Preboreal oscillations inferred from Arctica islandica sclerochronology

Geobios 47 (2014) 305–313

Available online at

ScienceDirect www.sciencedirect.com

Original article

Preboreal oscillations inferred from Arctica islandica sclerochronology§ Samuli Helama a, Jan Kresten Nielsen b,*, Jesper Kresten Nielsen c, Nils-Martin Hanken d, Kenneth Evison d,e a

Arctic Centre, University of Lapland, Rovaniemi, Finland Su¨leyman Demirel University, Department of Geological Engineering, 32260 Isparta, Turkey c North Energy ASA, New Area Exploration, P.O. Box 1243, NO-9504 Alta, Norway d Department of Geology, University of Tromsø, Dramsveien 201, NO-9037 Tromsø, Norway e Baker Hughes Norge AS, Tanangervegen 501, NO-4056 Tananger, Norway1 b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 21 January 2014 Accepted 20 July 2014 Available online 16 September 2014

An increasingly important source of annually resolved palaeoenvironmental proxy data originates from cross-dated incremental chronologies summarizing the shell growth of several individuals. Here, we have analysed annual increment variations in a collection of radiocarbon-dated shells of ocean quahog (Arctica islandica) from early Holocene prodelta deposits in north Norway. Radiocarbon dating of the shell material showed that the increments were formed contemporaneously during Preboreal times. The biologically youngest shell contains 35 annual increments, whereas the oldest shell shows 169 increments. Time-series of annual increments demonstrated clear age trends with the widest increments during the very early years of bivalve life, followed by a notable decline in increment widths as the bivalves aged. Subsequent to removing these biological trends from the series, a sclerochronological cross-dating was carried out and resolved the temporal alignments of the shell growth increment records relative to each other. The resulting shell growth increment chronology evidences vigorous growth variations. Spectral analysis of the chronology revealed 3.7- and 4.3-year periodicities, indicative of Preboreal environmental oscillations. Periodicities of longer period were not detected. Our results prove the value of radiocarbon-dated shell assemblages to build ‘‘floating’’ geochronologies for periods and regions where dead shells from museum collections or seabed are not obtainable. Increasing constructions of such chronologies enhance the potential of sclerochronological cross-dating of annual shell growth increment chronologies to depict and detail annually-resolved climate variability not only for late Holocene, as previously illustrated, but also for early Holocene times, when large-scale oscillations punctuated the global climate dynamics. Development of longer chronologies with higher sample replication remains an attainable interdisciplinary target. ß 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Early Holocene Geochronology Sclerochronology Climate variability North Atlantic Oscillation

1. Introduction Annual shell growth increments form the basis of sclerochronological tools for construction of long, high-resolution records. Annually resolved proxies of this type originate from sclerochronological analyses of bivalve shells (Jones, 1983). Multicentennial and even millennial annually-resolved chronologies constructed from growth increment series of several individual organisms are constructed from marine (Marchitto et al., 2000; Strom et al., 2004, 2005; Helama et al., 2006; Butler et al., 2010, 2013) and freshwater §

Corresponding editor: Fre´de´ric Quille´ve´re´. * Corresponding author. E-mail address: [email protected] (J.K. Nielsen). 1 Current address.

http://dx.doi.org/10.1016/j.geobios.2014.07.003 0016-6995/ß 2014 Elsevier Masson SAS. All rights reserved.

(Helama et al., 2009a, 2009b) shells. Averaging the individual shell growth increment series into a mean chronology improves the chronology confidence; however, before averaging the individual series ought to be cross-dated, after which the mean of several sample series may be regarded as sclerochronology (Helama et al., 2006). The method is similar to tree-ring dating in as much as the science of sclerochronology could be referred to as marine (Jones, 1983; Marchitto et al., 2000) or aquatic (Helama et al., 2006) counterparts of dendrochronology (Fritts, 1976). Despite advances in producing cross-dated late Holocene sclerochronologies, especially within the North Atlantic realms (Butler et al., 2009, 2010, 2013), no replicated bivalve sclerochronologies for the early Holocene have been published so far. Here, we apply the sclerochronological methods to a hitherto unstudied collection of radiocarbon-dated Preboreal (i.e., early

306

S. Helama et al. / Geobios 47 (2014) 305–313

Holocene) shells of ocean quahog (Arctica islandica) excavated from prodelta deposits in north Norway. Sclerochronological benefits of A. islandica include its distinct annual increments and its supercentenarian longevity (Jones, 1980; Thompson et al., 1980; Ropes et al., 1984; Butler et al., 2013), as well as the suggested connectivity of its growth variations to both climatic and oceanographic variations, particularly the North Atlantic oscillation (NAO; Scho¨ne et al., 2003; Helama et al., 2007b; Dunca et al., 2009; Wanamaker et al., 2009; Butler et al., 2010, 2013; Helama and Hood, 2011). Moreover, the shell growth variations can be used to show whether the oceanographic setting the bivalves populated was impacted by solar or multidecadal fluctuations of the North Atlantic (Butler et al., 2010, 2013). Several geological and glaciological records show that the North Atlantic climate system underwent drastic changes through the early Holocene (Bjo¨rck et al., 1996, 1997; Rasmussen et al., 2007; Hansen et al., 2011; Hanken et al., 2012). Climatic downturns are evident in independent proxy sources of Greenland ice cores (Rasmussen et al., 2007; Kobashi et al., 2008), marine sediment cores (Bond et al., 2001; Berner et al., 2010), stalagmite records (Lauritzen and Lundberg, 1999; Boch et al., 2009), and terrestrial proxies revealing lacustrine and vegetation changes (Bjo¨rck et al., 2001; Bos et al., 2007). Gathering evidence demonstrates that Earth’s climate system was highly unstable during both late glacial and early Holocene times and that it underwent recurring abrupt coolings at centurial to millennial scales. Yet, the analyses detailing the variations in the shorter term may reveal the relative roles of the climatic forcing behind the long-term events (Andrews et al., 2003; Berner et al., 2010; Schmidt et al., 2012). Sclerochronology may provide ideal records for analyses of such variations at annual resolution. Our primary aim was to test whether a dead assemblage of A. islandica shells, as unearthed from the early Holocene deposits in north Norway during sedimentological fieldwork, could be used as a basis for sclerochronological investigations. For this purpose, the exploited assemblage was rather small, containing only five individual shells to be radiocarbon-dated and examined for their annual shell growth increments through sclerochronological cross-dating. We also aimed at portraying the high-resolution

variability in the early Holocene palaeoenvironmental variations from inter-annual to longer timescales in the North Atlantic surroundings and to test the solar and NAO hypotheses as agents regulating the unstable Preboreal climate variability.

