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A MODERN MULTICENTENNIAL RECORD OF RADIOCARBON VARIABILITY FROM AN EXACTLY DATED BIVALVE CHRONOLOGY AT THE TREE NOB SITE (ALASKA COASTAL CURRENT)

Published online by Cambridge University Press:  10 November 2022

David C Edge Open the ORCID record for David C Edge [Opens in a new window] ,
Alan D Wanamaker Open the ORCID record for Alan D Wanamaker [Opens in a new window] ,
Lydia M Staisch Open the ORCID record for Lydia M Staisch [Opens in a new window] ,
David J Reynolds Open the ORCID record for David J Reynolds [Opens in a new window] ,
Karine L Holmes Open the ORCID record for Karine L Holmes [Opens in a new window]  and
Bryan A Black Open the ORCID record for Bryan A Black [Opens in a new window]
David C Edge*
Affiliation:
Laboratory of Tree Ring Research, University of Arizona, Bryant Bannister Tree Ring Building, 1215 E Lowell St, Tucson, AZ 85721, USA
Alan D Wanamaker
Affiliation:
Department of Geological and Atmospheric Sciences, Iowa State University, 2237 Osborn Dr, Ames, IA 50011, USA
Lydia M Staisch
Affiliation:
Geology, Minerals, Energy, and Geophysics Science Center, United States Geological Survey, Moffett Field - Building 19, 345 Middlefield Road MS973, Menlo Park, CA 94025, USA
David J Reynolds
Affiliation:
Centre for Geography and Environmental Science, Department of Earth and Environmental Science, University of Exeter, Penryn Campus, Treliever Road, Penryn, Cornwall, TR10 9FE, UK
Karine L Holmes
Affiliation:
Department of Geological and Atmospheric Sciences, Iowa State University, 2237 Osborn Dr, Ames, IA 50011, USA
Bryan A Black
Affiliation:
Laboratory of Tree Ring Research, University of Arizona, Bryant Bannister Tree Ring Building, 1215 E Lowell St, Tucson, AZ 85721, USA
*
*Corresponding author. Email: dedge@arizona.edu
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Abstract

Quantifying the marine radiocarbon reservoir effect, offsets (ΔR), and ΔR variability over time is critical to improving dating estimates of marine samples while also providing a proxy of water mass dynamics. In the northeastern Pacific, where no high-resolution time series of ΔR has yet been established, we sampled radiocarbon (14C) from exactly dated growth increments in a multicentennial chronology of the long-lived bivalve, Pacific geoduck (Paneopea generosa) at the Tree Nob site, coastal British Columbia, Canada. Samples were taken at approximately decadal time intervals from 1725 CE to 1920 CE and indicate average ΔR values of 256 ± 22 years (1σ) consistent with existing discrete estimates. Temporal variability in ΔR is small relative to analogous Atlantic records except for an unusually old-water event, 1802–1812. The correlation between ΔR and sea surface temperature (SST) reconstructed from geoduck increment width is weakly significant (r2 = .29, p = .03), indicating warm water is generally old, when the 1802–1812 interval is excluded. This interval contains the oldest (–2.1σ) anomaly, and that is coincident with the coldest (–2.7σ) anomalies of the temperature reconstruction. An additional 32 14C values spanning 1952–1980 were detrended using a northeastern Pacific bomb pulse curve. Significant positive correlations were identified between the detrended 14C data and annual El Niño Southern Oscillation (ENSO) and summer SST such that cooler conditions are associated with older water. Thus, 14C is generally relatively stable with weak, potentially inconsistent associations to climate variables, but capable of infrequent excursions as illustrated by the unusually cold, old-water 1802–1812 interval.

Keywords

ΔR chronologynortheast pacificpaleoceanographysclerochronology
Type
Research Article
Information
Radiocarbon , Volume 65 , Issue 1 , February 2023 , pp. 81 - 96
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Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2022. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona

INTRODUCTION

Radiocarbon (14C) is produced in the upper atmosphere from the interaction between cosmic radiation and nitrogen atoms, and due to its predictable rate of decay, is widely used as a geochronometer for dating organic material (e.g., Schuur et al. Reference Schuur, Druffel and Trumbore2016). The rate of 14C production, however, varies over time, as has been quantified by measuring levels in exactly dated tree rings over the past several millennia (Stuiver et al. Reference Stuiver, Kromer, Becker and Ferguson1986a). Information on this year-to-year variability in atmospheric 14C is now used to increase dating accuracy (Büntgen et al. Reference Büntgen, Wacker, Galván, Arnold, Arseneault, Baillie, Beer, Bernabei, Bleicher and Boswijk2018; Pearson et al. Reference Pearson, Salzer, Wacker, Brewer, Sookdeo and Kuniholm2020; Reimer et al. Reference Reimer, Austin, Bard, Bayliss, Blackwell, Ramsey, Butzin, Cheng, Edwards and Friedrich2020). In the marine system, 14C dating is complicated by the time necessary for atmospheric 14C to equilibrate across surface-ocean environments. Dating is further complicated due to the mixing of water masses, some of which may have been isolated from the surface and therefore relatively depleted in 14C (Stuiver et al. Reference Stuiver, Pearson and Braziunas1986b). This so-called marine radiocarbon reservoir effect (Stuiver et al. Reference Stuiver, Pearson and Braziunas1986b; Alves et al. Reference Alves, Macario, Ascough and Bronk Ramsey2018) can add 1000 or more years of uncertainty to dating estimates and varies considerably over a range of spatial scales. In the northeast (NE) Pacific north of 40°N, average radiocarbon ages are 600–1000 years older than contemporaneous terrestrial samples (McNeely et al. Reference McNeely and McCuaig1991). Significant spatial heterogeneity in 14C content has been observed on the order of 200 radiocarbon years per 40 km, likely due to differences in upwelling strength (Robinson and Thompson Reference Robinson and Thompson1981; McNeely et al. Reference McNeely and McCuaig1991; Jones and Jones Reference Jones and Jones1992; Panich et al. Reference Panich, Schneider and Engel2018; Hutchinson Reference Hutchinson2020). Thus, correction for this reservoir effect is critical for accurate radiometric dating of marine samples.

