JOURNAL OF QUATERNARY SCIENCE (2005) 20(6) 511–518Copyright � 2005 John Wiley & Sons, Ltd.Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.927

Testing solar forcing of pervasive Holoceneclimate cycles

CHRIS TURNEY,1,2* MIKE BAILLIE,2 STEVE CLEMENS,3 DAVID BROWN,2 JONATHAN PALMER,2 JONATHAN PILCHER,2

PAULA REIMER2,4 and HANNS HUBERT LEUSCHNER5

1 GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522,Australia2 Palaeoecology Centre, School of Archaeology and Palaeoecology, Queen’s University, Belfast BT7 1NN, UK3 Geological Sciences, Brown University, Providence, RI 02912-1846, USA4 Centre for Accelerator Mass Spectrometry L-397, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA5 Laboratory for Dendrochronology and Dendroclimatology, Subdepartment of Palynology and Quaternary Sciences, University ofGottingen, D-37075 Gottingen, Germany

Turney, C., Baillie, M., Clemens, S., Brown, D., Palmer, J., Pilcher, J., Reimer, P. and Leuschner, H. H. 2005. Testing solar forcing of pervasive Holocene climate cycles.J. Quaternary Sci., Vol. 20 pp. 511–518. ISSN 0267-8179.

Received 28 September 2004; Revised 16 February 2005; Accepted 2 March 2005

ABSTRACT: The temporal and spatial extent of Holocene climate change is an area of considerableuncertainty, with solar forcing recently proposed to be the origin of cycles identified in the NorthAtlantic region. To address these issues we have developed an annually resolved record of changesin Irish bog tree populations over the last 7468 years which, together with radiocarbon-dated bog andlake-edge populations, extend the dataset back to �9000 yr ago. The Irish trees underpin the inter-nationally accepted radiocarbon calibration curve, used to derive a proxy of solar activity, and allowus to test solar forcing of Holocene climate change. Tree populations and age structures provideunambiguous evidence of major shifts in Holocene surface moisture, with a dominant cyclicity of800 yr, similar to marine cycles in the North Atlantic, indicating significant changes in the latitudeand intensity of zonal atmospheric circulation across the region. The cycles, however, are not coher-ent with changes in solar activity (both being on the same absolute timescale), indicating thatHolocene North Atlantic climate variability at the millennial and centennial scale is not driven bya linear response to changes in solar activity. Copyright � 2005 John Wiley & Sons, Ltd.

KEYWORDS: Irish bog oak and pine; radiocarbon production rate; North Atlantic thermohaline circulation; millennial-scale climate change; Lough

Neagh.

Introduction

The present interglacial was until recently considered to be aperiod of exceptional climatic stability (Dansgaard et al.,1993). Although it was first suggested by Denton and Karlen(1973) that the Holocene was not as climatically stable asimplied by terrestrial and marine records it was not until the1990s that this view was seriously challenged. Changes inthe concentration of lithic grains through Holocene NorthAtlantic sediments have been suggested to represent pervasive900- and 500-yr cycles (loosely bundled together as a quasi-periodic ‘1500-yr’ cycle), and appear to record the southwardadvection of cold, ice-bearing waters from the Labrador andNordic Seas (Bond et al., 1993, 2001) (Fig. 1). Similar quasi-cycles have now been recorded throughout the northern hemi-

sphere (O’Brien et al., 1995; Bianchi and McCave, 1999; Huet al., 2003; Langdon et al., 2003; Magny et al., 2003), consis-tent with the pacing of Dansgaard–Oeschger oscillationswithin oxygen isotope stages 2 and 3 (Bond et al., 1993). Thesequasi-cycles have been interpreted as representing a climatepacemaker, independent of glacial–interglacial cycles andanthropogenic forcing, that appear to be driven by relativelysmall changes in solar activity (Bond et al., 2001), the lattermeasured by cosmogenic isotopes recorded either in theannually resolved tree rings (14C) (Bond et al., 2001) or theGreenland ice core (10Be) (Finkel and Nishiizumi, 1997). Pos-sible explanations to account for the relatively large responseto small changes in solar activity include amplification by otherfactors (such as stratospheric ozone concentration; Shindellet al., 1999), though recently some workers have tended tofavour cycles different from those implied for solar activity(Schulz and Paul, 2002).

Several attempts have been made to test the hypothesis ofsolar forcing of Holocene climate change (van Geel et al.,1996; Karlen and Kuylenstierna, 1996; Bond et al., 2001;Andrews et al., 2003; Hu et al., 2003; Magny et al., 2003; Yu

* Correspondence to: Chris Turney, GeoQuEST Research Centre, School of Earthand Environmental Sciences, University of Wollongong, Wollongong, NSW2522, Australia. E-mail: turney@uow.edu.au

et al., 2003), yet virtually all of these studies have used radio-carbon to derive a chronology for correlating palaeoclimaticdatasets to proxies of past solar activity. A major uncertaintyhas been the extent to which high-precision correlation canbe effected between palaeoclimatic and solar datasets owingto inherent uncertainties in derived calendar ages as a resultof calibrating radiocarbon ages (Stuiver et al., 1998). Ideally,both dataset types should be derived from the same sequence,since this keeps potential stratigraphic uncertainties to a mini-mum while leaving only time-lags inherent to the climate sys-tem. The number of such suitable sequences is extremelylimited.

