Estimating Marine Reservoir Effects in Archaeological Chronologies: Comparing Δr Calculations in Prince Rupert Harbour, British Columbia, Canada

Abstract

The best method for quantifying the marine reservoir effect (MRE) using the global IntCal Marine13 calibration curve remains unresolved. Archaeologists frequently quantify uncertainty on MRE values as errors computed from single pairs of marine-terrestrial radiocarbon ages, which we argue significantly overstates their accuracy and precision. Here, we review the assumptions, methods, and applications of estimating MRE via an estimate of the additional regional offset between the marine and terrestrial calibration curves (ΔR) for the Prince Rupert Harbour (PRH) region of British Columbia, Canada. We acknowledge the influence on ΔR of MRE variation as (1) a dynamic oceanographic process, (2) its variable expression in biochemical and geochemical pathways, and (3) compounding errors in sample selection, measurement, and calculation. We examine a large set of marine-terrestrial pairs (n= 63) from PRH to compare a common archaeological practice of estimating uncertainty from means that generate an uncertainty value of ±49 years with a revised, more appropriate estimate of error of ± 230 years. However, we argue that the use of multiple-pair samples estimates the PRH ΔR as 273 ± 38 years for the last 5,000 years. Calculations of error that do not consider these issues may generate inaccurate age estimates with unjustifiable precision.

Spatiotemporal Variation in ΔR on the West Coast of North America in the Late Holocene: Implications for Dating the Shells of Marine Mollusks

“…The applicability of these estimates of ΔR in geographic gaps in the prebomb shell distribution pattern, or to epochs prior to the nineteenth century, may be assessed by dating paired samples of shell and wood-charcoal from the same cultural or geological stratum (e.g., Deo et al 2004; Hutchinson et al 2004; Ingram 1998; Kennedy et al 2005; Martindale et al 2018; Southon and Fedje 2003; Taylor et al 2007). ΔR estimates derived from these shell-wood pairs are likely, however, to be less accurate than those from prebomb shell because the radiocarbon ages of both the shell and the charcoal partner are inherently uncertain.…”

“…I excluded samples that were clearly anomalous, but there is no objective test by which such “old-shell/old-wood” anomalies can be identified. Their incidence may be reduced by dating charred twigs (e.g., Kennedy et al 2005) rather than large charcoal fragments of uncertain origin, and anomalous values are more likely to be identified if several shell and charcoal samples from the same stratum are dated (e.g., Cook et al 2015; Martindale et al 2018).…”

The Chronology of Medieval Copenhagen

“…Exploration of aquatic resources being either freshwater or marine may significantly influence the accuracy of 14 C age determinations (Olsen et al 2010;Wood et al 2013;Martindale et al 2018). Thus, to adequately calibrate 14 C ages of human individuals it is necessary to reconstruct their dietary habits using stable isotope analysis, i.e.…”

“…Moreover, Dahlström et al 2018 based the early onset of Copenhagen on human remains from two cemeteries. Humans are notoriously difficult to radiocarbon date because of utilization of marine or freshwater resources which may result in significant 14 C age offset due to different ages of either marine or freshwater reservoirs (Olsen et al 2010;Wood et al 2013;Martindale et al 2018). We therefore provide a detailed account and interpretation of the diet of the individuals used for radiocarbon dating from both the Town Hall Square and St. Clemens cemeteries.…”

Marine Reservoir Effects in Seal (Phocidae) Bones in the Northern Bering and Chukchi Seas, Northwestern Alaska

“…Several attempts have been made at providing corrective MRE values for 14 C dates on marine species and human remains from populations that were highly reliant on marine-derived food sources from Arctic coastal zones. Marine mollusks are a focus in many MRE studies and useful in establishing local variations for oceanographic purposes (e.g., Kuzmin et al 2007;McNeely et al 2006;Pearce et al 2017;Martindale et al 2018). However, marine mollusks are generally not important to Arctic coastal populations as a dietary resource, as indicated by the dearth of mollusks in the archaeological record.…”