2. Geological setting Significant volumes of sediments of Weichselian and Holocene age are present in north Norwegian valleys and fjords (Corner et al., 1990; Vorren and Plassen, 2002; Corner, 2006; Eilertsen et al., 2006). The Breivikeidet Valley, situated at the head of Ullsfjord in Troms County (Fig. 1), is characterized by extensive postglacial shallow-marine deposits overlain by delta deposits. Glacioeustatic uplift of the landmass following glacier retreat caused the postglacial river to gradually cut down into Holocene marine deposits of the former fjord head (Corner and Fjalstad, 1993; Nielsen et al., 2004; Evison, 2012). Early Holocene prodelta mud has thus been raised above sea level and exposed in river banks (Corner and Fjalstad, 1993), allowing detailed studies of litho- and biofacies. The bottomsets are fine-grained and massive to thinly bedded; the beds may be bioturbated. The foresets, which consist of coarse-grained sediment are massive, normal and inverse beds. The foresets formed on a delta slope and grade into the bottomsets that denoted the prodelta. They formed in a fjord deltaic setting where fluvial discharge conveyed large amounts of sediments that probably became redistributed by tidal and wave currents. These currents may well have instigated cohesionless debris flows (grain flows), surge-type turbidity currents and suspension fallout forming the large-scale foresets of the delta slope. Suspension fallout from plumes caused deposition across the prodelta forming its bottomsets. Such depositional processes have been interpreted by Eilertsen et al. (2011) from successions of steep-faced fjord deltas in the Ma˚lselv Valley in north Norway. Also, Corner and Fjalstad (1993) interpreted the processes of suspension fallout and sediment gravity flow based on a river section through deltaic bottomsets and foresets about 750 m NNE from our fieldwork site (Fig. 1). A 4 m-thick succession of bottomsets is present at the geographical coordinates 69.657008N, 19.580868E, equivalent to

Fig. 1. Location of the fieldwork site (star) at Breivikelva, Breivikeidet, Troms County, north Norway. Contour lines: 10 m.

S. Helama et al. / Geobios 47 (2014) 305–313

UTM 0677584.18E, 7734284.55N (Kartverket; http://www.norgeskart.no). The bottomsets comprise of clayey silt and very finegrained sand (Fig. 2). The sediment is partly weakly bedded and partly mottled by bioturbation. The shell material for this study was taken from an interval 2.20–2.45 m below the overlying coarse-grained bed. The interval has a high diversity and density of shelly and skeletal material of marine invertebrates. The majority of them are particularly well preserved and show no evidence of current transportation. The overall absence of abrasion and fragmentation features – and the commonness of articulated specimens with pieces of ligaments and periostraca – indicates preservation within the habitat. Dissolution features are absent to rare in appearance. The bivalves Macoma calcarea, Arctica islandica, Mya truncata and Chlamys islandicus, as well as acorn barnacles are common. Deteriorated remains of barnacles, calcareous algae and herbivore gastropods reworked from coastal habitat may also be present.

3. Material and methods 3.1. Retrieving the shell material The above-mentioned succession of fjord deltaic bottomsets yielded ample shell material for this study (Fig. 2). Well-preserved valves were hand-picked from the succession. The valves were gently brushed and washed in water to remove the enclosed sediment. 3.2. Radiocarbon dating One sample from each of the five shells of A. islandica was radiocarbon dated at the Poznan´ Radiocarbon laboratory (Poland). The samples originated from the ventral shell margin of the outer shell layer. The 14C AMS dates were calibrated using OxCal v3.10 (Bronk Ramsey, 1995, 2001) by the marine radiocarbon calibration

307

curve (Hughen et al., 2004) and adopting a reservoir correction of DR = 65  37 (Mangerud and Gulliksen, 1975). These reservoir correction estimates originate from a local (Troms) shell of Chlamys islandica, which is a suspension feeding bivalve similar to A. islandica. 3.3. Incremental dating The shells were externally strengthened with epoxy and crosssectioned along the line of maximum (height) growth. The crosssectioned surface was ground on progressively finer grades (up to 1200 grit) of carborundum paper attached to a grinding machine and polished using diamond paste. Prior to sclerochronological analysis, the polished surfaces were etched in a 1:1 solution of glutaraldehyde (25%) and acetic acid (1%) with a trace of alcian blue maintained at 38 8C for 25 min (Mutvei et al., 1994, 1996; Scho¨ne et al., 2005; Nielsen and Nielsen, 2009). Time-series for sclerochronological analyses were obtained by measuring consecutive increment widths from the shell margin of each specimen (Fig. 3). Increment widths were measured from the photographic enlargements perpendicular to the winter-lines, analogous to ‘‘GI I’’ sensu Jones (1980). Before the attempts at cross-dating the incremental series, the biological trends were removed from the series. This was done using an indexing approach where S-year spline functions (Cook and Peters, 1981) of predetermined stiffness (S = 8), purpose-made for the study species (Helama et al., 2006), were fitted to each series as detrending. Dimensionless indices were derived from the curve by divisions between the increment width and curve values. The cross-dating approach was applied to the time-series of the individual shells and, iteratively, between the undated sample and the master chronology (the average of all other cross-dated sample series). In the course of this process, each pairwise statistical chronological comparison producing conspicuously high correlations was judged by visual assessment. The resulting incremental chronology was assessed using two statistical measures. Mean

Fig. 2. The fieldwork site. a. Measured section of fine-grained bottomset beds. The shell material for this study was sampled from the interval 0.8–1.05 m above the river level at the time of sampling, corresponding to 2.20–2.45 m below the coarse-grained bed. Grain size definitions after the Udden-Wentworth scale. b. Excavated site with the measured succession of bluish muddy bottomsets overlaid by sandy foresets. Measure rod: 1 m.