Ireland’s maritime climate is dominated by prevailing wes-terly airflow and is effectively free of any influence of majorice masses and continents, making it extremely sensitive to pastfluctuations in the strength of thermohaline circulation and topast freshwater pulses into the North Atlantic (McDermott et al.,2001). We have developed Irish tree-ring chronologies usingbog-grown oaks (Quercus spp.) and pines (Pinus sylvestris L.)(Fig. 1), the former contributing to the internationally acceptedradiocarbon calibration curve (Pearson et al., 1986; Stuiveret al., 1998). The marginal environments of these species grow-ing on bogs coupled with the effectively random nature of thesampling (in both space and time) allows us to test robustlywhether tree populations in Ireland changed in response tovarying hydrological conditions associated with North Atlanticmillennial-scale cycles, while the 14C dataset can be used toderive a measure of past solar activity.

Methods

A continuous bog oak chronology of >750 trees exists back to7468 yr (Pilcher et al., 1984), obtained from numerous bog andlake sites across northern Ireland (Fig. 1). The synchronisationof sequences was achieved by visual and statistical correlationof tree ring-width patterns (Pilcher et al., 1984; Brown et al.,1986) over a period of 30 yr. The method is extremely robustand error-free, and as a result has been used to provide pre-cisely dated bidecadal wood samples for the internationalradiocarbon calibration curve (Pearson et al., 1986; Stuiver

et al., 1998). Included in the oak chronology are trees collectedfrom lake margin locations below present-day water levels. Inaddition, there are Scots pine chronologies from bogs and lake-side sites, dated by dendrochronology (Pilcher et al., 1995), orcalibrated radiocarbon determinations against the INTCAL98dataset (Stuiver et al., 1998). All ages reported are relative toAD 1950 (henceforth referred to as ‘years ago’).

An ocean–atmosphere box diffusion model (Oeschger et al.,1975) was used to calculate 14C production rate from theINTCAL98 atmospheric 14C dataset, the same as that reportedin earlier studies (Bond et al., 2001; Hu et al., 2003). As imple-mented by Stuiver and Braziunas (1993), the model consists ofone box for the atmosphere, one for the ocean mixed layer, 37boxes for the thermocline, five boxes for the deep ocean andtwo for the biosphere (short and long residence time)(Table 1). The climate-influenced mixing parameters (air-gassea exchange, eddy diffusivity, and biospheric uptake andrelease) were held constant through the run. The model wasparameterised to produce a pre-industrial marine mixed layer�14C of �46.5 % and a deep ocean value of �190% at AD

1830 for the 1998 marine calibration dataset, Marine98(Stuiver et al., 1998). This resulted in a reservoir age of 39014C yr for AD 1830 and an average reservoir age of 360 forthe period 7050 BC to AD 1850 when compared to the atmo-spheric values from INTCAL98 (Table 1).

0 100 200

N

MC21GGC22

MC52VM29-191

12

3

4

Lough Neagh

GISP2

km

Temple Hill

Moss

Figure 1 Location of major IRD and Holocene climate cycle records in North Atlantic and principal sites used to develop oak and tree populationrecords in Northern Ireland. Numbered sites refer to the following: (1) Garry Bog, (2) Ballymacombs More, (3) Sluggan Bog, and (4) south Lough Neaghfenlands

Table 1 Parameters used in the ocean–atmosphere box diffusionmodel (Oeschger et al., 1975) to determine decadal 14C productionrates

Parameter Marine98

Air-gas sea exchange 19 mol m�2 yr�1

Eddy diffusivity 4000 m2 yr�1

Pre-industrial atmospheric 280 ppm (Stuiver et al., 1984; Neftel[CO2] et al., 1985)Biosphere residence time and 2.7 yr; 0.34 atmreservoir size (fraction of 80 yr; 2.86 atmthe atmosphere) (Emanuel et al., 1984)Ocean CO2 concentration 2.31 mol m�3 (Takahashi et al., 1981)Initial atmospheric �14C 90%

512 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

Results and discussion

Figures 2(A) and (B) show plots of the population frequency ofthe Irish tree cohorts. The oak dataset commences 7468 yearsago and records a cyclic pattern throughout its length that weinterpret to represent changes in moisture delivery to Ireland.Under ‘dry’ conditions, water table levels decrease, allowingthe surface to be colonised by tree populations (Pilcher et al.,1996). The peaks in bog oak populations are complemented bythe presence of stands of dendrochronologically dated oakidentified below current lake levels in northern Ireland, provid-ing unambiguous evidence for drier conditions. The bog oaksand pines used to construct the above chronologies were bytheir nature dependent on post-glacial climate ameliorationand the development of marginal vegetation and were hencenot prevalent in the early Holocene landscape (Pilcher et al.,1996). Earlier cycles in the Holocene are represented by exten-sive finds of bog pine and also a radiocarbon-dated cohort ofwillow trees (Salix spp.) below the current lake level in LoughNeagh (Smith et al., 1974), consistent with peaks in tree popu-lations during dry phases (Figs 2(A) and (B)). We undertookmean age analyses (Leuschner et al., 2002) to identify whetherthe cycles had an effect on the population dynamics of the bogoaks (Fig. 2(C)) and find that troughs in tree populations coin-cide in the first instance with peaks in the mean age of thepopulations, indicating recruitment failure. Saplings couldnot establish under pervasive ‘wet’ conditions, as a result skew-ing the mean age to older members of the population, whichsubsequently died, resulting in a significant drop in the meanage (Leuschner et al., 2002). Following climatic amelioration(interpreted here to be ‘drying’), the trees regenerate, returning

