ABSTRACTQuantifying 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.

AbstractThe cooling transition into the Little Ice Age was the last notable shift in the climate system prior to anthropogenic global warming. It is hypothesised that sea-ice to ocean feedbacks sustained an initial cooling into the Little Ice Age by weakening the subpolar gyre circulation; a system that has been proposed to exhibit bistability. Empirical evidence for bistability within this transition has however been lacking. Using statistical indicators of resilience in three annually-resolved bivalve proxy records from the North Icelandic shelf, we show that the subpolar North Atlantic climate system destabilised during two episodes prior to the Little Ice Age. This loss of resilience indicates reduced attraction to one stable state, and a system vulnerable to an abrupt transition. The two episodes preceded wider subpolar North Atlantic change, consistent with subpolar gyre destabilisation and the approach of a tipping point, potentially heralding the transition to Little Ice Age conditions.

AbstractThe climate varies due to human activity, natural climate cycles, and natural events external to the climate system. Understanding the different roles played by these drivers of variability is fundamental to predicting near-term climate change and changing extremes, and to attributing observed change to anthropogenic or natural factors. Natural drivers such as large explosive volcanic eruptions or multidecadal cycles in ocean circulation occur infrequently and are therefore poorly represented within the observational record. Here we turn to the first high-latitude annually-resolved and absolutely dated marine record spanning the last millennium, and the Paleoclimate Modelling Intercomparison Project (PMIP) Phase 3 Last Millennium climate model ensemble spanning the same time period, to examine the influence of natural climate drivers on Arctic sea ice. We show that bivalve oxygen isotope data are recording multidecadal Arctic sea ice variability and through the climate model ensemble demonstrate that external natural drivers explain up to third of this variability. Natural external forcing causes changes in sea-ice mediated export of freshwater into areas of active deep convection, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) and thereby northward heat transport to the Arctic. This in turn leads to sustained anomalies in sea ice extent. The models capture these positive feedbacks, giving us improved confidence in their ability to simulate future sea ice in in a rapidly evolving Arctic.

There is an increasing use of marine species as palaeoclimate recorders for the marine realm. These archives provide novel baseline records of past oceanographic variability in regions devoid of instrumental observations. Here we report results of a study of the ontogenetic growth pattern of two Antarctic marine bivalve molluscs: Yoldia eightsi and Laternula elliptica from West Antarctic Peninsula populations using negative exponential detrending technique and multi-taper method spectral analysis. Our data show that the growth of both Y. eightsi and L. elliptica follows a general negative exponential trend over their longevity. However, our analyses also identified an innate 9.06 year periodic endogenous growth rhythm in the growth increment pattern of Y. eightsi and two innate periodic growth rhythms, 5 and 6.6 year period, were found in L. elliptica. We hypothesize that the Y. eightsi endogenous growth rhythm may be related to the reallocation of energetic resources between somatic growth and gametogenesis although more biological data are required to test this hypothesis. Further work into L. elliptica biology is required to understand the possible meaning of the innate growth rhythms found for this species. The identification of growth rhythms is important not only for their biological significance but also in sclerochronological analysis because of their importance in developing palaeoenvironmental reconstructions.

Given the rapid increase in atmospheric carbon dioxide concentrations (pCO2) over the industrial era, there is a pressing need to construct long-term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the past biological and climate controls on the carbon isotopic (δ13Cshell) composition of the North Icelandic shelf waters over the last millennium, derived from the shells of the long-lived marine bivalve mollusk Arctica islandica. Variability in the annually resolved δ13Cshell record is dominated by multidecadal variability with a negative trend (−0.003 ± 0.002‰ yr−1) over the industrial era (1800–2000 Common Era). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (δ13C of CO2) derived from the burning of fossil fuels. Comparison of the δ13Cshell record with Contemporaneous proxy archives, over the last millennium, and instrumental data over the twentieth century, highlights that both biological (primary production) and physical environmental factors, such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and sea surface temperature and salinity of the subpolar gyre, are the likely mechanisms that contribute to natural variations in seawater δ13C variability on the North Icelandic shelf. Contrasting δ13C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated marine Suess effect signal at this locality.