308

S. Helama et al. / Geobios 47 (2014) 305–313

Fig. 3. a. Sketch of the cross-section showing the internal structure of Arctica islandica shell (modified after Haugen and Sejrup, 1990). b. Microscopic view to cross-section of a shell, surrounded by epoxy, showing the annual increment widths (bars) as measured perpendicularly to winter-lines.

inter-series correlation (RBAR) was calculated over the early, middle, and late years of the chronology when the chronology showed maximum sample sizes of three, four, and three series respectively. Subsequently, the expressed population signal (EPS) was computed as a function of the mean inter-series correlation and the sample size (Wigley et al., 1984; Briffa and Jones, 1990). 3.4. Analysing the chronology Once the dating of the individual series was established by radiocarbon and cross-dating techniques, it became justified to rebuild the chronology using stiffer detrending curves (Helama et al., 2006; Helama and Nielsen, 2008). The ontogenetically youngest increments occurring prior to the widest increment in each series were omitted (Nielsen et al., 2008). Here, this procedure resulted in the omission of the two youngest increments only. These were the increments that predated the maximum juvenile growth phase as their behaviour could not be sufficiently accounted by a deterministic model in the form of a modified negative exponential function (Fritts et al., 1969; Cook et al., 1990a). This model was fitted to the time-series of incremental widths of individual shells. Then, the annual growth indices were derived as ratios between the value of measured growth and the growth trend model. To improve the chronology determination, a calculation of the final chronology by median (Helama et al., 2009a; Helama, 2011), instead of mean, was performed, as the number of the series remained inherently below six (Mosteller and Tukey, 1977; Cook et al., 1990b). Concentration of shell growth variability at different periodicities was estimated by multi-taper methods (MTM; Ghil et al., 2002). The MTM analysis was performed for the part of the chronology that exhibited a sample size of two or more cross-dated series. 4. Results 4.1. Incremental series The mean length of the series was 104.8 years. The sampled shells contained between 35 and 169 annual increments. All the studied shells exhibited a trend in their increment widths as a function of the bivalve ontogeny (Fig. 4(a)). The width of the increments was highest during the very early years of shell growth,

Fig. 4. a. Sample series of individual Arctica islandica (AI) shells as a function of their ontogenetic ages. b. Summary curve of the five series as estimated using median (see text for details).

S. Helama et al. / Geobios 47 (2014) 305–313

with a decline towards narrower increment widths over the first two to five decades of the growth histories. Subsequent to this exponential decline, the growth history of each shell showed more gradual declines in its increment widths. These ontogenetic features of shell growth also became evident for the summary curve of ageing, calculated as the median of the five studied annual shell growth increment series that were calculated as a function of bivalve ontogeny (Fig. 4(b)). Moreover, this curve clearly displayed the concave shape of the growth decline in the increment formation through the ageing of the organisms. 4.2. Chronology statistics Calibrated radiocarbon dates provided low-resolution chronological assessments for the absolute ages. These ages demonstrate an early Holocene age for the shells. The calibrated radiocarbon ages show that the bivalves lived during the Preboreal period with temporally overlapping life-spans (Table 1). This was further confirmed by sclerochronological cross-dating, providing the temporal alignments of the shell growth increment records relative to each other (Fig. 5). This synchronization was performed using the shell growth index series and resulted in a multi-shell chronology where the individual series are markedly well correlated with the resulting master chronology. Subsequent to cross-dating, we calculated Pearson correlations between the individual samples. Coefficients of correlation averaged 0.36 (Fig. 5(a)). Even more notable growth synchronies between individual series and the master chronologies were illustrated (Fig. 5(b)), where the mean correlation between the individual sample series and the master chronology was 0.42 (Table 1). The dating yielded a sclerochronology with a full length of 191 years. The sample size was at least two samples over 168 years. The sample size of the chronology obviously depends on the temporal alignment of the individual samples (Fig. 5(c)). The statistics of the mean inter-series correlation (RBAR) and the expressed population signal (EPS) were computed separately over the early (incremental years 1–61), middle (62–112), and late (113–191) parts of the chronology when the sample size reaches a maximum of three, four and three shells, respectively. The obtained statistics (RBAR/EPS) over these periods were 0.14/ 0.332, 0.432/0.753, and 0.312/0.576, with a concomitant indication that the most reliable sclerochronological variations were determined over the middle part of the chronology. Of note, none of these EPS values exceed the predetermined level of 0.85, commonly used as a rough limit for a nonsignificant reduction in chronology reliability (Wigley et al., 1984). It is likely that improvement in the sample size would raise the EPS above that threshold. 4.3. Growth variations As in previous sclerochronological investigations, the incremental series showed a negative exponential decline in their Table 1 The five shell samples and their radiocarbon and calibrated dating, including the laboratory number of the sample according to the radiocarbon laboratory, the radiocarbon 14C age with associated 1-s error (years BP), the 2-sigma range of the calibrated 14C AMS dates (years BC), the ontogenetic age (A) obtained from the increment number, and the correlation between the sample series and the master chronology (rM). Sample

Lab number

14

TMU-JKN-AI-001 TMU-JKN-AI-002 TMU-JKN-AI-003 TMU-JKN-AI-004 TMU-JKN-AI-005

Poz-37566 Poz-37567 Poz-37568 Poz-37569 Poz-37570

9980  50 10,010  50 10,050  50 10,080  50 10,090  50

C Age

cal BC

A

rM

9130–8700 9160–8750 9190–8810 9220–8880 9230–8900

35 137 169 78 100

0.52 0.37 0.43 0.41 0.39

309

widths (Fig. 4). In order to remove this trend from the series, a negative exponential curve was fitted individually to each crossdated series (Fig. 6(a)) and a new set of ratio-based index series was produced (Fig. 6(b)). The resulting incremental chronology exhibited notable incremental fluctuations. This new bivalve sclerochronology implied a concentration of spectral power with indication of significant periodic variations between three and four years (Fig. 7). Of note, the spectral analysis did not reveal statistically significant oscillations of longer than multi-annual periodicities – i.e., the strongest multi-annual periodicities were found at 3.7 and 4.3 years. Regarding growth variations of even longer term, it is noteworthy that the chronology was not expected to display centurial trends owing to the chosen detrending method that was used to remove the biological trend of ageing from the individual series. The three highest annual growth values were observed for the incremental years 101, 162, and 126 of the ‘‘floating’’ chronology (see Fig. 6(b)), with sclerochronological indices reaching dimensionless values around 1.3. The poorest shell growth occurred in incremental years 98, 27, and 187, with sclerochronological indices around 0.65. The growth vigour is further demonstrated as the date of poorest shell growth (associated with the ‘‘floating’’ year 98) was followed by the 10-year-period of most enhanced shell growth, taking place over the incremental years between 99 and 108. On the other hand, the decadal period of diminished growth occurred between the years 181 and 190, and thus, in association with the third of the weakest growth years (187).

