Ultra-distal tephra deposits and Bayesian modelling constrain a variable marine radiocarbon offset in Placentia Bay, Newfoundland

• Published in: Geochronology
• Main Authors: Monteath, Alistair J., Bolton, Matthew S. M., Harvey, Jordan, Seidenkrantz, Marit-Solveig, Pearce, Christof, Jensen, Britta
• Format: Text
• Language: English
• Published: 2023
• Subjects: Newfoundland; Pacific; North Atlantic; North West Atlantic
• Online Access: https://doi.org/10.5194/gchron-5-229-2023; https://gchron.copernicus.org/articles/5/229/2023/

Radiocarbon dating marine sediments is complicated by the strongly heterogeneous age of ocean waters. Tephrochronology provides a well-established method to constrain the age of local radiocarbon reservoirs and more accurately calibrate dates. Numerous ultra-distal cryptotephra deposits (non-visible volcanic ash more than 3000 km from source) have been identified in peatlands and lake sediments across north-eastern North America and correlated with volcanic arcs in the Pacific north-west. Previously, however, these isochrons have not been identified in sediments from the north-west Atlantic Ocean. In this study, we report the presence of two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these well-dated isochrons to constrain the local marine radiocarbon reservoir offset ( Δ R) and develop a robust Bayesian age–depth model with a Δ R that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was - 126 ± 151 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="cae5bdecf7af0978e53d2c4c9289ec09"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-5-229-2023-ie00001.svg" width="58pt" height="10pt" src="gchron-5-229-2023-ie00001.png"/></svg:svg> years (relative to the Marine20 calibration curve) at the time of Mazama Ash deposition (7572 ± 18 yr BP) and −396 ± 144 years at the time of WRAe deposition (1098–1097 yr BP). Changes in Δ R appear to coincide with inferred shifts in relative influences of the inner Labrador Current and the Slopewater Current in the bay. An important conclusion is that single-offset models of Δ R are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short timescales. Acknowledging potentially varying offsets is critical when ocean circulation and ventilation characteristics ...