In addition to spatial variability, the marine radiocarbon reservoir effect at a given location also fluctuates over time with respect to currents, vertical mixing of deep, 14C-depleted water, and the volume and source of freshwater input, which in most cases mixes in 14C-enriched water. However, robust estimates of the temporal variability of the marine radiocarbon reservoir effect in many regions suffer from the pooling of samples across large geographic region with differing ocean dynamics, the difficulty of sampling consistently through time at a specific location (Ascough et al. Reference Ascough, Cook and Dugmore2005; Hutchinson Reference Hutchinson2020), and the relative amount of time represented. Archaeological sites can provide some degree of repeated sampling if accurate dates can be established for terrestrial samples known to be contemporaneous with marine samples (Southon et el. Reference Southon, Nelson and Vogel1992; Ascough et al. Reference Ascough, Cook and Dugmore2005). A relatively new approach to establishing 14C variability is to sample carbonate from the absolutely dated annual increments of long-lived marine bivalves (Butler et al. Reference Butler, Scourse, Richardson, Wanamaker, Bryant and Bennell2009; Scourse et al. Reference Scourse, Wanamaker, Weidman, Heinemeier, Reimer and Butler2012; Wanamaker et al. Reference Wanamaker, Butler, Scourse, Heinemeier, Eiríksson, Knudsen and Richardson2012; Lower-Spies et al. Reference Lower-Spies, Whitney, Wanamaker, Griffin, Introne and Kreutz2020). Indeed, annual increments in bivalves can be exactly placed in time via the dendrochronology technique of crossdating to generate continuous, annually resolved, multicentennial-length chronologies. From these measurements of 14C in bivalve increments, the marine radiocarbon reservoir effect can be quantified over time for the same location (Butler et al. Reference Butler, Scourse, Richardson, Wanamaker, Bryant and Bennell2009; Wanamaker et al. Reference Wanamaker, Butler, Scourse, Heinemeier, Eiríksson, Knudsen and Richardson2012; Lower-Spies et al. Reference Lower-Spies, Whitney, Wanamaker, Griffin, Introne and Kreutz2020). To date, this has been successfully applied in the North Atlantic to explore carbon cycling and ocean circulation by acting as a tracer of relatively “old” water depleted of 14C vs. relatively “young” water more recently mingled with the atmospheric 14C reservoir (Butler et al. Reference Butler, Scourse, Richardson, Wanamaker, Bryant and Bennell2009; Wanamaker et al. Reference Wanamaker, Butler, Scourse, Heinemeier, Eiríksson, Knudsen and Richardson2012; Lower-Spies et al. Reference Lower-Spies, Whitney, Wanamaker, Griffin, Introne and Kreutz2020).

In the northeast Pacific, temporal variability in the marine radiocarbon reservoir remains poorly quantified. To address this issue, we sample growth increments at approximately decadal intervals from a crossdated chronology of Pacific geoduck, a long-lived bivalve (Paneopea generosa) abundant from approximately Puget Sound, WA through Kodiak, AK, that occur from the intertidal zone to 60 meters depth. They are typically buried about 1 meter in mud-and-sand sediments and feed by extending a siphon into the water column (Goodwin Reference Goodwin1973). Geoduck shell growth, as measured in the hinge area, is rapid in the first 10–15 years of life, declining exponentially thereafter, while year-to-year growth responds to environmental conditions, primarily water temperature (Cerrato Reference Cerrato2000; Strom et al. Reference Strom, Francis, Mantua, Miles and Peterson2004). The chronology was developed from samples collected near the Tree Nob Islands in northern British Columbia, Canada, and continuously spans 1725 to 2008 CE (Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Growth-increment widths from this chronology were used to develop a sea surface temperature reconstruction, which is closely tied to NE Pacific variability as reflected by a strongly positive correlation (r = 0.62, p < 1.0e-5) with the leading principal component of SST gridded data across the northeast Pacific (Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Indeed, the Tree Nob chronology has some of the strongest region-wide climate relationships of any of the network of eight geoduck chronologies developed to date (Strom et al. Reference Strom, Francis, Mantua, Miles and Peterson2004; Black Reference Black, Copenheaver, Frank, Stuckey and Kormanyos2009; Black et al. Reference Black, Dunham, Blundon, Raggon and Zima2010; Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Given this apparent sensitivity to regional climate variability as well as exceptional length, the Tree Nob chronology was chosen for assessing the relationships of local and basin-scale climate indicators with 14C reservoir variability. We also utilized a previously published series of 14C measurements (Kastelle et al. Reference Kastelle, Helser, Black, Stuckey, Gillespie, McArthur, Little, Charles and Khan2011) from these shells sampled through the “bomb pulse” interval (1950–1982) to provide a finer-scale assessment of the link between 14C variability and instrumental climate records. In total, the pre-bomb data augments the finer-scale modern data during the bomb-pulse to provide complementary and longer-term perspectives on marine radiocarbon reservoir variability and relationships to climate and ocean dynamics.