Reconstructing regional to hemispheric-scale climate variability requires the application of spatially representative and climatically sensitive proxy archives. Large spatial networks of dendrochronologies have facilitated the reconstruction of atmospheric variability and inferred variability in the Atlantic Ocean system. However, the marine environment has hitherto lacked the direct application of the spatial network approach because of the small number of individual absolutely-dated marine archives. In this study we present the first analyses of a network of absolutely-dated annually-resolved growth increment width chronologies from the marine bivalves Glycymeris glycymeris and Arctica islandica. The network contains eight chronologies spanning > 500 km along the western British continental shelf from the southern Irish Sea to North West Scotland. Correlation analysis of the individual chronologies and a suite of climate indices, including the Atlantic Multidecadal Oscillation (AMO), Central England surface air temperature (CET), northeast Atlantic sea surface temperatures (SST's) and the winter North Atlantic Oscillation (wNAO), demonstrates that, despite the large geographical distances been sites and the heterogeneous nature of the marine environment, the increment width variability in these series contains an element of coherence likely driven by a common response to changing environmental forcing. A nested Principal component analysis (PCA) was used to construct five composite series which explain between 31% and 74% of the variance across the individual chronologies. Linear regression analyses indicate that the composite series explain up to 41% of the variance in Northeast Atlantic SSTs over the calibration period (1975–2000). Calibration verification (reduction of error [RE] and coefficient of efficiency [CE]) statistics indicate that the composite series contains significant skill at reconstructing multi-decadal northeast Atlantic SST variability over the past two centuries (1805–2010). These data suggest that composite series derived from sclerochronology networks can facilitate the robust reconstruction of marine climate over past centuries to millennia providing invaluable baseline records of natural oceanographic variability.

Owing to the lack of absolutely dated oceanographic information before the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions (for example, Medieval Climate Anomaly-Little Ice Age, MCA-LIA). Here we present analyses of a millennial-length, annually resolved and absolutely dated marine δ18O archive. We interpret our record of oxygen isotope ratios from the shells of the long-lived marine bivalve Arctica islandica (δ18O-shell), from the North Icelandic shelf, in relation to seawater density variability and demonstrate that solar and volcanic forcing coupled with ocean circulation dynamics are key drivers of climate variability over the last millennium. During the pre-industrial period (AD 1000-1800) variability in the sub-polar North Atlantic leads changes in Northern Hemisphere surface air temperatures at multi-decadal timescales, indicating that North Atlantic Ocean dynamics played an active role in modulating the response of the atmosphere to solar and volcanic forcing.

There is currently a deficiency of annually-resolved temperature series from the marine environment. We present a multiproxy reconstruction of Hebridean shelf sea (Tiree Passage; NW Scotland) spring sea surface temperatures (SSTs) for the period AD 1805-2010. The reconstruction is based on the growth increment series from the first absolutely dated annually-resolved multi-centennial Glycymeris glycymeris bivalve mollusc sclerochronology coupled with previously published stable oxygen isotope data (δ18O) from benthic foraminifera sampled from a dated sediment core from nearby Loch Sunart. The independent series contain significant correlations with SSTs across complementary frequency domains. The low frequency component of the sedimentary archive was combined with the mid and high frequency components of the G. glycymeris chronology indices to create a single multiproxy series. Split calibration-verification statistics (reduction of error, RE, coefficient of efficiency, CE, and R2) indicate that the multiproxy record, calibrated to local instrumental sea surface temperatures, contains significant precision and skill at reconstructing spring SSTs (RE=0.59, CE=0.26, R2=0.54). These data demonstrate that bivalve sclerochronologies, when combined with low frequency proxies such as sediment archives, can facilitate statistically robust reconstructions of palaeoceanographic variability during the late Holocene for hydrographically-significant regions of the temperate marine system previously void of annually-resolved archives. The reconstructed SSTs contain a general warming trend of 0.60±0.14°C per century. Only four years in the reconstructed period (1999, 2000, 2002 and 2003) exceed temperatures greater than two standard deviations higher than the reconstructed mean SST (9.03°C), whilst just three years in the first half of the 19th century (1835, 1838 and 1840) fall more than 2σ below the reconstructed mean (6.80°C). © 2013.