to intermediate values. Peaks in Irish bog and lake-edge treepopulations are centred on 800 years ago (correlative to theMedieval Warm Period), 1700 years ago (correlative to theRoman Optimum), 2650, 3200, 4100, 5000, 5600, 6200,7300, 8000 and 9000 years ago, indicating that moisture deliv-ery and/or water tables were significantly below present levelsat these times. Thus, we can recognise that oak and pine popu-lations obtained from bog and lake-edge contexts indicatealternating periods of sustained dry and wet conditions inIreland.

The above moisture cycles are consistent with those trendsobserved in ice, terrestrial and marine records across the NorthAtlantic region. In the glaciochemical series reported for theGISP2 ice core (Figs. 1 and 3), concentrations of sea salt varythrough the Holocene (O’Brien et al., 1995), with shifts invalues suggesting cyclic changes in the latitudinal extent ofthe polar vortex, changing the atmospheric loading of aerosolsover the ice. The most significant sustained periods ofincreased sodium (Na) concentration in the ice identified byO’Brien et al. (presented in Fig. 3) are associated withdecreases in bog tree populations (for instance 0–610 and5000–6100 years ago), although the most recent period isalmost certainly exaggerated by human activity in the Irishlandscape at this time (Pilcher et al., 1996). Changes in bog treepopulations and surface wetness have also been observedacross the British Isles, Europe and North America with com-parable trends to those reported here (Bridge et al., 1990; Bar-ber et al., 1994; Lageard et al., 1999; Langdon et al., 2003;Magny et al., 2003; Yu et al., 2003), though none of the pub-lished records have the chronological precision afforded by theIrish bog oaks. For instance, water table levels reconstructedfrom testate amoebae record changes in the depth of ‘surface’

No.

of d

endr

o-da

ted

Iris

h bo

g oa

ks

(cle

ar)

and

lake

oak

s (s

olid

) B0

40

60

20

No.

of I

rish

bog

pine

s (d

endr

o-da

ted,

cle

ar;

14C

-dat

ed, s

olid

) an

d 14

C-d

ated

lake

will

ow (

*)A

Ireland

Dry

Wet

0 2 4 6 8 10Calendar years, B.P.

40

60

20

*

0

C

0

300

400

200

Mea

n oa

k ag

e,ye

ars

100

Figure 2 Comparison between the number of Irish (A) Scots pine (*equals willow), (B) oak tree populations and (C) mean bog oak ages (Leuschneret al., 2002). Grey zonation denotes inferred dry phases in Ireland

TESTING SOLAR FORCING OF HOLOCENE CLIMATE CYCLES 513

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

moisture of up to 16 cm during the last 7500 years in TempleHill Moss, southeast Scotland (Langdon et al., 2003) (Figs 1and 3) and indicate that peaks in bog oak and pine populationsacross northern Ireland coincide (within dating uncertainties)with low water levels while near-surface water table depthsare associated with troughs (Fig. 3), supporting our contentionthat tree numbers are representative of changing surfacemoisture conditions.

Similar trends to the Irish dataset are recorded in the marineenvironment (Bianchi and McCave, 1999; Bond et al., 2001;Andrews et al., 2003; Sarnthein et al., 2003; Solignac et al.,2004) apparently embedded in long-term climate trends overthe Holocene (Andrews et al., 2003; Sarnthein et al., 2003).We have compared our record to the hematite-stained grains(a proxy for ice-rafted Debris; IRD) obtained from MC-52 andV29-191 (Bond et al., 2001) (Figs 1 and 3), the best-dated proxyrecord of drift ice in the North Atlantic and one of the originalkey sequences reported to contain millennial-scale cyclesthrough the Holocene. Despite the inherent uncertainties inthe chronology of the marine dataset (radiocarbon ages withcalibrated age ranges of >300 years and an uncertainty of�200 years in marine reservoir age) (Bond et al., 2001; Yuet al., 2003), the match between the IRD and tree dataset ismarkedly consistent, with collapses in tree populations coin-ciding with increases in hematite-stained grains, suggesting asouthward migration of polar waters and the associated move-ment of prevailing westerly airflow over Ireland from a more