5. Discussion 5.1. Radiocarbon dating and the Preboreal oscillation Radiocarbon dating of the shell material showed that our chronology postdates the younger Dryas-Preboreal climatic shift of 11,450 to 11,390  80 yr. BP (Bjo¨rck et al., 1996), and effectively overlaps Preboreal time (Mangerud et al., 1974). Thus, it is likely that the incremental chronology coincides, at least partly, with indications of a 150-year-long cooling in the early Preboreal dated to about 11,200–11,050 calendar years (cal. yr.) BP (Bjo¨rck et al., 1996). The dating of the single event, termed the Preboreal Oscillation (PBO), was derived by chronological matching by Bjo¨rck et al. (1996) of the similar cooling indications in radiocarbon-dated lacustrine sequences from Sweden, Greenland ice-core records, and German tree-ring chronology. Because of the revisions of the tree-ring chronologies (Spurk et al., 1998), the dating of the PBO was later shifted to about 11,300– 11,150 cal. yr. BP (Bjo¨rck et al., 1997). Accordingly, the PBO was recorded as a double-event of summer sea surface temperature cooling centred at 11.3 and 11.2 cal. kyr. BP in the Norwegian Sea (Berner et al., 2010). Even then, the dating of the PBO has not been fully stabilized. Based on their Greenland ice-core chronologies, Rasmussen et al. (2007) dated this cooling event to 11.5 to 11.4 cal. kyr. BP as indicated by lowered d18O values and reduced accumulation. This dating appears coincident with the presence of melting icebergs and lowered summer sea surface temperatures as indicated by palaeoceanographic proxies in the Nordic Seas (Rasmussen et al., 2011). In fact, the cooling of the PBO has been linked with a melt-water discharge event from a drainage of glacial Lake Agassiz (Canada) into the Arctic Ocean at about 11,335 cal. yr. BP, the discharge lasting until 10,750 cal. yr. BP (Fisher, 2002). On these grounds, the radiocarbon dating of the incremental chronology (Table 1) relative to PBO becomes rather complex, but it appears that the new chronology would suggestively postdate the culmination phase of this cooling event. A closer look at the combined evidence of the Greenland ice cores and palaeoclimatic and palaeoceanographic changes in the Nordic Seas shows that the Preboreal time was actually

310

S. Helama et al. / Geobios 47 (2014) 305–313

Fig. 6. a. Bivalve sclerochronology shown as the individual Arctica islandica measurement series and their modelled growth curves. b. Resulting shell growth (sclerochronological) index time-series.

overlaps this cooling period with indications of vigorous growth variations during the time of significant large-scale climate perturbations. 5.2. Shell growth variations in a Preboreal context

Fig. 5. a. Cross-dating between the five incremental time-series. This assessment is quantified using the mean correlation between the individual Arctica islandica (AI) series (rBS). b. Comparisons between the sample series (black line) and master chronologies (grey line). c. Sample size variations along the resulting 191-year sclerochronology.

unstable even beyond the PBO. While the PBO clearly represents the next major event of cooling in the North Atlantic sector since the Younger Dryas, albeit to a lesser degree, with general instability of the period also becomes obvious in the years following the culmination of the PBO (Bjo¨rck et al., 2001; Risebrobakken et al., 2003; Rasmussen et al., 2007; Kobashi et al., 2008; Berner et al., 2010). Greenland ice-core and European stalagmite evidence agree in showing climatic coolings around 9.3 or 9.1 and 10.0 or 9.95 cal. kyr. BP (Rasmussen et al., 2007; Boch et al., 2009). Moreover, the period of the PBO ended with an abrupt warming at 11,270 cal. yr. BP (Kobashi et al., 2008). The culmination of this warming at 11,150 cal. yr. BP, on the other hand, was followed by a gradual cooling, as recorded both in Greenland ice cores (Kobashi et al., 2008) and temperature changes in the Nordic Seas (Risebrobakken et al., 2003). Judging purely from the radiocarbon dating results, our chronology

This new sclerochronology of Arctica islandica demonstrates notable variable growth fluctuations during the Preboreal time. The spectral estimates of the A. islandica sclerochronology exhibit periodicities at 3.7 and 4.3 years (Fig. 7). Similar multi-annual to subdecadal signals have previously been found in A. islandica chronologies elsewhere around the North Atlantic during the late

Fig. 7. Spectral characteristics of the chronology estimated as MTM-spectrum where the horizontal lines show the median, 90% and 95% significance levels relative to the red noise. Estimation was carried out over a 168-year-interval with replication of two or more incremental series (see Fig. 5c).

S. Helama et al. / Geobios 47 (2014) 305–313

(Scho¨ne et al., 2003; Dunca et al., 2009; Wanamaker et al., 2009; Butler et al., 2010, 2013) and mid Holocene (Helama and Hood, 2011). Sclerochronological variations on these scales have so far been associated with the possible influence of the North Atlantic oscillation (NAO; Hurrell, 1995; Hurrell and Deser, 2010) over late (Wanamaker et al., 2009; Butler et al., 2010, 2013) and mid Holocene times (Helama and Hood, 2011). Similarly, the NAO influence has been demonstrated for the late Holocene sclerochronologies, showing statistically significant correlations with the instrumentally observed winter NAO indices (Scho¨ne et al., 2003; Helama et al., 2007b). According to Wanamaker et al. (2009), the periodicities between 2 and 6 years in the increment widths of A. islandica could generally be taken as an indication of NAO-like growth variations in the western North Atlantic. The multi-annual to sub-decadal periodicities found in the present shell growth incremental chronology are analogous to this band of frequencies. Alternatively, our results may be regarded as supportive of the multi-annual pattern of climate variability. An interesting feature of our chronology is the lack of decadal and multi-decadal variations in the spectral analysis (Fig. 7). Previously, periodicities around 10 years have been detected in the A. islandica sclerochronologies in parts of the eastern North Atlantic and these variations have been linked with the possible influence on climate of sunspots (Butler et al., 2010, 2013). Moreover, a periodicity of 43 years was found in the A. islandica sclerochronology from the North Icelandic shelf by Butler et al. (2013), who associated this variability with the Atlantic multidecadal oscillation (Gray et al., 2004). Nevertheless, we found no evidence of solar periodicities and thus, no implications for solar forcing on climate variability in our data as retrieved from this new sclerochronology. Similarly, the sclerochronology does not imply the influence of the Atlantic Multidecadal Oscillation in the studied palaeoenvironmental setting. In the present study, the biological trends of ageing are evident in the individual shell growth increment series (Fig. 4); they were modelled and further removed using negative exponential curves fitted separately to each series (Fig. 6(a)). As shown previously, this method may not provide an optimal way of preserving the growth variations of lowest frequencies (Strom et al., 2005; Helama et al., 2009a; Butler et al., 2010, 2013). Therefore, we further examined the dataset using another method, regional curve standardization (RCS; Briffa et al., 1992, 1996; Briffa and Melvin, 2011) to remove the biological growth variations from the series and to construct the chronology from the same raw data in order to investigate whether the absence of decadal and longer periodicities is a spurious result of the standardization procedure rather than actual lack of such variations in the data. In general, the RCS method is expected to demand a larger sample size in comparison to more conventional standardization methods that we have applied here by fitting a negative exponential curve individually to each serie (Fig. 6(a)), as already noticed in both the dendrochronological (Briffa et al., 1992, 1996; Helama et al., 2004) and sclerochronological (Helama et al., 2009a; Butler et al., 2010) literature. Regarding this sample-size limitation, the resulting RCS chronology ought to be considered with caution. Still, the RCS chronology reproduced the significant multiannual periodicities but did not indicate any periodicity on decadal to multidecadal scales (result not shown), similar to the presented chronology obtained from the multi-taper method (Fig. 7). The lack of such periodicities would thus stand out as an inherent characteristic of our A. islandica chronology. 5.3. Geochronological potential of sclerochronology Our A. islandica chronology was constructed using five large shells. We are under no illusion that this sample size is sufficient for conclusive palaeoclimatic implications to be drawn. Quite the contrary, we wish to emphasize the importance of sufficient