Cross-dated chronologies derived from internal growth increments in the shells of the long-lived bivalve, the dog cockle Glycymeris glycymeris (Linnaeus, 1758), live-collected from two different sites off the east (1997) and south (2009) coasts of the Isle of Man respectively, are described. The chronologies, developed from ten individuals from each site, were found to be statistically robust (Expressed Population Signal (EPS) = 0.87 and 0.94 respectively) with a significant common growth signal despite their location 27. km apart (R = 0.53; N = 49, P = < 0.0001). The period of common growth between the two chronologies is consistent with the 12-year difference in their dates of collection thus providing evidence of an annual periodicity of growth line formation. Significant positive correlations were identified between the chronology indices from both the southern (R = 0.55, N = 58, P = < 0.0001) and eastern sites (R = 0.64, N = 68, P = < 0.0001) and mean January to September sea surface temperatures. A significant positive correlation was also found between the southern site and the winter North Atlantic Oscillation index (R = 0.43; N = 49, P = 0.0009). These data indicate that annual growth increments in the shells of G. glycymeris have the potential to be used as a scleroclimatological archive. © 2012.

In order to assess its potential as a sclerochronological archive, we present statistical and geochemical analyses of internal growth increment series in shells of the heart cockle Glossus humanus (L.), a large marine bivalve. The investigated samples were collected from Loch Sunart and the Sound of Mull, Scotland, United Kingdom. High-resolution stable isotope (δ18O) analyses and radiocarbon (14C) determinations indicated that G. humanus forms annual growth lines. Examination of the growth increment series revealed that the maximum longevity of G. humanus in this region was 78 years. Radiocarbon dating and crossmatching techniques, derived from dendrochronology, were used to provide an estimation of the temporal distribution of the fossil G. humanus. of the shells that contained >25 growth increments, seven were found to statistically crossmatch, including shells from two distinct sites 15 km apart. The calibrated 14C determinations independently confirmed the crossmatching of three G. humanus shells from the Sound of Mull with a separately constructed Glycymeris glycymeris chronology and a further three G. humanus shells from site 3, in the main basin of Loch Sunart, but indicate a significant difference (site 1) in the antiquity of the two G. humanus populations. Radiocarbon dating indicated that, despite their fragile nature, G. humanus shells remain preserved in near original condition for at least 700 years. Given the small amount of available shell material, it is unlikely that G. humanus will become a key species for the construction of long absolutely dated sclerochronologies. However, these data do indicate that the annually resolved G. humanus growth series could be used to supplement series from other long-lived bivalves and facilitate the construction of a robust multispecies sclerochronology spanning the last 1000 years. © the Author(s) 2013.

A multicentennial and absolutely-dated shell-based chronology for the marine environment of the North Icelandic Shelf has been constructed using annual growth increments in the shell of the long-lived bivalve clam Arctica islandica. The region from which the shells were collected is close to the North Atlantic Polar Front and is highly sensitive to the varying influences of Atlantic and Arctic water masses. A strong common environmental signal is apparent in the increment widths, and although the correlations between the growth increment indices and regional sea surface temperatures are significant at the 95% confidence level, they are low (r~. 0.2), indicating that a more complex combination of environmental forcings is driving growth. Remarkable longevities of individual animals are apparent in the increment-width series used in the chronology, with several animals having lifetimes in excess of 300. years and one, at 507. years, being the longest-lived non-colonial animal so far reported whose age at death can be accurately determined. The sample depth is at least three shells after AD 1175, and the time series has been extended back to AD 649 with a sample depth of one or two by the addition of two further series, thus providing a 1357-year archive of dated shell material. The statistical and spectral characteristics of the chronology are investigated by using two different methods of removing the age-related trend in shell growth. Comparison with other proxy archives from the same region reveals several similarities in variability on multidecadal timescales, particularly during the period surrounding the transition from the Medieval Climate Anomaly to the Little Ice Age. © 2012 Elsevier B.V.

