Radiocarbon (14C) analyses are commonly used to determine the absolute age of floating tree-ring chronologies. At best, with the wiggle-matching method, a precision of 10 years could be achieved. For the early Middle Ages, this situation has been markedly improved by the discovery of rapid changes in atmospheric 14C concentrations in tree-rings dated to 774/775 and 993/994 AD. These high-resolution changes can be used to secure other floating tree-ring sequences to within 1-year accuracy. While a number of studies have used the 774 even to secure floating tree-ring sequences, the less abrupt 993 event has not been so well utilized. This study dates a floating pine chronology from Ujście in Wielkopolska (Greater Poland) (NW Poland), which covers the 10th century period and is critical for studies on the beginning of the Polish State to the calendar years 859–1085 AD using the changes in single year radiocarbon around 993/4 AD.

Miyake et al. (2012, 2013, 2014) described a sudden increase of radiocarbon (14C) concentration in annual tree rings of Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa) between AD 774 and 775 and between AD 993 and 994. In both analyzed periods, the sudden increase was observed almost in a single year. The increase in the 14C content was about 12‰ in the period AD 774–775 (Miyake et al. 2012) and about 11.3‰ in the period AD 993–994 (Miyake et al. 2013, 2014; Fogtmann-Schultz et al. 2017; Rakowski et al. 2018). A similar increase was observed in 660 BC, with a peak height of about 10‰ (Park et al. 2017). Single-year samples of dendrochronologically dated tree rings of deciduous oak (Quercus robur) from Grabie, a village near Krakow (SE Poland), spanning the years 670–652 BC, were collected and their 14C content was measured using an AMS technique. The results clearly show a rapid increase in the 14C concentration in tree rings around 660 BC similar to this observed in Park et al. (2017).

An increase in atmospheric radiocarbon (14C) content of about 11.3‰ in the period AD 993–994 was observed in annual tree rings from Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa) (Miyake et al. 2013, 2014). Single-year samples of dendrochronologically dated tree rings (English oak, Quercus robur) from Kujawy, a village near Krakow (SE Poland), spanning the years AD 981–1000, were collected, and their 14C content was measured using the AMS system in the Leibniz Laboratory. The results clearly show an increase of 6.2±1.6‰ in the 14C concentration in tree rings between AD 993 and 994, with a maximum increase of 10.9±1.7‰ between AD 991 and 994.