Ultra-distal tephra deposits and Bayesian modelling constrain a variable marine radiocarbon offset in Placentia Bay, Newfoundland
<p>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 (no...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-05-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/5/229/2023/gchron-5-229-2023.pdf |
| Summary: | <p>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 <span class="inline-formula">3000</span> 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 (<span class="inline-formula">Δ</span>R) and develop a robust Bayesian age–depth model with a
<span class="inline-formula">Δ</span>R that varies through time. Our results indicate that the marine
radiocarbon offset in Placentia Bay was <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">126</mn><mo>±</mo><mn mathvariant="normal">151</mn></mrow></math><span><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></span></span> years (relative to
the Marine20 calibration curve) at the time of Mazama Ash deposition
(7572 <span class="inline-formula">±</span> 18 yr BP) and <span class="inline-formula">−396</span> <span class="inline-formula">±</span> 144 years at the time of WRAe
deposition (1098–1097 yr BP). Changes in <span class="inline-formula">Δ</span>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 <span class="inline-formula">Δ</span>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 have
differed over time. The addition of tephra isochrons permits the calculation
of semi-independent reservoir corrections and verification of the single
<span class="inline-formula">Δ</span>R model.</p> |
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| ISSN: | 2628-3697 2628-3719 |