic plants and phytoplankton use DIC in photosynthesis, which transfers reservoir effects into the aquatic food chain. Living plants and animals from Alster and Trave, for example, can have apparent 14C ages of up to 2000–3000 years (Philippsen and Heinemeier 2013).
Experimental food crusts made from these organisms have similar 14C ages, and there are cases in this region where archaeological food crusts give 14C ages that are much older than expected, but within the range predicted for fish contemporary with the pottery (Philippsen 2010; Philippsen et al. 2010; 2012).