311

sample replication for building the incremental chronologies. That is to stress the importance of the law of large numbers as a premise for dendrochronological (Fritts and Swetnam, 1989) or sclerochronological (Helama et al., 2009a) analyses. Within our dataset, the chronology quality was inevitably at its highest level for the middle part of the chronology, where both the sample replication and the mean inter-series correlations reached their highest values, with a concomitant increase in the expressed population signal (EPS) value. Even over this range, the EPS statistic did not reach the critical value 0.85 that has been generally taken as a reasonable limit for chronology confidence (Wigley et al., 1984). Similar to our results, the annual shell growth increment chronologies of A. islandica from the North Sea did not obtain EPS values above 0.85 for chronologies with a sample size of eleven shells, while the corresponding EPS limit was reached in an incremental chronology constructed from fifteen A. islandica shells (Witbaard et al., 1997). The EPS statistic also depends on the mean inter-series correlation (RBAR). In this regard, our incremental sample set closely mimicked the RBAR values obtained for sclerochronological materials collected from coastal Norwegian waters. In comparison to the presented RBAR reaching a mean of 0.36 (Fig. 5(a)), the previous A. islandica chronologies have exhibited RBAR values of 0.33 (Helama et al., 2007) and 0.35 (Helama and Hoold, 2011). Interestingly, the theoretical estimated minimum number of sample series needed to obtain an EPS above 0.85 can be calculated for a certain RBAR (Wilson and Elling, 2004; Helama et al., 2009a). Obviously, at least 11 similarly correlating (RBAR 0.36) shells of A. islandica would be needed to fulfil the suggested EPS criterion. In previous studies, the sclerochronological datasets have often been characterized by relatively small sample sizes (Marchitto et al., 2000; Epple´ et al., 2006; Scourse et al., 2009). Even in this condition, they may provide invaluable sources of palaeoclimate information and can, if constructed as well as evaluated by their signals with the greatest care, help contribute significantly to an improved understanding of past climate systems (Helama et al., 2009a). While attaining larger sample sizes remains the only reasonable approach for enhancing the confidence of the summary (either mean or median) chronology, our incremental study has indeed demonstrated the geochronological potential of even relatively small assemblages of palaeontological shells to be used for constructing local shell growth increment chronologies via cross-dating. Similarly, this potential evokes the viewpoint that such death assemblages should not be neglected when considering the sclerochronological possibilities of the shell materials as excavated from sedimentological sections. This way, the shell collecting can be achieved at relatively low cost compared to seabed dredging during cruises by research vessels. Death assemblages of excavated shells may also result in sample sets with readily overlapping lifespans of bivalves, thus enabling incremental cross-dating, as demonstrated for man-made shell deposits following historical pearl-hunting (Helama et al., 2007a, 2009b; Helama and Nielsen, 2008), archaeological midden archives (Helama and Hood, 2011), and sedimentary sections (this study), thus, enabling incremental cross-dating. Radiocarbon dating to establish the contemporaneity of the shells to be incrementally cross-dated, as done in this study (Table 1), is vital for any cross-dating trials. Development of several such chronologies, similar to our chronology from prodelta deposits in north Norway may enable cross-dating between the local chronologies, thus, paving the way for enhanced and temporally elongated regional chronologies. In this regard, palaeontological shell materials may not represent the only possibility for constructing early- and mid-Holocene chronologies. Recently, an archaeological assemblage of A. islandica shells from a stone age midden deposit in northern Norway was

312

S. Helama et al. / Geobios 47 (2014) 305–313

successfully cross-dated to construct a 155-years radiocarbondated (ca. 3000 cal. yr. BC/BCE) ‘‘floating’’ shell growth incremental chronology (Helama and Hood, 2011). This chronology was built using seven shells containing between 35 and 74 annual increments. This archaeological chronology thus mimics our palaeontological chronology by its species, sample size, and length. Construction of these chronologies shows an encouraging opportunity to accomplish similar chronologies for periods and/or regions where dead shells from museum collections (Scho¨ne et al., 2003; Helama et al., 2007b) or dredged from the seabed (Butler et al., 2009, 2010, 2013) may not be obtainable. In addition to A. islandica shell growth increments, recent sclerochronological analyses have identified the sclerochronological potential of shells belonging to other marine bivalve species, such as heart cockle (Glossus humanus) and dog cockle (Glycymeris glycymeris) to build precisely dated annual shell growth incremental chronologies of centennial or even millennial length for the North Atlantic region (Brocas et al., 2013; Reynolds et al., 2013a, 2013b). These viewpoints highlight the utility of sedimentological fieldwork for obtaining larger shell collections of several bivalve species for sclerochronological research. Finally, the present results highlight the potential of crossdated sclerochronologies to depict and detail the past climate with high-resolution records. In the study region, extension of the existing chronology back in time could make it possible to portray the PBO in a sclerochronological context. Yet, construction of crossdated sclerochronologies for other intervals is needed to better understand the temporal and spatial climate variability within the North Atlantic realm for validation with model simulations and climate reconstructions. Acknowledgements Fig. 1 was drawn by Jan Petter Holm (University of Tromsø). Comments by three reviewers are acknowledged. We acknowledge the support (SH) from the Lapland Regional Fund of the Finnish Cultural Foundation, Waldemar von Frenckells stiftelse, and Maa- ja vesitekniikan tuki ry. The Julie von Mu¨llen’s Foundation (The Royal Danish Academy of Sciences and Letters) kindly gave JaKN a travel grant to visit the Department of Geology, University of Tromsø.