METHODS AND BACKGROUND

Oceanographic Setting

The NE Pacific consists of a subpolar, cyclonic gyre in the Gulf of Alaska (GoA) and a subtropical, anti-cyclonic gyre (Figure 1a). The subpolar Alaska Gyre (AG) consists of the North Pacific Current (NPC) in the south and the Alaska Coastal Current (ACC) along the North American coast, which quickens and narrows west of Kodiak Island to become the Alaska Stream (Dodimead and Hollister Reference Dodimead and Hollister1958). Variability of transport within the AG is related to fluctuations in the Pacific Decadal Oscillation (PDO), the dominant mode of SST variability in the North Pacific (20–60N), and ultimately to the Aleutian Low (AL; Newman et al. Reference Newman, Alexander, Ault, Cobb, Deser, Di Lorenzo, Mantua, Miller, Minobe and Nakamura2016; Hristova et al. Reference Hristova, Ladd and Stabeno2019). In addition to basin-scale phenomena, the GoA experiences local variability in the magnitude of spring runoff, up/down-welling, and mixing/stratification. Basin-scale patterns may contribute to fluctuations in the strength of the ACC, relative makeup of ACC source waters, up/down-welling in source-water regions, and vertical entrainment (Guilderson et al. Reference Guilderson, Roark, Quay, Page and Moy2006; Hristova et al. Reference Hristova, Ladd and Stabeno2019; Hutchinson Reference Hutchinson2020). Guilderson et al. Reference Guilderson, Roark, Quay, Page and Moy2006) have proposed a two-end-member mixing regime for the AG based on a linear relationship between 14C and potential density observed in samples collected during the summer of 2002, such that warm, 14C-enriched water enters the AG from the south, and the observed latitudinal gradient is due to vertical entrainment of 14C depleted water within the AG.

Figure 1 Study site: (a) Mixed layer ocean currents in the NE Pacific Ocean. Black streamlines indicate general surface currents based on drifting buoy data from the Atlantic Oceanographic and Meteorological Library, National Oceanic and Atmospheric Administration. Individual marine radiocarbon reservoir offset (ΔR) measurements from previous studies shown as colored points (McNeely et al. Reference McNeely and McCuaig1991; Robinson and Thompson Reference Robinson and Thompson1981; Jones and Jones Reference Jones and Jones1992; Panich et al. Reference Panich, Schneider and Engel2018). General location and direction of the Alaska Gyre (AG), North Pacific Current (NPC), and Alaska Coastal Current (ACC) shown in white. (b) Local bathymetry (Amante and Eakins Reference Amante and Eakins2009). Tree Nob geoduck collection site and measurement locations for Langara sea surface temperature and salinity (SST and SSS) and Prince Rupert sea level. Glaciers shown in bright blue. Skeena River is in dark blue. (Please see online version for color figures.)

The primary, proximate source of water at Tree Nob is from the south via Hecate Strait (HS), a shallow strait that shoals from 200 m in the south to just 50 m at its northern extent (Figure 1b). In 1983–1984 several current meters were deployed across three transects of HS, which allowed for accurate measurement of flow and the development of surrogate measures of HS flow approximated by three sea level gauges, one to the west and, two to the east (r = 0.81; Crawford et al. Reference Crawford, Huggett and Woodward1988). When sea level is high in the east and low in the west, geostrophic flow induces a northward current through HS.

The Tree Nob Island Group lies at the far northeastern extent of Hecate Strait. The Islands are bounded to the north by Brown Passage and to the south by Bell Passage. These waterways connect Chatham Sound, to the east, with Hecate Strait, Dixon Entrance, and the open Northeast Pacific to the west. Due to high winds and strong tides, the Tree Nob site is well mixed with the open ocean (Trites Reference Trites1956; Lin and Fissel Reference Lin and Fissel2018). And due to strong northerly flows, Tree Nob is not strongly impacted by freshwater inputs (Lin and Fissel Reference Lin and Fissel2018). Although the study site lies in a quasi-estuarine environment, the strong relationship of geoduck growth increment width with regional- to basin-scale climatic indicators (Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021) suggests that the Tree Nob geoduck integrate environmental conditions across a broad region. Furthermore, the absolutely dated, annually resolved carbonate spans nearly three centuries to yield a marine archive with a uniquely long timespan, precision, and replication in the northeast Pacific.

Pre-Bomb Radiocarbon

Geoduck form annual increments (Shaul and Goodwin Reference Shaul and Goodwin1982), with widths highly correlated to water temperature (Strom et al. Reference Strom, Francis, Mantua, Miles and Peterson2004; Black et al. Reference Black, Copenheaver, Frank, Stuckey and Kormanyos2009; Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021) that can be assigned exact calendar years through crossdating (Black et al. Reference Black, Gillespie, MacLellan and Hand2008; Kastelle et al. Reference Kastelle, Helser, Black, Stuckey, Gillespie, McArthur, Little, Charles and Khan2011). A crossdated chronology developed from live-collected shells in the Tree Nob Islands (Figure 1b), and later appended with dead-collected material, extends from CE 1725–2008 (Black et al. Reference Black, Copenheaver, Frank, Stuckey and Kormanyos2009; Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Live- and dead-collected shells were recovered in sand-and-mud substrate at approximately 10 m water depth. The carbonate of marine organisms is incorporated from the dissolved inorganic carbon (DIC) of ambient seawater and is thus expected to reflect local environmental conditions experienced during shell formation (Adkins et al. Reference Adkins, Griffin, Kashgarian, Cheng, Druffel and Boyle2002; Beirne et al. Reference Beirne, Wanamaker and Feindel2012).

14C samples were obtained from the Tree Nob geoduck shells over the pre-bomb chronology interval of 1725–1920. Crossdated annual increments were sampled to a depth of ∼600–800 μm from the shell hinge area using a Merchantek micromill and a Brasseler USAV scriber point (item #H1621.11.008). Samples were then pooled together to obtain ∼10 mg. The micromill was set to maximum drill speed, and several passes were performed ranging from 100–150 μm depth at 55 μm/s scan speed and 55 μm/s plunge speed. In total, 15 shell carbonate samples integrating ten to eleven annual increments were gathered. All samples were taken from the same cut plane used for increment-width measurement to ensure precise calendar-year dating of samples. Samples were sent to the National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS Woods Hole, Massachusetts, USA) for 14C analysis.

