A06_13

Ramped Pyrolysis Radiocarbon Dating of Lime Lumps: Establishing the Earliest Mortar-Based Construction Phase of Turku Cathedral, Finland

Barrett G¹,  Allen K¹, Reimer P¹, Ringbom Ų, Lindroos A²

¹14Chrono, Queen's University Belfast, Belfast, United Kingdom, ²Åbo Akademi, Turku, Finland

Ramped pyrolysis radiocarbon dating was carried out on lime lumps from the oldest remains of Turku Cathedral, Finland, the first sacristy.  Lime lumps from four sampling locations were analyzed.  For each sample, 5-6 fractions of CO₂ from different temperature fractions were trapped, converted to graphite and radiocarbon dated.

One of the four samples exhibited contamination for its lowest temperature fractions.  For the remaining samples, the age-temperature profiles were well-behaved, exhibiting a plateau of dates that were all in statistical agreement and indicative of a sample where only a single carbonate source (lime binder from the construction phase) is contributing to the radiocarbon dates.  For each of the four samples, the combined radiocarbon age resulted in a late 13th century calibrated age.  Combining the radiocarbon dates from all four samples (19 in statistical agreement, χ2-test: df=18, T = 4.5, 5% = 28.9) provided a calibrated age of 1271-1292 cal AD (95.4%). 

This set of results strengthens previous mortar dating results (acid hydrolysis on bulk and lime lumps) and confirms that Turku Cathedral was first constructed from stone and mortar in the late 13th century.  The results find remarkable convergence with written sources that suggest Bishop Magnus I may have been elected there in 1291 AD and that the building was inaugurated as a cathedral in 1300 AD.  Ramped pyrolysis combined with suitably selected lime lumps is shown, in this instance, to be a robust approach for dating lime-based mortars.