References Andrews, J.T., Hardadottir, J., Stoner, J.S., Mann, M.E., Kristjansdottir, G.B., Koc, N., 2003. Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12,000 cal yr: long-term North Atlantic oceanographic variability and solar forcing. Earth Planetary Sc Lett 210, 453–465. Berner, K.S., Koc¸, N., Godtliebsen, F., 2010. High frequency climate variability of the Norwegian Atlantic Current during the early Holocene period and a possible connection to the Gleissberg cycle. Holocene 20, 245–255. Bjo¨rck, S., Muscheler, R., Kromer, B., Andresen, C.S., Heinemeier, J., Johnsen, S.J., Conley, D., Koc¸, N., Spurk, M., Veski, S., 2001. High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger. Geology 29, 1107–1110. Bjo¨rck, S., Kromer, B., Johnsen, S., Bennike, O., Hammarlund, D., Lemdahl, G., Possnert, G., Rasmussen, T.L., Wohlfarth, B., Hammer, C.U., Spurk, M., 1996. Synchronized terrestrial-atmospheric deglacial records around the North Atlantic. Science 274, 1155–1160. ´ ., Funder, S., 1997. The Preboreal oscillation Bjo¨rck, S., Rundgren, M., Ingo´lfsson, O around the Nordic Seas: terrestrial and lacustrine responses. J Quaternary Sci 12, 455–465. Boch, R., Spo¨tl, C., Kramers, J., 2009. High-resolution isotope records of early Holocene rapid climate change from two coeval stalagmites of Katerloch Cave, Austria. Quaternary Sci Rev 28, 2527–2538. Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I., Bonani, G., 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294, 2130–2136. Bos, J.A.A., van Geel, B., van der Plicht, J., Bohncke, S.J.P., 2007. Preboreal climate oscillations in Europe: Wiggle-match dating and synthesis of Dutch highresolution multi-proxy records. Quaternary Sci Rev 26, 1927–1950.

Briffa, K.R., Jones, P.D., 1990. Basic chronology statistics and assessment. In: Cook, E., Kairiukstis, L.A. (Eds.), Methods of dendrochronology: applications in the environmental science. Kluwer Academic Publishers, Dordrecht, pp. 137–152. Briffa, K.R., Melvin, T.M., 2011. A closer look at regional curve standardization of tree-ring records: justification of the need, a warning of some pitfalls, and suggested improvements in its application. In: Hughes, M.K., Swetnam, T.W., Diaz, H.F. (Eds.), Dendroclimatology, progress and prospects. Springer, Dordrecht, pp. 113–145. Briffa, K.R., Jones, P.D., Bartholin, T.S., Eckstein, D., Schweingruber, H.F., Karle´n, W., Zetterberg, P., Eronen, M., 1992. Fennoscandian summers from AD 500: temperature changes on short and long timescales. Clim Dynam 7, 111–119. Briffa, K.R., Jones, P.D., Schweingruber, F.H., Karle´n, W., Shiyatov, S.G., 1996. Treering variables as proxy-climate indicators: problems with low-frequency signals. In: Jones, P.D., Bradley, R.S., Jouzel, J. (Eds.), Climate variations and forcings mechanisms of the last 2000 years. Springer-Verlag, Berlin, pp. 9–41. Brocas, W.M., Reynolds, D.J., Butler, P.G., Richardson, C.A., Scourse, J.D., Ridgway, I.D., Ramsay, K., 2013. The dog cockle, Glycymeris glycymeris (L.), a new annually-resolved sclerochronological archive for the Irish Sea. Palaeogeogr Palaeoclimatol Palaeoecol 373, 133–140. Bronk Ramsey, C., 1995. Radiocarbon calibration and analysis of stratigraphy: the OxCal program. Radiocarbon 37, 425–430. Bronk Ramsey, C., 2001. Development of the radiocarbon program OxCal. Radiocarbon 43, 355–363. Butler, P.G., Richardson, C.A., Scourse, J.D., Wanamaker Jr., A.D., Shammon, T.M., Bennell, J.D., 2010. Marine climate in the Irish Sea: analysis of a 489-year marine master chronology derived from growth increments in the shell of the clam Arctica islandica. Quaternary Sci Rev 29, 1614–1632. Butler, P.G., Scourse, J.D., Richardson, C.A., Wanamaker Jr., A.D., Bryant, C.L., Bennell, J.D., 2009. Continuous marine radiocarbon reservoir calibration and the 13C Suess effect in the Irish Sea: Results from the first multi-centennial shell-based marine master chronology. Earth Planetary Sc Lett 279, 230–241. Butler, P.G., Wanamaker Jr., A.D., Scourse, J.D., Richardson, C.A., Reynolds, D.J., 2013. Variability of marine climate on the North Icelandic Shelf in a 1357-year proxy archive based on growth increments in the bivalve Arctica islandica. Palaeogeogr Palaeoclimatol Palaeoecol 373, 141–151. Cook, E.R., Peters, K., 1981. The smoothing spline: a new approach to standardizing forest interior tree-ring width series for dendroclimatic studies. Tree-Ring Bull 41, 45–53. Cook, E., Briffa, K., Shiyatov, S., Mazepa, V., 1990a. Tree-ring Standardization and growth-trend estimation. In: Cook, E.R., Kairiukstis, L. (Eds.), Methods of dendrochronology: applications in the environmental sciences. Kluwer Academic Publishers, Dordrecht, pp. 104–123. Cook, E., Shiyatov, S., Mazepa, V., 1990b. Estimation of the mean chronology. In: Cook, E.R., Kairiukstis, L. (Eds.), Methods of dendrochronology: applications in the environmental sciences. Kluwer Academic Publishers, Dordrecht, pp. 123–132. Corner, G.D., 2006. A transgressive-regressive model of fjord-valley fill: stratigraphy, facies and depositional controls. In: Dalrymple, R.W., Leckie, D.A., Tillmann, R.W. (Eds.), Incised valleys in time and space, 85, SEPM Special Publication, pp. 161–178. Corner, G.D., Fjalstad, A., 1993. Spreite trace fossils (Teichichnus) in a raised Holocene fjord-delta, Breidvikeidet, Norway. Ichnos 2, 155–164. Corner, G.D., Nordahl, E., Munch-Ellingsen, K., Robertsen, K.R., 1990. Morphology and sedimentology of an emergent fjord-head Gilbert-delta, Alta delta, Norway. In: Collella, A., Prior, D.B. (Eds.), Coarse-grained deltas, 10, IAS Special Publication, pp. 155–168. Dunca, E., Mutvei, H., Go¨ransson, P., Mo¨rth, C.-M., Scho¨ne, B.R., Whitehouse, M.J., Elfman, M., Baden, S.P., 2009. Using ocean quahog (Arctica islandica) shells to ¨ resund, Kattegat and Skagerrak, Sweden. reconstruct palaeoenvironment in O Int J Earth Sci 98, 3–17. Eilertsen, R., Corner, G.D., Aasheim, O., Andreassen, K., Kristoffersen, Y., Ystborg, H., 2006. Valley-fill stratigraphy and evolution of the Ma˚lselv fjord valley, northern Norway. In: Dalrymple, R.W., Leckie, D.A., Tillman, R.W. (Eds.), Incised valleys in time and space, 85, SEPM Special Publication, pp. 179–195. Eilertsen, R.S., Corner, G.D., Aasheim, O., Hansen, L., 2011. Facies characteristics and architecture related to palaeodepth of Holocene fjord-delta sediments. Sedimentology 58, 1784–1809. Epple´, V.M., Brey, T., Witbaard, R., Kuhnert, H., Pa¨tzold, J., 2006. Sclerochronological records of Arctica islandica from the inner German Bight. Holocene 16, 763–769. Evison, K., 2012. Sedimentologiske, Paleoøkologiske og Diagenetiske Undersøkelser av Holocene Deltaavleiringer ved Breivikeidet, Troms. M. Sc. thesis, University of Tromsø. Fisher, T., 2002. Preboreal oscillation caused by a glacial Lake Agassiz flood. Quaternary Sci Rev 21, 873–878. Fritts, H.C., 1976. Tree rings and climate. Academic Press, London. Fritts, H.C., Mosimann, J.E., Bottorff, C.P., 1969. A revised computer program for standardizing tree-ring series. Tree-Ring Bull 29, 15–20. Fritts, H.C., Swetnam, T.W., 1989. Dendroecology - a tool for evaluating variations in past and present forest environments. Adv Ecol Res 19, 111–188. Ghil, M., Allen, M.R., Dettinger, M.D., Ide, K., Kondrashov, D., Mann, M.E., Robertson, A.W., Saunders, A., Tian, Y., Varadi, F., Yiou, P., 2002. Advanced spectral methods for climatic time series. Rev Geophys 40, (3–1–3–41). Gray, S.T., Graumlich, L.J., Betancourt, J.L., Pederson, G.T., 2004. A tree-ring based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D. Geophys Res Lett 31, L12205, http://dx.doi.org/10.1029/2004GL019932. Hanken, N.-M., Uchman, A., Jakobsen, S.L., 2012. Late Pleistocene-early Holocene polychaete borings in NE Spitsbergen and their palaeoecological and climatic implications: an example from the Basissletta area. Boreas 41, 42–55.

