Quaternary Science Reviews 41 (2012) 94e103 Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev Evaluating periodicities in peat-based climate proxy records Graeme T. Swindles a,*, R. Timothy Patterson b, Helen M. Roe c, Jennifer M. Galloway d a School of Geography, University of Leeds, Leeds, LS2 9JT, United Kingdom b Ottawa-Carleton Geoscience Centre and Department of Earth Sciences, Carleton University, Ottawa, Ontario, K1S 5B6, Canada c School of Geography, Archaeology and Palaeoecology, Queen’s University Belfast, Belfast, BT7 1NN, United Kingdom d Natural Resources Canada/Ressources naturelles Canada, Geological Survey of Canada/Commission géologique du Canada, 3303-33rd Street N.W., Calgary, Alberta, T2L 2A7, Canada article info abstract Article history: Proxy records derived from ombrotrophic peatlands provide important insights into climate change over Received 4 October 2011 decadal to millennial timescales. We present mid- to late- Holocene humification data and testate Received in revised form amoebae-derived water table records from two peatlands in Northern Ireland. We examine the repli- 2 March 2012 cation of periodicities in these proxy climate records, which have been precisely linked through teph- Accepted 9 March 2012 rochronology. Age-depth models are constructed using a Bayesian piece-wise linear accumulation model Available online xxx and chronological errors are calculated for each profile. A Lomb-Scargle Fourier transform-based spectral analysis is used to test for statistically significant periodicities in the data. Periodicities of c. 130, 180, 260, Keywords: Holocene 540 and 1160 years are present in at least one proxy record at each site. The replication of these peri- Climate change odicities provides persuasive evidence that they are a product of allogenic climate controls, rather than Peatlands internal peatland dynamics. A technique to estimate the possible level of red-noise in the data is applied Periodicities and demonstrates that the observed periodicities cannot be explained by a first-order autoregressive Lomb-Scargle Fourier spectral analysis model. We review the periodicities in the light of those reported previously from other marine and Solar forcing terrestrial climate proxy archives to consider climate forcing parameters. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction (water-shedding) peatlands are directly coupled to the atmosphere and can provide continuous palaeohydrological records spanning 1.1. Background much of the Holocene (Barber, 1981; Hughes et al., 2000; Langdon et al., 2003; Chambers and Charman, 2004). The recognition and understanding of periodicities in Holocene Palaeohydrological reconstructions from ombrotrophic peat- climate records are amongst the most important challenges facing lands agree well with other proxy climate data (Baker et al., 1999; climate scientists. Numerous studies from the Northern Hemi- Charman et al., 2001; Booth et al., 2004) and instrumental climate sphere now provide clear evidence for persistent climate period- records (Charman et al., 2004; Hendon and Charman, 2004; icities operating on decadal to millennial timescales, some of which Schoning et al., 2005). It has been suggested that estimates of bog have been linked with recognised drivers, for example, celestial surface wetness can be utilised as a proxy of summer water deficit, factors (e.g. Haigh, 1994; Yu and Ito, 1999; Chapman and which is controlled by summer rainfall in oceanic locations, with Shackleton, 2000; Bond et al., 2001; Chambers and Blackford, summer temperature playing a greater, albeit subsidiary, role in 2001; Cumming et al., 2002; Hu et al., 2003; Patterson et al., higher-latitude, continental settings (Charman, 2007; Charman 2007; Wanner et al., 2008; Gray et al., 2010). Understanding the et al., 2009). timing, character and causes of these periodicities is fundamental Well established peat-based palaeohydrological proxies include for improving our knowledge of global climate system dynamics. In plant macrofossils (Barber et al., 1994), testate amoebae (Woodland the terrestrial realm, a proliferation of studies have focused on et al., 1998) and colorimetric peat humification data (Blackford and peatlands as archives of Holocene climate change (Blackford, 2000; Chambers,1993). These methods have been used to generate mid to Chambers and Charman, 2004; Blundell and Barber, 2005; late Holocene palaeohydrological records from peatlands in NW Mauquoy et al., 2008; Nichols and Huang, 2012). Ombrotrophic Europe (e.g. Blundell and Barber, 2005; Mauquoy et al., 2008), North America (e.g. Booth and Jackson, 2003; van Bellen et al., 2011) and the Southern Hemisphere (e.g. Mauquoy et al., 