Laboratory derived error was provided by NOSAMS based on 10 separate measurements of each sample. NOSAMS estimates an additional error of 2.6‰ for replicate samples due to variability in sample collection, processing, and homogeneity. The errors were combined to present a total measurement error of radiocarbon age (NOSAMS 2020).

ΔR is given by the difference between measured and “expected” radiocarbon age. Each milled geoduck shell sample spanned approximately 10 years, with an average date of formation which corresponds to a date on the Marine20 curve. The Marine 20 curve provides reservoir age estimates in ten-year intervals. Linear interpolation was used to better match these decadally reported radiocarbon age estimates to the average calendar year represented by each sample milled from the geoduck shells. The radiocarbon age of the sample as “expected” by Marine20 was then subtracted from the value measured by NOSAMS to give the ΔR. (Stuiver Reference Stuiver, Pearson and Braziunas1986b; Stuiver and Braziunas Reference Stuiver and Braziunas1993; Heaton et al. Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Ramsey, Grootes, Hughen and Kromer2020), where:

$${\rm{\Delta R\;}} = {{\rm{\;}}^{14}}{\rm{C\;age\;measured\;}}-{{\rm{\;}}^{14}}{\rm{C\;age\;modeled\;by\;Marine}}20$$

The Marine20 curve (Heaton et al. Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Ramsey, Grootes, Hughen and Kromer2020) accounts for variability in atmospheric 14C production and climate as well as interactions among the ocean, atmosphere, and biosphere. Therefore, the ΔR time series may better represent changes in local 14C content than age-corrected Δ14C by removing as many other sources of variability as possible.

Radiocarbon and Climate Covariability

For the pre-bomb dataset, 14C values were compared by linear and polynomial regression to reconstructed, seasonal (mean Apr–Nov) sea surface temperature derived from geoduck growth-increment width as published in Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). 14C measurements were compared to the Northern Hemisphere (NH) volcanic explosivity index (VEI) by Pearson correlation given the likely influence of such events on ocean circulation and SST (Gao et al. Reference Gao, Robock and Ammann2008).

Unlike pre-bomb data, the bomb pulse 14C data could be compared directly to instrumental climate records. The first of the instrumental records is sea level, which serves as a proxy for HS flow as described by Crawford et al. (Reference Crawford, Huggett and Woodward1988). Data were obtained from the Canadian Hydrographic Service. Only the Prince Rupert gauge covers the full period of the bomb-pulse data and is thus the only station used, though the pairwise correlations with the other two sites suggest this gauge is representative (r=0.949, 0.921, 0.892; p<0.00001). The upwelling index, as calculated by the National Oceanic and Atmospheric Administration (NOAA) Pacific Fisheries Environmental Laboratory (PFEL) was averaged across 51°N, 131°W and 54°N, 134°W, the two stations nearest to Tree Nob. Monthly freshwater discharge from the Skeena River, just 40 km distant and the second largest river draining British Columbia, was downloaded from the Department of Environment and Natural Resources, Canada (https://wateroffice.ec.gc.ca/report/data_availability_e.html?type=historical&station=08EF001&parameter_type=Flow+and+Level). Monthly mean SST and sea surface salinity (SSS) data, collected at the Langara Lighthouse station, were obtained from Fisheries and Oceans Canada (https://open.canada.ca/data/en/dataset/719955f2-bf8e-44f7-bc26-6bd623e82884; Figure 1b). Finally, bomb-pulse 14C data were compared to three basin-scale indices. Niño 3.4 is a measure of SST in the central, equatorial Pacific with strong connections to northeast Pacific SST and coastal sea surface heights (ñhttps://psl.noaa.gov/gcos_wgsp/Timeseries/Data/Niño34.long.data). The North Pacific Index is a measure of sea surface pressure in the northeast Pacific which serves as a gauge of the strength of the AL (NPI; Trenberth and Hurrell Reference Trenberth and Hurrell1994; https://climatedataguide.ucar.edu/sites/default/files/npindex_monthly.txt). Finally, the PDO index (https://www.ncdc.noaa.gov/teleconnections/pdo/) is the leading principal component of SSTs in the North Pacific and is closely linked to atmospheric pressure (Mantua et al. Reference Mantua, Hare, Zhang, Wallace and Francis1997; Newman et al. Reference Newman, Alexander, Ault, Cobb, Deser, Di Lorenzo, Mantua, Miller, Minobe and Nakamura2016).

Monthly climate data were averaged over 3-year intervals to match the temporal resolution of the 14C sampling (Kastelle et al. Reference Kastelle, Helser, Black, Stuckey, Gillespie, McArthur, Little, Charles and Khan2011) such that climate data for June of 1964 were represented by the average of June 1963, 1964, and 1965 and used for comparison with the 1964-centered 14C value. Correlations to the bomb-pulse 14C data were performed in the R package TreeClim (Zang and Biondi Reference Zang and Biondi2015). Significance (α = .01) was calculated by bootstrapping based on methods adapted from DENDROCLIM2002 (Biondi and Waikul Reference Biondi and Waikul2004).

Bomb-Pulse Radiocarbon Data

Samples spanning the bomb-pulse period were obtained from the Tree Nob geoduck shells in a previous study (Kastelle et al. Reference Kastelle, Helser, Black, Stuckey, Gillespie, McArthur, Little, Charles and Khan2011). The 32 bomb-pulse samples aggregate an average of three years of growth. The age-corrected Δ14C data were detrended by a latitude-specific, empirically derived marine radiocarbon curve (Helser et al. Reference Helser, Kastelle and Lai2014) to remove the bomb signal. Given that individual, complete increments were not sampled, the mean calendar year represented by a given sample was often a decimal. To facilitate direct comparison to instrumental data, the detrended 14C data were combined in three-year bins by weighted mean over the interval from 1952-1972 (Figure S1). Weights were assigned proportional to the square of the temporal proximity such that a 14C datum centered at 1965.9, being 0.4 years from 1965.5, was assigned a weight of $$\left( {1.5 - 0.4} \right)$$ 2.