northerly location. Although the overall trends appear to becoherent, discrepancies do exist when the two records arecompared in detail. For instance, the correlation appears poorbetween 2000 and 2500 years ago, where MC-52 and V29-191indicates relatively warm conditions throughout (low IRD),while onshore the Irish record suggests significant changes insurface moisture, and consistent with changes in deep waterflow in the North Atlantic (Bianchi and McCave, 1999) andsea-surface temperature in the Barents Sea (Sarnthein et al.,2003). Furthermore, the precise relationship to the 37-yr long‘8200 yr’ event (McDermott et al., 2001; Baldini et al., 2002)is unclear. Unfortunately, no Irish dendro-dated trees spanthe period of interest, so we cannot provide an absolutecalendar age for the event (Barber et al., 1999). Stratigraphi-cally, the location of the event is recorded in the GISP2 icecore as a shift to more negative �18O values (data not shown)but this does not obviously relate to either a long-term shift inIRD (Bond et al., 2001) or expansion of the polar vortex(O’Brien et al., 1995), suggesting it was independent of theNorth Atlantic cycles identified here (Hu et al., 1999). Overall,however, it appears that changes in bog and lakeside treepopulations (representative of varying surface moisture condi-tions in Ireland) were associated with variations in the NorthAtlantic.

To test for cyclicity in the trends in the Irish tree dataset, weundertook spectral analysis using the Blackman–Tukeyapproach (Jenkins and Watts, 1968) on decadal means of the

20100

North Atlantic hematite-stained grains, %

(MC52/VM29-191)

No. of Irish dendro-dated bog oaks (clear),

14C-dated bog pines (solid) and 14C-dated willow (*)

4060 20

*

00

1

2

3

4

5

6

7

8

9

10

Cal

. yea

rs, B

.P.

Warm Cold

Testate amoebae reconstructed water table depth, cm(Temple Hill Moss,

southeast Scotland)

-20 -15 -10 -5 0

Dry Wet

14C production(atoms/cm2/sec)

0.5 1.0 1.5 2.0 2.5 10 15 20 25

GISP2 10Be (103 atom/g)

4 6 82

GISP2 Na, ppb

10

1

2

34

5

6

7

8

9

10

Dry Wet

1

2

34

5

6

7

8

9

10

Reducedpolar vortex

Expandedpolar vortex

8.2 ka8.2 ka8.2 ka8.2 ka

Figure 3 Comparison of the summary of Irish bog oaks, pines and willow populations to the 14C production rate (the latter partially derived fromradiocarbon measurements obtained on the Irish trees) (Pearson et al., 1986; Stuiver et al., 1998), with the quantified reconstruction of water tablelevels in southeast Scotland (Langdon et al., 2003) and the North Atlantic hematite-stained grains record obtained from MC52/VM29–191 (Bond et al.,2001) plotted with their 95% calibrated age confidence limits. In addition, the 10Be record from GISP2 is shown (Finkel and Nishiizumi, 1997). Thedual numbering of peaks in the 14C and 10Be curves allows time-parallel correlation between the absolutely dated Irish tree-ring chronology andGISP2 ice core, with a mean difference of 58 years (see Table 2). Sodium (Na) content of the GISP2 ice core is given as a proxy of sea salt aerosolloading of the atmosphere over Greenland (O’Brien et al., 1995) as dark grey banding (decadal values over sampling resolution) and black (meancentennial values). The stratigraphic location of the 8200-yr event in GISP2 is also given (denoted as 8.2 ka)

514 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

Irish bog oak populations. There is a statistically significantpeak in the bog oak populations equivalent to 800 yr, withsmaller peaks at 478 and 1538 yr (Fig. 4(A)).

Recently, it has been argued that some Holocene cycles are aresult of changes in solar activity (Bond et al., 2001; Hu et al.,2003). As a measure of the latter, we looked at the decadal pro-duction rate of 14C, a cosmogenic radionuclide, which isrelated to solar activity. Reduced solar activity results in higherproduction rates. As a result of the above, high-precision com-parisons are possible as the absolutely dated Irish oak chronol-ogies reported here underpin the radiocarbon calibration curvefor this time period, providing a unique opportunity to test pre-cisely the hypothesis of solar forcing. Although the GISP2 ice

core provides an opportunity to test the hypothesis of solar for-cing of climate, the record of the cosmogenic isotope 10Be isonly reported up to 3300 years ago with a significant hiatusover the period 8000 to 9400 years ago (Finkel and Nishiizumi,1997), precluding precise comparison of the dataset to proxiesof palaeoclimatic change. Despite the relatively coarse sam-pling resolution of the GISP2 record, clear correlations canbe made between the 14C and 10Be datasets (Fig. 3 andTable 2). In virtually all instances, the GISP2 core is older thanthe absolutely dated tree-ring chronology by up to 120 ice-coreyears, with a mean of 58 years (probably as a result of errors inthe construction of the ice core timescale) (Southon et al.,2002). Nevertheless, because the pattern of peaks in 14C

800 years1538

years

80% level coherency CI

Non-zero coherency (80%)

14C production

Oak Trees

Coherency

FreqPeriod

Bandwidth for 450 lags on 705 values at ∆t = 10.00

.000.1428

.2856

.4284

.5711

.7139

.8567

.9995

.000 .0005 .0010 .0015 .0020 .0025 .0030 .0035 .0040 .0045 .00502004. 1001. 667.1 500.3 400.2 333.4 285.8 250.1 222.3 200.0

