An Icelandic freshwater radiocarbon reservoir effect: Implications for lacustrine 14 C chronologies

A freshwater radiocarbon (¹⁴C) reservoir effect (FRE) is a ¹⁴C age offset between the atmospheric and freshwater carbon reservoirs. FREs can be on the order of 10 000 ¹⁴C yr in extreme examples and are a crucial consideration for ¹⁴C dating of palaeoenvironmental and archaeological samples. Correction for a FRE may be possible, provided the FRE and the proportion of FRE-affected carbon within a sample can be accurately quantified. However, although such correction is desirable for affected samples, it is essential that such correction is accurate in order to produce useful chronological information. Accuracy of FRE correction can be limited by spatial variation in FRE within a freshwater system, but despite this there is currently a paucity of information to identify and quantify such variability within affected systems. Here we present results of a study that investigates the effects of spatial FRE variation upon dating accuracy within the freshwater system of Lake Mývatn, northern Iceland. A substantial FRE (>10 000 ¹⁴C yr) has previously been identified in archaeological and modern samples from the region, which shows the potential for considerable spatial variability. The study also assesses the use of δ¹³C and δ¹⁵N in age correction of affected samples. The results show that benthic detritus and organisms at primary trophic levels from locations within the lake are affected by a FRE of at least 3500 ¹⁴C yr, with clear spatial variation resulting in ¹⁴C age differences of up to 7670 ¹⁴C yr between samples. There is a broad correlation between stable isotope values and FRE within the data set. However, large associated uncertainties currently preclude highly accurate and precise stable isotope-based quantification of the proportion of FRE-affected carbon within archaeological and palaeoenvironmental samples from Mývatn and the surrounding region.