RESULTS

Pre-Bomb Radiocarbon

The average ΔR for all pre-bomb (1725–1920) Tree Nob geoduck 14C samples is +256 years (σ = 22.3 yr, n=15) and is relatively stable over time. Only one value differs from the mean by more than 2σ, corresponding to increments formed from 1802–1812 (Figure 2a; Table 1), though this sample is not an outlier with respect to the distribution of all samples based on Grubb’s test (critical value=2.55, N=15, α=.05). Three samples fall outside 1σ of the mean ΔR and correspond to the years 1784–1794, 1842–1852, and 1902–1912 while all other radiocarbon ages strongly agree with Marine 20 values after ΔR adjustment.

Figure 2 Pre-bomb radiocarbon: (a) Tree Nob radiocarbon ages relative to the Marine 20 curve; points above (below) line indicate older (younger) water. Red bars: radiocarbon age of Tree Nob samples that are not significantly different from Marine 20 (Heaton et al. Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Ramsey, Grootes, Hughen and Kromer2020). Width of bar indicates the range of calendar years sampled. Green bar: 14C sample (middle year of 1807) that is significantly different from average ΔR (2σ). See Methods for error calculation. Blue line: Average radiocarbon age for the mixed layer given by Marine20, corrected by average ΔR of all samples (+256 years). Dark blue shading: 1σ sample error of ΔR; light blue shading is 2σ. (b) ΔR vs SST. SST is the 11-year averaging of Langara SST reconstruction, z-scored (Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Coloring of points as in panel (a). Red dashed line: least squares linear fit of red points. (Please see online version for color figures.)

Table 1 Radiocarbon data.

ID: Accession number. δ13C: measured by NOSAMS. Sclero Years: Calendar years (CE) of carbonate sample given by chronology. Conventional radiocarbon age: age (BP) based on Libby standard. Fm: Fraction modern carbon (corrected for fractionation by NOSAMS). Fm error: laboratory error reported. Δ14C: age-corrected based on crossdating. Δ14C error: laboratory error in ‰ units. ΔR: see “Pre-Bomb Radiocarbon” section for calculation. Marine20 ΔR error: given by Marine20 (Heaton et al. Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Ramsey, Grootes, Hughen and Kromer2020).

Radiocarbon and Climate Covariability

A linear regression of ΔR onto SST with the 1802–1812 sample removed is marginally significant and has a positive slope (r2=0.29, p = 0.03; Figure 2b). The 1802–1812 interval contains the most extreme SST and 14C values in the record and suggests an opposing SST-radiocarbon relationship. The Tree Nob ΔR value for 1802–1812, which is greater than the mean ΔR by 2.1σ, coincides with the coldest period in the SST record reconstructed from geoduck (Figure 3). Temperatures are below 2σ between the years 1808–1812 with the lowest value in 1810 of 3.4σ below the mean. This is coincident with a volcanic eruption in late 1808 with a VEI of 5.5, though other highly explosive eruptions appear to have no relationship to Tree Nob 14C (Gao et al. Reference Gao, Robock and Ammann2008; Guevara-Murua et al. Reference Guevara-Murua, Williams, Hendy, Rust and Cashman2014; Figure 3).

Figure 3 Tree Nob and volcanic proxy records: (a) ΔR in Tree Nob shell material; horizontal uncertainty reflects the 10–11 years represented by each sample while vertical bars are laboratory error. Dark and light gray bands indicate 1- and 2-σ from the mean (white dotted line) of Marine20. (b) Reconstructed Langara SST and 50% prediction interval from Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021. Dark and light gray bands indicate 1- and 2-σ from the mean (white dotted line) (c) NH VEI based on Gao et al. Reference Gao, Robock and Ammann2008. Yellow highlighting identifies an interval of synchronous anomalies across all three indicators and corresponds to the years 1802–1812. (Please see online version for color figures.)

Bomb-Pulse Radiocarbon Data

Among regional climate indicators, the relationship between geoduck 14C in the bomb-pulse interval and SST is perhaps the strongest and most consistent with significant and positive correlations from July through September (Figure 4). Sea level is positively correlated in February and March while Skeena River discharge negatively correlates in July and August (Figure 4). The correlation with the NOAA upwelling index is inconsistent with a significant negative relationship for February (r=-0.77, p<0.01) but a positive relationship for June (r=0.65, p<0.01) (Figure 4). The variable with strongest and most consistent relationships with geoduck 14C is the Niño3.4 index, which positively correlates for almost every month from May through December and is the only variable with a significant annual relationship (r=0.62, p=0.0056; Figure 4). The NPI negatively correlates in February and September while the PDO positively correlates in February and October, suggesting young water coincident with a deeper AL and positive PDO (Figure S2).

Figure 4 Bomb 14C correlations with monthly-averaged climate indicators. The level of significance is p <.01 and is calculated by bootstrapping. Significant correlations are shown in black, non-significant in grey. Correlations with annually-averaged (Jan–Dec) climate values are shown to the right of plots, bold font indicates significance at p < .05. Climate data sources given in the “Radiocarbon and Climate Covariability” section.

DISCUSSION

The ΔR value at Tree Nob of 256 ± 22 years is consistent with regional measurements in both the 20th century, 249 ± 158 years (McNeely et al. Reference McNeely and McCuaig1991; http://calib.org/marine/, n=12, average distance of 280 ± 138 km, recalculated with Marine20), and throughout the Holocene, 250 ± 195 years (Schmuck et al. Reference Schmuck, Reuther, Baichtal and Carlson2021). The Tree Nob average is also consistent with the geographically nearest 20th century measurement of 247 ± 50 (McNeely et al. Reference McNeely and McCuaig1991) and the geographically nearest archaeological site, a late-Holocene study only 30 km distant with an estimated 273 ± 38-year reservoir (Edinborough et al. Reference Edinborough, Martindale, Cook, Supernant and Ames2016). These Tree Nob values fit into a broader 14C gradient along the NE Pacific Coast (Figure 1a), with young water in the California Current (37–38°N, average ΔR = 26 years, σ = 103 years, n = 10; Robinson and Thompson Reference Robinson and Thompson1981; Ingram and Southon Reference Ingram and Southon1996; Panich et al. Reference Panich, Schneider and Engel2018), older water along Vancouver Island to the north (48–49°N, average ΔR = 147 years, σ = 73 years, n = 10; McNeely et al. Reference McNeely and McCuaig1991; Robinson and Thompson Reference Robinson and Thompson1981), and even older water in the ACC (54–58°N, average ΔR = 346 years, σ = 83 years, n = 10; McNeely et al. Reference McNeely and McCuaig1991).

