Ultra-distal tephra deposits and Bayesian modelling constrain a variable marine radiocarbon offset in Placentia Bay, Newfoundland
Abstract. 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 >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 never 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 (ΔR) and develop a robust Bayesian age-depth model with a ΔR that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was -126±151 years (relative to the Marine20 calbration curve) at the time of Mazama Ash deposition (7622±18 C.E.) and -396±144 years at the time of WRAe deposition (852–853 C.E.). Changes in ΔR coincide with inferred shifts in relative influences of the Labrador Current and the Slopewater curret in the bay. An important conclusion is that single-offset models of ΔR are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short time scales. 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 ΔR model.
Alistair Monteath et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1273', Anonymous Referee #1, 06 Jan 2023
- AC1: 'Reply on RC1', Alistair Monteath, 10 Mar 2023
RC2: 'Comment on Monteath et al egusphere-2022-1273', Paul Zander, 24 Jan 2023
- AC2: 'Reply on RC2', Alistair Monteath, 10 Mar 2023
Alistair Monteath et al.
Alistair Monteath et al.
Viewed (geographical distribution)
Monteath and co-authors present compositional data from two well-dated North American cryptotephra deposits in a marine sediment core from Placentia Bay, Newfoundland. By incorporating the ages of these tephra layers with previous radiocarbon dates from the same sediment core, and application of Bayesian age modeling, the authors make inferences about the past variability of the local marine radiocarbon offset (deltaR) through the Holocene. I found the paper to be well-written with clear accompanying figures and datasets. The methods are strong, particularly the Bayesian statistics, although I have a suggestion related to PCA analyses using compositional major oxide data (see L162-163 below). Beyond this, I only have minor suggestions and think that a revised manuscript would be appropriate for publication in Gchron. I look forward to seeing future applications of cryptotephra layers in marine sediment records to constrain variable deltaR like this!
L36: It would be helpful for non-14C specialists to know what range of average reservoir ages are here for context.
L91: Since you refer to this data later on in the discussion, please clarify what foram analyses were performed. Assemblages, isotopes, etc?
L122: Please specify what this single reservoir correction is here for reference.
L162-163: Have the authors considered the use of log-transformation ratios to better separate tephra sources? Especially when it comes to statistical analyses like PCA, these analyses cannot be performed on compositional oxide data due to the constant sum constraint, i.e., the co-dependency of variables. In other words, if one variable changes, they all do as they are fractional abundances. In addition, it would be helpful to have more information on the similarity coefficient analyses performed. Any references for the software, R packages, etc that were used would be important to include here as well.
L173-175: Which Mazama Ash date was used for the age model?
L184-187: Which WRAe date was used for the age model?
L199: Please provide depth of 14C layers in core and/or uncorrected age here for context. Also please include the depth for the WRAe tephra layer. Maybe just put in paratheses following the sample name.
L201: I wouldn’t use the word ‘substantially’ as it is rather subjective and the deltaR uncertainty overlap between Early and Middle Holocene, making them not statistically different. Also please provide the general difference for reference.
L200-203: This sentence seems out of place as the discussion before was focused on the deltaR around the WRAe, and then you led this sentence with ‘therefore’. Consider rephrasing for clarity. Or do you mean it was lower in the Late Holocene compared to the Early-Mid Holocene?
L228: It would be helpful if you provided what sort of proxy evidence these paleoceanographic changes are based on for reference.
L264: In regard to background noise, could you try change-point analyses, or something similar, to objectively determine when concentrations rise substantially? As I am not a crypto specialist, what is considered background versus ‘primary’ airfall? Since there are tephra shards always present in your record, at what threshold do you decide to increase sampling resolution for EMPA analysis? Some sort of objective statistical test would be nice to see applied. Additionally, what do you suspect is the source of constant tephra deposition if Placentia Bay is presumably sheltered oceanographically
L267: But the East Greenland Current carries substantial amounts of sea ice. Are you sure no tephra shards are deposited in the sea ice source areas?
L18: CE should be BP for Mazama Ash age.
L20: ‘Current’ is misspelled.
L160 and 262: Discrete is misspelled.