S. Helama et al. / Geobios 47 (2014) 305–313 Hansen, J., Hanken, N.-M., Nielsen, J.K., Nielsen, J.K., Thomsen, E., 2011. Late Pleistocene and Holocene distribution of Mytilus edulis in the Barents Sea region and its palaeoclimatic implications. J Biogeogr 38, 1197–1212. Haugen, J.E., Sejrup, H.P., 1990. Amino acid composition of aragonitic conchiolin in the shell of Arctica islandica. Lethaia 23, 133–141. Helama, S., 2011. Sclerochronology – mussels as bookkeepers of aquatic environment. In: McGevin, L.E. (Ed.), Mussels: anatomy, habitat and environmental impact. Nova Science Publishers, Hauppauge, pp. 395–412. Helama, S., Hood, B.C., 2011. Stone Age midden deposition assessed by bivalve sclerochronology and radiocarbon wiggle-matching of Arctica islandica shell increments. J Archaeol Sci 38, 452–460. Helama, S., Nielsen, J.K., 2008. Construction of statistically reliable sclerochronology using subfossil shells of river pearl mussel. J Paleolimnol 40, 247–261. Helama, S., Lindholm, M., Timonen, M., Eronen, M., 2004. Detection of climate signal in dendrochronological data analysis: a comparison of tree-ring standardization methods. Theor Appl Climatol 79, 239–254. Helama, S., Scho¨ne, B.R., Black, B.A., Dunca, E., 2006. Constructing long-term proxy series for aquatic environments with absolute dating control using a sclerochronological approach: introduction and advanced applications. Mar Freshw Res 57, 591–599. Helama, S., Nielsen, J.K., Valovirta, I., 2007a. Conchology of endangered freshwater pearl mussel: conservation palaeobiology applied to museum shells originating from northern Finland. B Malacol 43, 161–170. Helama, S., Scho¨ne, B.R., Kirchhefer, A.J., Nielsen, J.K., Rodland, D.L., Janssen, R., 2007b. Compound response of marine and terrestrial ecosystems to varying climate: pre-anthropogenic perspective from bivalve shell growth increments and tree-rings. Mar Environ Res 63, 185–199. Helama, S., Nielsen, J.K., Macias Fauria, M., Valovirta, I., 2009a. A fistful of shells: amplifying sclerochronological and palaeoclimate signals from molluscan death assemblages. Geol Mag 146, 917–930. Helama, S., Nielsen, J.K., Valovirta, I., 2009b. Evaluating contemporaneity and postmortem age of malacological remains using sclerochronology and dendrochronology. Archaeometry 51, 861–877. Hughen, K.A., Baillie, M.G.L., Bard, E., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Kromer, B., McCormac, G., Manning, S., Bronk Ramsey, C., Reimer, P.J., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., van der Plicht, J., Weyhenmeyer, C.E., 2004. Marine04 marine radiocarbon age calibration, 0–26 cal ky BP. Radiocarbon 46, 1059–1086. Hurrell, J.W., 1995. Decadal trends in the North Atlantic oscillation, regional temperatures and precipitation. Science 269, 676–679. Hurrell, J.W., Deser, C., 2010. North Atlantic climate variability: the role of the North Atlantic oscillation. J Mar Syst 79, 231–244. Jones, D.S., 1980. Annual cycle of shell growth increment formation in two continental shelf bivalves and its paleoecologic significance. Paleobiology 6, 331–340. Jones, D.S., 1983. Sclerochronology: reading the record of the molluscan shell. Am Sci 71, 384–391. Kobashi, T., Severinghaus, J.P., Barnola, J.-M., 2008. 4  1.5 8C abrupt warming 11,270 years ago identified from trapped air in Greenland ice. Earth Planet Sc Lett 268, 397–407. Lauritzen, S.-E., Lundberg, J., 1999. Calibration of the speleothem delta function: an absolute temperature record for the Holocene in northern Norway. Holocene 9, 659–669. Mangerud, J., Gulliksen, S., 1975. Apparent radiocarbon ages of recent marine shells from Norway, Spitsbergen, and Arctic Canada. Quaternary Res 5, 263–273. Mangerud, J., Andersen, S.T., Berglund, B.E., Donner, J.J., 1974. Quaternary stratigraphy of Norden, a proposal for terminology and classification. Boreas 3, 109–128. Marchitto, T.M., Jones, G.A., Goodfriend, G.A., Weidman, C.R., 2000. Precise temporal correlation of Holocene mollusk shells using sclerochronology. Quaternary Res 53, 236–246. Mosteller, F., Tukey, J.W., 1977. Data analysis and regression: a second course in statistics. Addison-Wesley, Reading. Mutvei, H., Dunca, E., Timm, H., Slepukhina, T., 1996. Structure and growth rates of bivalve shells as indicators of environmental changes and pollution. Bull Inst Oceanogr (Monaco) 14, 65–72. Mutvei, H., Westermark, T., Dunca, E., Carell, B., Forberg, S., Bignert, A., 1994. Methods for the study of environmental changes using the structural and chemical information in molluscan shells. Bull Inst Oceanogr (Monaco) 13, 163–186 (No. Spec). Nielsen, J.K., Hanken, N.-M., Nielsen, J.K., 2004. The relationships between early diagenetic calcite concretions and shell dissolution in subaerially exposed Holocene marine sediments, North Norway. 32nd International Geological Congress, Scientific Sessions, Abstracts 32 (1), 166.