+ + + + + +

+

+

+

+

+++

++

+

++

++

+

++

+

++

++++

+

++++

+

+

+

++

+

++

+

+

+ +

+

+

+

+

++++

++

+

++

+

+++

++

+

+

++

+

+

+

+

+

+

+

+

+

++

+

+

++

+

+

+

+

+

+

++

+

478 years

357 years

500 years

714-952 years

A

B

14C production, first derivative

Oak Trees

Coherency+ + + + + +

Bandwidth for 450 lags on 705 values at ∆t = 10.00 80% level coherency CI

Non-zero coherency (80%)

FreqPeriod

.000 .0005 .0010 .0015 .0020 .0025 .0030 .0035 .0040 .0045 .00502004. 1001. 667.1 500.3 400.2 333.4 285.8 250.1 222.3 200.0

.000.1428

.2856

.4284

.5711

.7139

.8567

.9995

Figure 4 Cross-spectral of the Irish tree and 14C production for the last 7468 yr. Spectra of Irish bog oak numbers, 14C production rate (A) and the firstderivative of 14C production (B) using the Blackman–Tukey approach (Jenkins and Watts, 1968). Spectral and cross-spectral analysis of the datasetswas undertaken using the ARAND software package (http://www.ngdc.noaa.gov/paleo/softlib/). Spectral density (�2/f, where �2¼ variance andf¼ frequency¼1 per period) is normalised and plotted on a logarithm scale. The periods (years) of spectral peaks (significant at the 95% confidencelevel) are labelled. Also shown is the degree of coherence between the two datasets; a measure of similarity in amplitude modulation and phase

TESTING SOLAR FORCING OF HOLOCENE CLIMATE CYCLES 515

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

production mirror closely the 10Be values, we can utilise thecosmogenic isotopes as time-parallel marker horizons betweenIreland and Greenland (Fig. 3). The correlation between peaksprovides greater confidence in the correlation between Green-land sea salt variations with Irish tree populations and that theobserved cycles form part of a regional signal (particularly clearduring sustained high Na concentration values). A more com-prehensive test of solar forcing, however, can only be underta-ken using the Irish trees. Visual comparison of the two datasets(Fig. 3), however, suggests little if any obvious correlationbetween solar activity and Irish tree populations.

To test the above observation, we spectrally analysed 14Cproduction over the same interval as that represented by theIrish oaks, and found spectral peaks centred on 357 yr and500 yr, as well as a broad peak spanning the period 714–952 yr (Fig. 4(A)). Intriguingly, a 14C cyclicity centred on900–1100 yr has been reported for the interval spanning the

last 11 400 yr using a similar approach (Bond et al., 2001) butis identified here with a slightly shorter periodicity, suggestingthat this cycle is not stable through time. Both oak and 14C pro-duction have spectral variance near the 500- and 800-yr peri-ods although coherence between the two records at theseperiods is low (Fig. 4(A)), reflecting differences in the amplitudemodulation and/or phase.

Based on studies of the periods 400, 2800 and 11 400 yearsago, it has been suggested that climatic shifts accompaniedchanging 14C production, rather than maxima and minimavalues of the latter (van Geel et al., 1996, 1999; van der Plichtet al., 2004). To test this hypothesis, we spectrally analysed thefirst derivative of 14C (representing changes in the radiocarbonproduction) (Fig. 4(B)). This analysis had no influence on thelocation of the spectral peaks for the 14C values (cf. Fig. 4(A))and the coherence with the Irish oaks remains poor (Fig. 4(B)),indicating no significant relationship.

To evaluate the spectra further and judge the significance ofspectral peaks identified by the Blackman–Tukey method, weundertook the higher-resolution Multitaper Method (MTM;Thomson, 1982) on the 14C production and Irish oak popula-tion data. The MTM plots of oak and 14C (Fig. 5) support thecross-spectral plots in Fig. 4(A) as they share similar variancein the 475–500-yr period range. The coherence is not high(barely 80% CI) in the cross-spectral plot and this is reflectedin the MTM plots as differences in the peak centres (oak and14C are centred on 476 and 503 yr respectively). Furthermore,in the 700–950-yr range, the cross-spectrum in Fig. 4(A) showsno coherence which is reflected in the MTM plots as one largeoak peak at 784 yr (Fig. 5(A)) versus two small 14C peaks at 714and 860 yr (Fig. 5(B)). As a result, the high-resolution MTMspectra supports the lack of coherence between the two recordsshown in the cross-spectrum.

To test for periodicity, a statistical F-test was performed onthe MTM amplitude. Four amplitudes in the oak data (1670,

Table 2 Irish tree and GISP2 ages for cosmogenic isotopic peaks iden-tified in Fig. 3

Cosmogenic isotopic Tree-ring GISP2 age Differencepeak (14C and 10Be) age range (BP) range (BP)

1 3320–3330 3399–3389 79–482 4410–4420 4452–4477 67–323 5280–5290 5291–5330 50–14 5440–5450 5495–5523 83–455 5580–5590 5620–5653 73–306 6270–6280 6345–6390 120–657 7140–7150 7175–7220 80–258 7560–7570 7622–7649 89–529 9400–9410 9445–9477 77–3510 9880–9890 9920–9959 79–30