Although the Tree Nob sample site is outside the area recommended for use with the Marine20 curve given the potential impacts of sea ice (Heaton et al. Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Ramsey, Grootes, Hughen and Kromer2020), the 14C values observed in the geoduck increments fit remarkably well with predicted values. Indeed, ΔR showed a notable lack of variability over time, straying relatively little outside sample error estimates. The only exception is the excursion during the early 1800s for which age calculations based only on 14C decay could lead to dating errors of 50 years or more (Table 1; Figures 2a and 3a). Data from elsewhere in the GoA region also suggests little variability in ΔR over the Holocene, though accuracy in these studies may be impacted by relatively small sample sizes and collections over relatively large spatial domains (Southon et al. Reference Southon, Nelson and Vogel1990; Hutchinson Reference Hutchinson2020; Schmuck et al. Reference Schmuck, Reuther, Baichtal and Carlson2021). Estimates of 14C variability at Tree Nob of 2.9‰ in Δ14C and 22 years ΔR (1σ) are also similar in magnitude to the 1σ decadal variability measured in tropical Pacific corals of 2–3‰ (e.g., Druffel Reference Druffel, Griffin, Guilderson, Kashgarian, Southon and Schrag2001; Grottoli et al. Reference Grottoli, Gille, Druffel and Dunbar2003). This variability is much less than that of similar bivalve radiocarbon records from Arctica islandica shells in the North Atlantic where 1σ variability of ΔR is on the order of 40–60 years (eg. Butler et al. Reference Butler, Scourse, Richardson, Wanamaker, Bryant and Bennell2009; Wanamaker et al. Reference Wanamaker, Butler, Scourse, Heinemeier, Eiríksson, Knudsen and Richardson2012; Lower-Spies et al. Reference Lower-Spies, Whitney, Wanamaker, Griffin, Introne and Kreutz2020). Thus, ΔR at Tree Nob appears to be relatively stable, especially compared to published sclerochronological records from the North Atlantic, where water mass variability is likely much greater, but with the potential for significant, if infrequent, excursions as occurred in the early 1800s.

The coupled nature of the new ΔR data and existing Tree Nob geoduck growth-increment width based seasonal (Apr–Nov) SST reconstruction (Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021) provides a unique opportunity to evaluate the connections between marine radiocarbon and climatic variability in this region. In coastal areas currents, vertical mixing, and the volume and source of freshwater runoff may cause temporal variability in 14C (Allen et al. Reference Allen, Newberger and Federiuk1995; Hickey and Banas Reference Hickey and Banas2008; Schmuck et al. Reference Schmuck, Reuther, Baichtal and Carlson2021). Although instrumental records of these more proximal drivers of 14C are not available prior to about 1950, SST is an environmental indicator likely influenced by some combination of these processes (Lagerloef Reference Lagerloef1995; Hristova et al. Reference Hristova, Ladd and Stabeno2019). The subtle variability in 14C over time, uncertainties in the 14C measurements and SST estimates, and decadal averaging of all data may, in part, mask the relationship between 14C and SST. Yet there was still a significant linear relationship between ΔR and SST when calculated without the highly anomalous 1802–1812 sample (Figure 2b). This relationship suggests old water is relatively warm, counter to expectations, and may represent radiocarbon-old freshwater contributions from radiocarbon-depleted glacial melt or carbonate weathering, though no substantial connections were found to river flows in the higher resolution bomb-pulse data. In contrast, the 1802–1812 datum is consistent with the expectation that colder water, often of relatively deep or more northerly origin, is depleted in 14C. Indeed, anomalously old ΔR and slow geoduck growth in the early 1800s co-occurred with the most extreme cold climate event of the pre-bomb data.

The 1802–1812 14C anomaly is coincident with the largest Northern Hemisphere volcanic eruption of the Tree Nob 14C period of record, the “Unknown” eruption of 1808, which led to significant cooling of the Northern Hemisphere accompanied by other climatic anomalies (Moberg et al. Reference Moberg, Sonechkin, Holmgren, Datsenko and Karlén2005; Gao et al. Reference Gao, Robock and Ammann2008; Cole-Dai et al. Reference Cole-Dai, Ferris, Lanciki, Savarino, Baroni and Thiemens2009). Climate simulations of the North Pacific response to large tropical volcanic eruptions over the last 600 years show greatly enhanced upwelling-favorable winds at Tree Nob in the years following eruptions (Wang et al. Reference Wang, Otterå, Gao and Wang2012; Zanchettin et al. Reference Zanchettin, Timmreck, Graf, Rubino, Lorenz, Lohmann, Krüger and Jungclaus2012). The Tree Nob region is strongly dominated by downwelling with an average annual upwelling index of –32 m3/s per 100 m of coastline (max = –12 m3/s/100 m, min = –69 m3/s/100 m, 1947–2020; NOAA PFEL). Yet the 1802–1812 interval may have been associated with an anomalously strong period of upwelling, bringing cold, 14C-depleted water to the surface (Wang et al. Reference Wang, Otterå, Gao and Wang2012; Zanchettin et al. Reference Zanchettin, Timmreck, Graf, Rubino, Lorenz, Lohmann, Krüger and Jungclaus2012). In addition, wind reversals in the Tree Nob region evident in model simulations of the Unknown eruption may have reduced the advection of warmer, 14C-enriched water into the region, further enhancing “old and cold” conditions. However, extreme 14C anomalies are not evident with other major volcanic events such as Tambora or Krakatoa, possibly due to the seasonality, location, or nature of the eruptions. Thus, despite the coincidence, the relationship between extreme 14C and the Unknown eruption of 1808 may be spurious. Ultimately, a mechanism for this “old and cold” event of the early 1800s cannot be identified from the radiocarbon history, though it does underscore that infrequent yet significant excursions in 14C can occur and appear to be coincident with climate extremes.