313

Nielsen, J.K., Helama, S., Scho¨ ne, B., 2008. Shell growth history of geoduck clam (Panopea abrupta) in Parry Passage, British Columbia, Canada: temporal variation in annuli and the Pacific Decadal Oscillation. J Oceanogr 64, 951–960. Nielsen, J.K., Nielsen, J.K., 2009. Geoducks (Panopea abrupta) as isotopic bioarchives of seasonality in the climate of British Columbia. Ecol Res 24, 987–995. Rasmussen, S.O., Vinther, B.M., Clausen, H.B., Andersen, K.K., 2007. Early Holocene climate oscillations recorded in three Greenland ice cores. Quaternary Sci Rev 26, 1907–1914. Rasmussen, T.L., Thomsen, E., Nielsen, T., Wastega˚rd, S., 2011. Atlantic surface water inflow to the Nordic seas during the Pleistocene–Holocene transition (mid–late Younger Dryas and Pre-Boreal periods, 12 450-10 000 a BP). J Quaternary Sci 26, 723–733. Reynolds, D.J., Butler, P.G., Williams, S.M., Scourse, J.D., Richardson, C.A., Wanamaker Jr., A.D., Austin, W.E.N., Cage, A.G., Sayer, M.D.J., 2013a. A multiproxy reconstruction of Hebridean (NW Scotland) spring sea surface temperatures between AD 1805 and 2010. Palaeogeogr Palaeoclimatol Palaeoecol 386, 275–285. Reynolds, D.J., Richardson, C.A., Scourse, J.D., Butler, P.G., Wanamaker Jr., A.D., Ridgway, I., Sayer, M.D.J., Gulliver, P., 2013b. The potential of the marine bivalve mollusc Glossus humanus (L.) as a sclerochronological archive. Holocene 23, 1711–1720. Risebrobakken, B., Jansen, E., Andersson, C., Mjelde, E., Hevrøy, K., 2003. A highresolution study of Holocene paleoclimatic and paleoceanographic changes in the Nordic Seas. Paleoceanography 18, 1017, http://dx.doi.org/10.1029/ 2002PA000764. Ropes, J.W., Jones, D.S., Murawski, S.A., Serchuk, F.M., Jearld, A., 1984. Documentation of annual growth lines in ocean quahogs, Arctica islandica (Linne´). Fishery Bull 82, 1–19. Schmidt, M.W., Weinlein, W.A., Marcantonio, F., Lynch-Stieglitz, J., 2012. Solar forcing of Florida Straits surface salinity during the early Holocene. Paleoceanography 27, PA3204, http://dx.doi.org/10.1029/2012PA002284. Scho¨ne, B.R., Oschmann, W., Ro¨ssler, J., Freyre Castro, A.D., Houk, S.D., Kro¨ncke, I., Dreyer, W., Janssen, R., Rumohr, H., Dunca, E., 2003. North Atlantic oscillation dynamics recorded in shells of a long-lived bivalve mollusk. Geology 31, 1237–1240. Scho¨ne, B.R., Dunca, E., Fiebig, J., Pfeiffer, M., 2005. Mutvei’s solution: an ideal agent for resolving microgrowth structures of biogenic carbonates. Palaeogeogr Palaeoclimatol Palaeoecol 228, 149–166. Scourse, J., Richardson, C., Forsythe, G., Harris, I., Heinemeier, J., Fraser, N., Briffa, K., Jones, P., 2009. First cross-matched floating chronology from the marine fossil record: data from growth lines of the long-lived bivalve mollusc Arctica islandica. Holocene 16, 967–974. Spurk, M., Friedrich, M., Hofmann, J., Remmele, S., Frenzel, B., Leuschner, H.H., Kromer, B., 1998. Revisions and extension of the Hohenheim oak and pine chronologies: new evidence about the timing of the younger Dryas/Preboreal transition. Radiocarbon 40, 1107–1116. Strom, A., Francis, R.C., Mantua, N.J., Miles, E.L., Peterson, D.L., 2004. North Pacific climate recorded in growth rings of geoduck clams – a new tool for paleoenvironmental reconstruction. Geophys Res Lett 31, L06206, http://dx.doi.org/ 10.1029/2004GL019440. Strom, A., Francis, R.C., Mantua, N.J., Miles, E.L., Peterson, D.L., 2005. Preserving lowfrequency climate signals in growth records of geoduck clams (Panopea abrupta). Palaeogeogr Palaeoclimatol Palaeoecol 229, 167–178. Thompson, I., Jones, D.S., Dreibelbis, D., 1980. Annual internal growth banding and life history of the ocean quahog Arctica islandica (Mollusca: Bivalvia). Mar Biol 57, 25–34. Vorren, T.O., Plassen, L., 2002. Deglaciation and palaeoclimate of the AndfjordVa˚gsfjord area, North Norway. Boreas 31, 97–125. Wanamaker Jr., A.D., Kreutz, K.J., Scho¨ne, B.R., Maasch, K.A., Pershing, A.J., Borns, H.W., Introne, D.S., Feindel, S., 2009. A late Holocene paleo-productivity record in the western Gulf of Maine, USA, inferred from growth histories of the longlived ocean quahog (Arctica islandica). Int J Earth Sci 98, 19–29. Wigley, T.M.L., Briffa, K.R., Jones, P.D., 1984. On the average value of correlated time series, with applications in dendroclimatology and hydrometeorology. J Clim Appl Meteorol 23, 201–213. Wilson, R., Elling, W., 2004. Temporal instability in tree-growth/climate response in the Lower Bavarian Forest region: implications for dendroclimatic reconstruction. Trees 18, 19–28. Witbaard, R., Duineveld, G.C.A., de Wilde, P.A.W.J., 1997. A long-term growth record derived from Arctica islandica (Mollusca, Bivalvia) from the Fladen Ground (Northern North Sea). J Mar Biol Assoc United Kingdom 77, 801–816.