2004). * Corresponding author. Tel.: þ44 (0) 113 3439127. Several authors have applied spectral analysis to such hydro- E-mail address: [email protected] (G.T. Swindles). climate proxy data, which has revealed millennial, centennial and 0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2012.03.003 G.T. Swindles et al. / Quaternary Science Reviews 41 (2012) 94e103 95 sub-centennial periodicities (Wijmstra et al., 1984; Barber et al., 1994; Chambers et al., 1997; Hughes et al., 2000; Chambers and Blackford, 2001; Langdon et al., 2003; Blundell and Barber, 2005; Borgmark, 2005; Swindles et al., 2007a). However, the precise Slieveanorra inter-site replication of these periodicities has been limited by Dead Island chronologies based on radiocarbon dates. Peatlands in Northern Ireland have an advantage over those in many other regions as they contain multiple cryptotephra layers enabling direct correlation of Belfast palaeoclimate records. This is necessary for testing the replicability of periodicities between sites. 1.2. Research aim In this study we present new 4500-year peat humification records from two ombrotrophic peatlands in Northern Ireland, Dublin which are supplemented with published water table reconstruc- tions from the same sites (Swindles et al., 2010). The aim of the study is to test the replicability of periodicities in these tephra- linked proxy climate records. If similar periodicities are found in both sites, it is reasonable to conclude that they have been driven by allogenic climatic controls rather than site-specific factors. We N place this research into context by providing an overview of the climate periodicities documented in other key terrestrial and marine records from the Northern Hemisphere and consider potential forcing parameters. 0 50km 2. Materials and methods 0 25 miles 2.1. Field and laboratory methods Fig. 1. Map showing the location of study sites: Slieveanorra (SA), County Antrim (55 0400 2200 N, 06 130 3900 W) and Dead Island (DI), County Londonderry (54 530 1500 Two ombrotrophic peatlands were investigated: Dead Island N, 06 320 5100 W). (DI), a lowland raised bog in County Londonderry, and Slieveanorra (SA), an upland raised bog and blanket peat complex in County Antrim (Fig. 1). Both sites have an intact dome, although their Testate amoebae-derived water table reconstructions from each margins/laggs have been damaged locally by peat cutting in recent site have been published previously (Swindles et al., 2010). The centuries. Since these peripheral, cut-over areas are limited in reconstructions were carried out using the ACCROTELM pan- relation to the total areas of the bogs, we assume that their impact European transfer function, which is based on weighted aver- on the hydrology of the cupola area has been minimal (Swindles, aging tolerance-downweighted regression with inverse deshrink- 2006). ing (Charman et al., 2007). Sample-specific reconstruction errors A Russian peat corer was used to extract cores from lawn were calculated through 1000 bootstrap cycles (Line et al., 1994). microforms on the main bog cupola at the two sites following the Plant macrofossil data were not integrated as the analysis was only parallel-hole method (with 20 cm overlap). The cores were wrap- carried out at key intervals and is not a continuous dataset ped in aluminium foil, returned to the laboratory, and stored in (Swindles, 2006; Swindles et al., 2007a). refrigeration at 4 C. The cores were sub-sampled at consecutive 1- Chronological control for the records of DI and SA is based on 14 cm intervals. The peat subsamples (2 cm3) were dried at 105 C for tephrochronology, AMS C dating and spheroidal carbonaceous approximately 16 h and then combusted in a muffle furnace at particle (SCP) stratigraphies as described in Swindles et al. (2010). 450 C for 8 h to calculate percentage loss-on-ignition (Schulte and The tephrochronological framework enables precise correlation of Hopkins, 1996). the proxy climate records. Radiocarbon dates were calibrated with Peat humification analysis is based on alkali extraction of humic IntCal09 (Reimer et al., 2009) and expressed with 2s ranges acids and spectrophotometry (Aaby, 1976). The principle behind (Supplementary material B). humification analysis is that during periods of drier climate, bog Probability distributions for calibrated radiocarbon dates were water tables will be lower and it will take longer for litter to reach used for the estimation of the age-depth models along with the the anoxic catotelm, resulting in greater decomposition. During tephra and SCP determinations. The calendar year of AD 2003 was wetter periods, high water tables result in peat being less decom- attributed to the top of