The bomb-pulse 14C data provides a complementary perspective on the relationship between 14C and climate at somewhat finer temporal scales within an era spanned by the instrumental record. Positive correlations between SST and 14C run counter to the relationship between SST and 14C over the pre-bomb record and are consistent with the “old and cold” hypothesis. Bomb-pulse 14C data are not significantly correlated with annual mean SST, but instead correlate during the summer months, which is when shells are most actively growing and incorporating carbonate (Shaul and Goodwin Reference Shaul and Goodwin1982; Edge et al. Reference Edge, Reynolds, Wanamaker, Griffin, Bureau, Outridge, Stevick, Weng and Black2021). Beyond temperature, positive ΔR correlation with sea level suggests that the advection of water masses into the region also influences 14C. High sea level anomalies indicate geostrophic flow from the south from where water is likely 14C-enriched (Figure 1a). These significant correlations occur in the winter, which is when this transport is likely to be strongest (Crawford et al. Reference Crawford, Huggett and Woodward1988), moving water masses into the region that may persist into the growing season. The correlation between upwelling index and 14C is also significant in the winter and could reflect the importance of vertical water movements. This region is almost exclusively dominated by downwelling, which is at its most intense during the winter months with peak mean values from November through February. The negative correlation between February upwelling and geoduck ΔR is consistent with the tendency of warmer, shallower water to be 14C-enriched relative to upwelled water. The cause of the June positive correlation between upwelling and ΔR is less clear and may be spurious. One possibility is that upwelling may encourage stratification during the annual freshet, which typically peaks in June, while downwelling, especially during the annual freshet, forces lighter water under denser water to thereby enhance vertical mixing (Austin and Lentz Reference Austin and Lentz2002). Finally, positive correlations with Skeen River discharge in July and August, the warmest and driest months of the year, may reflect inputs of 14C-depleted glacial melt. This relationship is not likely an artifact of freshwater stratification, as summer Skeena River flow is inversely correlated to local SST (r=–0.47, p=0.0078, 1940:2017 JJA Langara SST).

Correlations between basin-scale indicators and 14C are consistent with those between more local indicators and 14C. For example, positive Niño3.4 values indicate El Niño events, which are associated with warmer water in the region (r=0.62, p<0.00001, 1940–2017 Langara annual SST) and are thus consistent with positive correlations between 14C and SST (Figure 4). Correlations with Niño3.4 persist through the growing season and beyond. However, lagged correlations into the fall are likely due to lags in climate signals from the tropical Pacific, where the index is calculated, from reaching the mid-latitudes of the NE Pacific. Furthermore, modelling work demonstrates a strengthening of the ACC during El Niño events (Melsom et al. Reference Melsom, Meyers, O’Brien, Hurlbur and Metzger1999), which would increase the advection of more southerly, 14C-enriched water into the study region. A metanalysis of Holocene-timescale 14C variability in the northeast Pacific suggests ENSO may be the most predictive climate variable for coastal 14C, which is reflected by the strong correlations observed here (Hutchinson Reference Hutchinson2020). In contrast, 14C correlations with PDO and NPI are considerably weaker and less consistent with ENSO. However, the nature of the correlations is consistent with overall patterns of temperature and transport in the NE Pacific. Positive values of the PDO are associated with lower atmospheric pressure over the NE Pacific and relatively strong advection of water from the south along the cost and thus through Hecate Strait. The NPI is closely related to the PDO, but more directly measures regional pressure, and is opposite in sign, explaining its negative correlation with 14C relative to a positive correlation with PDO. Thus, NPI and PDO likely reflect the influence of the Aleutian Low with its ties to both AG advection (Hristova et al. Reference Hristova, Ladd and Stabeno2019) and SST (Newman et al. Reference Newman, Alexander, Ault, Cobb, Deser, Di Lorenzo, Mantua, Miller, Minobe and Nakamura2016). Indeed, the intensity of the Aleutian Low is greatest in the winter, which coincides with the seasonality of the relationship with 14C for both indicators.

Bomb-pulse 14C are important confirmatory data for the pre-bomb but must be interpreted with caution. There may be biases in the 14C bomb-pulse model used to detrend the data. Also, the bomb pulse data are from a very limited temporal window that spans a single cool regime in the North Pacific that began in 1946 and lasted through 1976 (Miller et al. Reference Miller, Cayan, Barnett, Graham and Oberhuber1994; Mantua et al. Reference Mantua, Hare, Zhang, Wallace and Francis1997). Thus, the bomb-pulse data lack the variability in environmental conditions covered by the pre-bomb data, absent the contrast of a warm ocean regime let alone climatic extremes such as the cold period of the early 1800s. This may help explain why the PDO, with energy in interdecadal timescales, did not correlate as strongly with the bomb-pulse data as ENSO, which has greater energy on interannual timescales. These differing temporal resolutions might also change the 14C-SST relationships. Finally, the bomb-pulse itself also changes the ocean-atmosphere exchange dynamics by enhancing Δ14C difference between the two reservoirs, which may affect relationships between 14C and climate. Because the bomb-pulse 14C time series is short (n=21) and a large number of correlation analyses were performed without a Bonferroni correction, the significance of these monthly correlations should be interpreted cautiously. Therefore, to reduce the number of spurious results, we implemented an α=.01 threshold for significance testing in TreeClim. Yet, despite these potential shortcomings, positive bomb-pulse 14C correlations with SST are consistent with the “old is cold” hypothesis in which colder water masses tend to be of deeper or more northerly origin and depleted of 14C. This relationship, as well as the correlations with sea level, NPI, PDO, and ENSO are also consistent with the two-end-member mixing regime proposed by Guilderson et al. Reference Guilderson, Roark, Quay, Page and Moy2006). Notably, this is opposite to the relationship in the pre-bomb data, wherein cold water is coincident with radiocarbon enrichment. Yet relationships between climate and ΔR are relatively weak, as is the variability in ΔR over time, suggesting that during most years radiocarbon is generally stable and minimally affected, if at all, by environmental variability at this site. The radiocarbon excursion in the early 1800s and co-occurrence with unusual cold does, however, indicate that the system is subject to anomalies consistent with the expectation of “old and cold” water masses.

