A lake fortress, a floating chronology, and an atmospheric anomaly: the surprising results of a radiocarbon wiggle-match from Āraiši, Latvia

Abstract

An Iron Age timber settlement which, in view of the defensive structures uncovered, is described as a lake fortress, on an island in Lake Āraiši, north-eastern Latvia, was excavated in 1965-69 and 1975-79 by teams led by Jānis Apals, who distinguished five construction phases. Dendrochronological analysis produced a c. 100-year floating chronology for Norway spruce (Picea abies (L.) Karst.) timbers from the earliest phase. A 14 C wiggle-match was undertaken to obtain an absolute date range for the final year of the floating chronology, and thus for the construction of the settlement. Ten blocks of wood from one timber, each comprising 6-11 years and collectively spanning the whole 93-year tree-ring series, were dated by AMS. Using the IntCal13 calibration data, there is a 95% probability that the felling date of this timber falls in the range 775-784 cal AD. It appears, however, that the AD 775 spike in the atmospheric 14 C level occurred within the 6-year span of the last sample. On this basis, we can narrow the date of construction to 776-780 cal AD. This date is significantly earlier than those reported in previous publications.

SINGLE-YEAR 14C DATING OF THE LAKE-FORTRESS AT ĀRAIŠI, LATVIA

“…Four contiguous biennial samples (KIA-50671-74; Table 1) from the oldest decade of the Āraiši timber (years n-79/80 to years n-85/86) were extracted to α-cellulose in Kiel to check the accuracy of 14 C ages from an ABA extract (KIA-49360, years n-77 to n-84), which were rejected by Meadows and Zunde (2014); a single target from each sample was measured in early 2016 and the results appeared to confirm the 2014 wiggle-match. In 2019, therefore, a second attempt was made to locate the AD 774/5 Miyake event by extracting each of the last 11 annual rings (year n-10 to year n) to α-cellulose and dating 2 targets made from separate combustions of the same extract on different AMS target wheels (KIA-54189-99).…”

“…In 2012-2013, a spruce timber from the log platform was dated at the Kiel AMS 14 C laboratory, by sampling 10 contiguous multi-annual blocks collectively spanning 92 years, ending with the final ring formed before tree-fall, "year n". Following the publication of the IntCal13 calibration curve (Reimer et al 2013), a wiggle-match felling date of cal AD 775-784 (95% probability) was proposed (Meadows and Zunde 2014). It should therefore have been possible to locate the AD 774/5 14 C production spike (Miyake et al 2012) in single-year samples from the final decade of growth, and thus to date the Āraiši floating tree-ring chronologies to the exact year.…”

“…It should therefore have been possible to locate the AD 774/5 14 C production spike (Miyake et al 2012) in single-year samples from the final decade of growth, and thus to date the Āraiši floating tree-ring chronologies to the exact year. Indeed, inconsistency between 2 14 C measurements (KIA-47639, 1265 ± 25 and 1150 ± 20 BP) of unhomogenized acid-base-acid (ABA) residue from the last 6 annual rings suggested that the Miyake event occurred between year n-5 and year n (Meadows and Zunde 2014). Concerns about the reproducibility of 14 C measurements in Kiel at the time (Meadows et al 2015) delayed the dating of single-year samples from Āraiši until after the AD 774/5 Miyake event had been successfully located in known-age oak (Rakowski et al 2015).…”

Power Centres and Marginal Landscapes: Tracking Pre- and Post-Conquest (Late Iron Age and Medieval) Land-Use in the Cēsis Castle Hinterland, Central Latvia

“…These include the important fortified lake settlement at Āraiši (former German Arrash, ca. 8km south-east of Cēsis), occupied from the 9 th to mid-10 th century (Apals 2012, Meadows andZunde 2014), and later the site of a medieval castle founded in the 14 th century.…”

“…Pollen and fungal spore analysis from Lake Āriašu, 6.5 km south of Cēsis, revealed significant evidence for human activity during the late Iron Age (Stivriņs et al 2015) associated with a lake settlement occupied from c. AD 780 to the early/mid-eleventh century (Apals 2002;Meadows and Zunde 2014;Punning et al 1968). Although the first appearance of Available radiocarbon dated pollen studies from southern Livonia support the picture of a heavily wooded landscape across much of the landscape (see Brown and Pluskowski 2014 for a broader discussion of the available pollen data from Livonia), at least beyond the immediate locales of settlements, with several sequences from peripheral locations, including Rozu and Zilais mires, showing little or no evidence for human activity during either the Iron Age or medieval periods (Kalniņa et al 2014;Ozola 2013).…”