Mean 58

A

B

Period, years

Am

plit

ud

eA

mp

litu

de

F-testF-test

1670

Period, years

1203

784

476

244

214

503

358291 228

209

0

0.005

0.01

0.015

0.02

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Amplitude

Amplitude

F-test

F-test

1

0.98

0.96

0.94

0.92

0.90

0.88

0.86

1

0.98

0.96

0.94

0.92

0.90

0.88

0.86

1000 900 800 700 600 500 400 300 200

1000 900 800 700 600 500 400 300 200

714860

Figure 5 Spectrum of the (A) Irish tree and (B) 14C production rate using the high-resolution Multitaper Method (MTM; Thompson, 1982). The left axisis the estimated MTM amplitude and the right axis is the statistical F-test

516 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

784, 476 and 214 yr) were identified with >95% significance(Fig. 5(A)) and suggest periodicity through the dataset at thesebands. In contrast, many of the peaks identified in the 14Crecord appear to be aperiodic, with only amplitudes centredon 358 and 291 yr exceeding the 95% significance level(Fig. 5(B)). This result is confirmed by wavelet analyses of bothtime-series (Figs. 6(A) and (B)). The Irish oak record shows thestrongly periodic signal of the 1538–1670-yr cyclicity to beactive largely prior to 4000 years ago with a reduced periodi-city of this cycle afterwards (Schulz et al., 1999). In contrast,the 784–800-yr cycle has maximum amplitude after 5000 yearsago. This pattern is clearly not evident in the 14C productionrecord, with none of the above cycles recording statisticallysignificant F-test values (Fig. 5(B)) or visible in the wavelet ana-lysis (Fig. 6(B)), i.e. they are aperiodic. In particular, the 800-and 500-yr variance in the 14C and oak records do not modu-late similarly in amplitude, accounting for the lack of coher-ence at these periods in the cross-spectrum, supporting thecontention that there is no linear relationship between palaeo-climate and solar activity.

The timing of the shift in periodicity in the oak record duringthe mid-Holocene as demonstrated by the wavelet analysis(Fig. 6(A)) is intriguing. This period is known to fall on the tran-sition between maximum and minimum solar insolation values(decreasing at 65� N and increasing at 65� S; Berger, 1978) sug-gesting an orbital modulation of the mechanisms driving mil-lennial- and centennial-scale climatic change through theHolocene. Mechanisms that might be modulated by orbitalchanges include changes in the North Atlantic meridional cir-culation driven by changes in surface freshwater input or inter-nal dynamics (Bianchi and McCave, 1999; Schulz and Paul,2002), manifestation of a bipolar seesaw (Broecker et al.,1999) and/or teleconnection with the tropical Pacific Oceanthrough long-term changes in the El Nino–Southern Oscillation(Russell et al., 2003; Turney et al., 2004), though at present

none of these are widely accepted by the scientific community.Further work must now be undertaken to explore whether theseare responsible for driving climatic change at the millennial tocentennial scale observed in the North Atlantic region.

Conclusions

We conclude that North Atlantic climate change at the millen-nial to centennial scale is not driven by a linear response tochanges in solar activity. If solar activity is the primary forcingfor Holocene climate change, it enters the system elsewhereand is propagated internally to the North Atlantic via one ormore nonlinear mechanisms. Alternative potential mechan-isms must be investigated more fully before we can fullyexplain Holocene climate change and the potential influenceof these cycles on future climate change.

Acknowledgements We thank John O’Neill for reading an earlierdraft of the manuscript. Our thanks to G. Bond and P. Langdon forthe provision of the data for MC52/VM29-191 and Temple Hill Mossrespectively, and to Bas van Geel and an anonymous referee for helpfulcomments during the review process. This research was part-funded bythe Natural Environment Research Council and the US Department ofEnergy (the latter by the University of California, Lawrence LivermoreNational Laboratory under Contract No. W-7405-Eng-48).

References

Andrews JT, Hardardottir J, Kristjansdottir GB, Gronvold K, Stoner JS.2003. A high-resolution Holocene sediment record from Huna-floaall, N. Iceland margin: century- to millennial-scale variabilitysince the Vedde tephra. The Holocene 13: 625–638.

Baldini JUL, McDermott F, Fairchild IJ. 2002. Structure of the 8200-year cold event revealed by a speleothem trace element record.Science 296: 2203–2206.

Barber DC, Dyke A, Hillaire-Marcel C, Jennings AE, Andrews JT,Kerwin MW, Bilodeau G, McNeely R, Southon J, Morehead MD,Gagnon J-M. 1999. Forcing of the cold event of 8,200 years ago bycatastrophic drainage of Laurentide lakes. Nature 400: 344–348.

Barber KE, Chambers FM, Maddy D, Stoneman R, Brew JS. 1994. A sen-sitive high-resolution record of late Holocene climatic change from araised bog in northern England. The Holocene 4: 198–205.

Berger A. 1978. Long-term variations of daily insolation and Quatern-ary climatic change. Journal of Atmospheric Sciences 35: 2362–2367.

Bianchi GG, McCave IN. 1999. Holocene periodicity in North Atlanticclimate and deep-ocean flow south of Iceland. Nature 397: 515–517.