Ultimately, the Tree Nob 14C time series suggests ΔR is relatively stable in the decadally averaged timescales sampled here, but with the potential for significant excursions under climatic extremes such as the cold period of the early 1800s. The 1802–1812 anomaly may be related to a brief, volcanic-induced climate excursion, an example of an event which may not be captured when sampling a lower temporal resolution. However, this pattern is limited to one location in the shallow nearshore environment and therefore may not well represent deeper or offshore locations. Other 14C archives may better address these locations to provide a contrast for the nearshore. For example, Pacific rockfish can live for a century or longer, form annual increments that can be crossdated, and thus could provide as source of absolutely dated, offshore carbonate that would pre-date the bomb pulse (Black et al. Reference Black, Gillespie, MacLellan and Hand2008; Sydeman et al. Reference Sydeman, García-Reyes, Schoeman, Rykaczewski, Thompson, Black and Bograd2014; van der Sleen Reference van der Sleen, Dzaugis, Gentry, Hall, Hamilton, Helser, Matta, Underwood, Zuercher and Black2016 POP paper). Indeed, a network of rockfish and geoduck chronologies could be sampled for 14C along the NE Pacific to better quantify temporal and spatial patterns of 14C variability. The timescales involved in 14C analysis could also be refined if individual increments are sampled to reveal interannual variability rather than the decadal-scale resolution addressed here. Given that the Tree Nob chronology covers 58% of the past 1500 years, there is the possibility of greatly increasing the temporal depth of the 14C chronology as more subfossil shells are collected to fill gaps, which could provide further insight into 14C variability in the NE Pacific and potentially refine dating of other organic marine material of archaeological, geological, or climatic importance.

COMPETING INTERESTS

The authors declare no conflicts of interest.

ACKNOWLEDGMENTS

This work is funded by the National Science Foundation (AGS Award Number: 1855628 to BAB; Award Number: 1602751 to ADW).

This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Geological Survey or the United States Government. The publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Supplementary material

To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2022.83

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Figure 0

Figure 1 Study site: (a) Mixed layer ocean currents in the NE Pacific Ocean. Black streamlines indicate general surface currents based on drifting buoy data from the Atlantic Oceanographic and Meteorological Library, National Oceanic and Atmospheric Administration. Individual marine radiocarbon reservoir offset (ΔR) measurements from previous studies shown as colored points (McNeely et al. 1991; Robinson and Thompson 1981; Jones and Jones 1992; Panich et al. 2018). General location and direction of the Alaska Gyre (AG), North Pacific Current (NPC), and Alaska Coastal Current (ACC) shown in white. (b) Local bathymetry (Amante and Eakins 2009). Tree Nob geoduck collection site and measurement locations for Langara sea surface temperature and salinity (SST and SSS) and Prince Rupert sea level. Glaciers shown in bright blue. Skeena River is in dark blue. (Please see online version for color figures.)

Figure 1

Figure 2 Pre-bomb radiocarbon: (a) Tree Nob radiocarbon ages relative to the Marine 20 curve; points above (below) line indicate older (younger) water. Red bars: radiocarbon age of Tree Nob samples that are not significantly different from Marine 20 (Heaton et al. 2020). Width of bar indicates the range of calendar years sampled. Green bar: 14C sample (middle year of 1807) that is significantly different from average ΔR (2σ). See Methods for error calculation. Blue line: Average radiocarbon age for the mixed layer given by Marine20, corrected by average ΔR of all samples (+256 years). Dark blue shading: 1σ sample error of ΔR; light blue shading is 2σ. (b) ΔR vs SST. SST is the 11-year averaging of Langara SST reconstruction, z-scored (Edge et al. 2021). Coloring of points as in panel (a). Red dashed line: least squares linear fit of red points. (Please see online version for color figures.)

Figure 2

Table 1 Radiocarbon data.

Figure 3

Figure 3 Tree Nob and volcanic proxy records: (a) ΔR in Tree Nob shell material; horizontal uncertainty reflects the 10–11 years represented by each sample while vertical bars are laboratory error. Dark and light gray bands indicate 1- and 2-σ from the mean (white dotted line) of Marine20. (b) Reconstructed Langara SST and 50% prediction interval from Edge et al. 2021. Dark and light gray bands indicate 1- and 2-σ from the mean (white dotted line) (c) NH VEI based on Gao et al. 2008. Yellow highlighting identifies an interval of synchronous anomalies across all three indicators and corresponds to the years 1802–1812. (Please see online version for color figures.)

Figure 4

Figure 4 Bomb 14C correlations with monthly-averaged climate indicators. The level of significance is p <.01 and is calculated by bootstrapping. Significant correlations are shown in black, non-significant in grey. Correlations with annually-averaged (Jan–Dec) climate values are shown to the right of plots, bold font indicates significance at p < .05. Climate data sources given in the “Radiocarbon and Climate Covariability” section.

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