The spatial distribution, density, growth rate, longevity, mortality and recruitment patterns of the long-lived clam Arctica islandica in Belfast Lough, Northern Ireland, UK are described. The A. islandica population at Belfast Lough appears to be restricted to a small area at the mouth of the Lough. Additional searches for specimens further into the Lough and into deeper waters found no evidence of a larger more widespread population and we report population densities of 4.5 individuals m -2. The ages of the clams were determined from the number of internal annual growth lines in acetate peel replicas of shell sections. The population growth curve was fitted using the Von Bertalanffy growth equation: Lt = 93.7 mm (1-e -0.03(t-1.25)). Based on catch curve analysis, the Belfast Lough population has an estimated longevity of 220 years and a natural mortality rate of 0.02. We compare growth characteristics and life history traits in this population with other analogous A. islandica populations. The overall growth performance and the phi-prime index were used to compare growth parameters with data from the literature and we observed no significant relationship between the growth performance indices and longevity or latitude. Analysis of the age-structure and reconstructed dates of settlement indicate that this population has experienced almost continual recruitment over the last century with a gap in successful recruitment into the population 90-100 years ago and another 140-150 years ago. The size-structure revealed a scarcity of small individuals which we believe may be an artefact of the dredge sampling process. © 2011 Marine Biological Association of the United Kingdom.

The stable carbon isotope ratio in bivalve shells (δ13CS) is an enigmatic geochemical archive whose interpretation is often frustrated by the intrusion of variable and unpredictable vital effects which can influence the mix of metabolic and dissolved inorganic carbon (DIC) in the shell material. The impacts of vital effects and rapid changes in calcification rates on the variability and value of δ13CS have been described in a number of studies and in many bivalve species with typical lifespans between a few years and a few decades, δ13CS has been observed to change (usually decreasing) with biological age. Very long-lived animals, by contrast, spend most of their lives in the mature, slow-growing phase, and it might be expected that in these instances the effect of changes in calcification rates on δ13CS would be less marked, or even absent. Analysis of δ13CS in mature (>40years old) Arctica islandica, reported here, indicates that this is the case δ13CS in shell samples with biological ages between 42 and 391years from four distinct sites in the North Atlantic shelf seas (Gulf of Maine, north Icelandic shelf, Irish Sea and North Sea) shows no age-related trend. This suggests that metabolic vital effects in mature A. islandica may be reasonably stable at the population level. If the drivers of isotopic disequilibrium between shell and ambient environment can be identified and quantified, it may be feasible to adjust for them and use δ13CS in mature A. islandica to investigate long-term changes in nutrient sources and as a robust proxy for δ13C of environmental DIC. © 2010 Elsevier B.V.

In this article, we explore the use of fluorescence spectroscopy to image growth patterns in the marine bivalve Arctica islandica (L.). The method presented here does not require any chemical treatment of the polished shell section and yields results comparable to acetate peels of acid-etched shell sections and Mutveitreated shell sections. Further, our results indicate that the annual growth lines in A. islandica fluoresce in the blue light spectrum (450-490 nm), thus an ultraviolet source (mercury lamp) is not required. The reflected light entering the digital camera was filtered (510-540 nm) and later enhanced to emphasize the annual growth patterns. The fluorescence of annual growth lines was consistent among the four animals used in this study. Additionally, we measured growth increments in the umbo section of one A. islandica shell using both the traditional acetate method and fluorescence imaging. The two sets of measurements were highly correlated (r = 0.97; P < 0.0001). We suggest that the fluorescence imaging method presented here is a viable option for increment identification and measurement in this key marine archive. It is likely that the methods demonstrated here for A. islandica can easily be used/modified for other bivalve (mollusk) taxa. Fluorescence microscopy permits rapid analysis of shell growth patterns with minimal pretreatment and offers an objective method of determination of annual growth increments and lines. © 2009, by the American Society of Limnology and Oceanography, Inc.