Abstract
The Marine Radiocarbon Reservoir Effect (MRE) is a 14C 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 ΔR, occur. This study provides a comprehensive dataset of 21 ΔR 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 ΔR and MRE values, together with their associated uncertainties, calculated as standard errors for predicted values. The ΔR values range from −320 ± 35 to +150 ± 28 14C 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 ΔR values and again show no spatial or temporal trends. Results show that ΔR values calculated for a single site using statistically indistinguishable groups of terrestrial and marine radiocarbon age measurements can produce variability of up to 225 14C years. ΔR 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 14C 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 ΔR 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 ΔR data, presents a cautionary tale regarding the use of ΔR to infer large-scale oceanographic or climatic changes. Upon the exclusion of 5 outliers from the 42 values, the remaining ΔR values are statistically indistinguishable from one another and range from −142 ± 61 to +40 ± 47 14C years. 34 of these values are from Scottish archaeological sites and can be combined to produce a mean value for Scotland of −47 ± 5214C years for the period 3500 BC to 1450 AD, to be used only in the absence of site- and period-specific data.
Highlights
The Marine Radiocarbon Reservoir Effect (MRE) manifests itself as a 14C age offset between samples formed in the terrestrial biosphere and contemporaneous samples formed in the marine environment (Stuiver et al, 1986)
This study recalculated DR values that were previously published by Ascough et al (2004, 2006, 2007a,b, 2009) and Ascough (2005), as well as those published by Russell et al (2010, 2011a, 2011b) and Russell (2011), by employing the statistical methodology recommended by Russell et al (2011a)
A DR value chosen for calibration should be as close as possible in time and space to the site which is to be dated in order to achieve a representative estimate of the local MRE at that time. 42 new DR values are presented in this paper, which cover a large proportion of coastal Northern British Isles and a vast period of human occupation therein
Summary
The Marine Radiocarbon Reservoir Effect (MRE) manifests itself as a 14C age offset between samples formed in the terrestrial biosphere and contemporaneous samples formed in the marine environment (Stuiver et al, 1986). This occurs due to the difference in mixing rates and residence times of carbon atoms in the two reservoirs, while variations in local conditions and mixing rates prevent there from being a universal 14C offset from the atmosphere for all oceanic environments (Jones et al, 2007a,b; Gomez et al, 2008; Harkness, 1983).
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