A period of calm in Scottish seas: A comprehensive study of ΔR values for the northern British Isles coast and the consequent implications for archaeology and oceanography

The Marine Radiocarbon Reservoir Effect (MRE) is a C-14 age offset between contemporaneous marine-and terrestrially-derived carbon. In Northern Hemisphere surface waters it is of the order of 400 years but temporal and spatial deviations, known as Delta R, occur. This study provides a comprehensive dataset of 21 Delta R and MRE values for the east coast of Scotland and 21 recalculated values for the west coast of Scotland and Ireland, for the period c. 3500 BC to 1450 AD. They are presented as mean, site-specific Delta R and MRE values, together with their associated uncertainties, calculated as standard errors for predicted values. The Delta R values range from -320 +/- 35 to +150 +/- 28 C-14 years and show no spatial or temporal trends. The MRE values range from 59 +/- 40 to 531 +/- 26, show an almost identical distribution pattern to the Delta R values and again show no spatial or temporal trends. Results show that Delta R values calculated for a single site using statistically indistinguishable groups of terrestrial and marine radiocarbon age measurements can produce variability of up to 225 C-14 years. Delta R is an important factor in the accurate calibration of samples containing marine-derived carbon for archaeological interpretation but is often also used as an indicator of changes in C-14 specific activity of the oceans, and therefore a proxy for changes in ocean circulation and/or climate. Using the methods outlined in this paper, it is apparent that Delta R values for the northern part of the British Isles have been relatively stable, within our ability to quantify non-random variation in the data. The fact that significant climatic shifts have been recorded during this time, yet these are not visible in the Delta R data, presents a cautionary tale regarding the use of Delta R to infer large-scale oceanographic or climatic changes. Upon the exclusion of 5 outliers from the 42 values, the remaining Delta R values are statistically indistinguishable from one another and range from -142 +/- 61 to +40 +/- 47(14)C years. 34 of these values are from Scottish archaeological sites and can be combined to produce a mean value for Scotland of -47 +/- 52(14)C years for the period 3500 BC to 1450 AD, to be used only in the absence of site- and period-specific data. (C) 2015 The Authors. Published by Elsevier B.V.