Bond G, Broecker W, Johnsen S, McManus J, Labeyrie L, Jouzel J,Bonani G. 1993. Correlations between climate records from northAtlantic sediments and Greenland ice. Nature 365: 143–147.

Bond G, Kromer B, Beer J, Muscheler R, Evans MN, Showers W,Hoffman S, Lotti-Bond R, Hajdas I, Bonani G. 2001. Persistent solarinfluence on North Atlantic climate during the Holocene. Science294: 2130–2136.

Bridge MC, Haggart BA, Lowe JJ. 1990. The history and palaeoclimaticsignificance of subfossil remains of Pinus sylvestris in blanket peatsfrom Scotland. Journal of Ecology 78: 77–99.

Broecker WS, Clark E, McCorkle DC, Hajdas I, Bonani G. 1999. Coretop 14C ages as a function of latitude and water depth on the Ontong-Java plateau. Paleoceanography 14: 13–22.

Brown DM, Munro MAR, Baillie MGL, Pilcher JR. 1986. Dendrochro-nology—the absolute Irish standard. Radiocarbon 28: 279–283.

Dansgaard W, Johnsen SJ, Clausen HB, Dahl-Jensen D, GundestrupNS, Hammer CU, Hvidberg CS, Steffensen JP, Sveinbjornsdottir

Figure 6 Wavelet analysis of the Irish tree and 14C production rate forthe last 7468 yr. Wavelet power spectra illustrate the change in concen-tration of spectral power with time in (A) the Irish bog oaks and (B) the14C production rate. Datasets have been padded with zeros to reducethe variance (Torrence and Compo, 1998). The contour levels areselected so that 75%, 50%, 25% and 5% of the wavelet power is aboveeach level respectively. Analysis undertaken using interactive softwareavailable at http://paos.colorado.edu/research/wavelets/

TESTING SOLAR FORCING OF HOLOCENE CLIMATE CYCLES 517

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)

AE, Jouzel J, Bond G. 1993. Evidence for general instability of pastclimate from a 250-kyr ice-core record. Nature 364: 218–220.

Denton GH, Karlen W. 1973. Holocene climatic variations—their pat-tern and possible cause. Quaternary Research 3: 155–205.

Emanuel WR, Killough GG, Post WM, Shugart HH, Stevenson MP.1984. Computer implementation of a globally averaged model ofthe world carbon cycle. Carbon Dioxide Research Division, USDepartment of Energy: Washington, DC.

Finkel RC, Nishiizumi K. 1997. Beryllium 10 concentrations in theGreenland Ice Sheet Project 2 ice core from 3–40 ka. Journal of Geo-physical Research 102: 26 699–26 706.

Hu FS, Slawinski D, Wright HE, Ito E, Johnson RG, Kelts KR, McEwanRF, Boedigheimer A. 1999. Abrupt changes in North American cli-mate during early Holocene times. Nature 400: 437–440.

Hu FS, Kaufman D, Yoneji S, Nelson D, Shemesh A, Huang Y, Tian J,Bond G, Clegg B, Brown T. 2003. Cyclic variation and solar forcingof Holocene climate in the Alaskan subarctic. Science 301: 1890–1893.

Jenkins GM, Watts DG. 1968. Spectral Analysis and its Applications.Holden Day: Oakland, CA.

Karlen W, Kuylenstierna J. 1996. On solar forcing of Holocene climate:evidence from Scandinavia. The Holocene 6: 359–365.

Lageard JGA, Chambers FM, Thomas PA. 1999. Climatic significanceof the marginalization of Scots pine (Pinus sylvestris L.) c. 2500 BC atWhite Moss, south Cheshire, UK. The Holocene 9: 321–331.

Langdon PG, Barber KE, Hughes PDM. 2003. A 7500-year peat-basedpalaeoclimatic reconstruction and evidence for an 1100-year cycli-city in bog surface wetness from Temple Hill Moss, Pentland Hills,southeast Scotland. Quaternary Science Reviews 22: 259–274.

Leuschner HH. Sass-Klaassen U, Jansma E, Baillie MGL, Spurk M.2002. Subfossil European bog oaks: population dynamics andlong-term growth depression as indicators of changes in the Holo-cene hydro-regime and climate. The Holocene 12: 695–706.

Magny M, Begeot C, Guiot J, Peyron O. 2003. Contrasting patterns ofhydrological changes in Europe in response to Holocene climatecooling phases. Quaternary Science Reviews 22: 1589–1596.

McDermott F, Mattey DP, Hawkesworth C. 2001. Centennial-scaleHolocene climate variability revealed by a high-resolution spe-leothem �18O record from SW Ireland. Science 294: 1328–1331.

Neftel A, Moor E, Oeschger H, Stauffer B. 1985. Evidence from polarice cores for the increase in atmospheric CO2 in the past two centu-ries. Nature 315: 45–47.

O’Brien SR, Mayewski PA, Meeker LD, Meese DA, Twickler MS,Whitlow SI. 1995. Complexity of Holocene climate as reconstructedfrom a Greenland ice core. Science 270: 1962–1964.

Oeschger H, Siegenthaler U, Schotterrer U, Gugelmann A. 1975. A boxdiffusion model to study the carbon dioxide exchange in nature.Tellus 27: 168–192.

Pearson GW, Pilcher JR, Baillie MGL, Corbett DM, Qua F. 1986. High-precision C-14 measurement of Irish oaks to show the natural C-14variations from AD 1840 to 5210 BC. Radiocarbon 28: 911–934.

Pilcher JR, Baillie MGL, Schmidt B, Becker B. 1984. A 7,272-year tree-ring chronology for western Europe. Nature 312: 150–152.

Pilcher JR, Baillie MGL, Brown DM, McCormac FG, MacSweeney PB,McLawrence AS. 1995. Dendrochronology of subfossil pine in thenorth of Ireland. Journal of Ecology 83: 665–671.

Pilcher JR, Baillie MGL, Brown DM, McCormac FG. 1996. Hydrologi-cal data from the long Irish subfossil oak records. In Tree Rings, Envir-onment and Humanity, Dean JS, Meko DM, Sweatnam TW (eds).Radiocarbon: Arizona; 259–264.

Russell JM, Johnson TC, Talbot MR. 2003. A 725 yr cycle in the climateof central Africa during the late Holocene. Geology 31: 677–680.

Sarnthein M, van Kreveld S, Erlenkeuser H, Grootes PM, Kucera M,Pflaumann U, Schulz M. 2003. Centennial-to-millennial-scale peri-odicities of Holocene climate and sediment injections off the wes-tern Barents shelf, 75�N. Boreas 32: 447–461.

Schulz M, Paul A. 2002. Holocene climate variability on centennial-to-millennial time scales: 1. Climate records from the North-Atlanticrealm. In Climate Development and History of the North AtlanticRealm, Wefer G, Berger W, Behre K-E, Jansen E (eds). Springer-Ver-lag: Berlin; 41–54.

Schulz M, Berger WH, Sarnthein M, Grootes PM 1999. Amplitude var-iations of 1470-year climate oscillations during the last 100,000years linked to fluctuations of continental ice mass. GeophysicalResearch Letters 26: 3385–3388.

Shindell D, Rind D, Balachandran N, Lean J, Lonergan P. 1999. Solarcycle variability, ozone, and climate. Science 284: 305–308.

Smith AG, Pearson GW, Pilcher JR. 1974. Belfast radiocarbon dates VII.Radiocarbon 16: 269–276.

Solignac S, de Vernal A, Hillaire-Marcel C. 2004. Holocene sea-sur-face conditions in the North Atlantic–contrasted trends and regimesin the western and eastern sectors (Labrador Sea vs. Iceland Basin).Quaternary Science Reviews 23: 319–334.

Southon J. 2002. A first step to reconciling the GRIP and GISP2 ice-corechronologies, 0–14,500 yr B.P. Quaternary Research 57: 32–37.

Stuiver M, Braziunas TF. 1993. Modeling atmospheric C-14 influencesand C-14 ages of marine samples to 10,000 BC. Radiocarbon 35:137–189.

Stuiver M, Burk RL, Quay PD. 1984. 13C/12C ratios in tree rings and thetransfer of biospheric carbon to the atmosphere. Journal of Geophy-sical Research 89: 731–748.

Stuiver M, Reimer PJ, Bard E, Beck JW, Burr GS, Hughen KA, Kromer B,McCormac FG, van der Plicht J, Spurk M. 1998. INTCAL98 radiocar-bon age calibration 24,000—0 cal BP. Radiocarbon 40: 1041–1083.

Takahashi T, Broecker WS, Bainbridge AE. 1981. The alkalinity andtotal carbon dioxide concentration of the world oceans. In CarbonCycle Modelling, Bolin, B (ed.). Wiley: New York.

Thomson DJ. 1982. Spectrum estimation and harmonic analysis.Proceedings of the IEEE 70: 1055–1096.

Torrence C, Compo GP. 1998. A practical guide to wavelet analysis.Bulletin of the American Meteorological Society 79: 61–78.

Turney CSM, Kershaw P, Clemens S, Branch N, Moss P, Fifield LK.2004. Millennial and orbital variations of El Nino/Southern Oscilla-tion and high-latitude climate in the last glacial period. Nature 428:306–310.

van der Plicht J, van Geel B, Bohncke SJP, Bos JAA, Blaauw M,Speranza AOM, Muscheler R, Bjorck S. 2004. The Preboreal climatereversal and a subsequent solar-forced climate shift. Journal of Qua-ternary Science 19: 263–269.

van Geel B, Buurman J, Waterbolk HT. 1996. Archaeological andpalaeoecological indications of an abrupt climate change in TheNetherlands, and evidence for climatological teleconnectionsaround 2650 BP. Journal of Quaternary Science 11: 451–460.

van Geel G, van der Plicht J, Renssen H. 1999. Comment on ‘A largeCalifornia flood and correlative global climatic events 400 years ago’(Schimmelmann et al., 1998). Quaternary Research 51: 108–110.

Yu Z, Campbell ID, Campbell C, Vitt DH, Bond GC, Apps MJ. 2003.Carbon sequestration in western Canadian peat highly sensitive toHolocene wet–dry climate cycles at millennial timescales. TheHolo-cene 13: 801–808.

518 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 20(6) 511–518 (2005)