Peatlands As a Model System for Exploring and Reconciling Quaternary Chronologies
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Peatlands as a model system for exploring and reconciling Quaternary chronologies maarten blaauw1, J.a. chriSten2 and d. mauquoy3 1School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, UK; [email protected] 2Centre for Mathematical Research, Guanajuato, Mexico; 3Geography and Environment, University of Aberdeen, UK Recent developments in statistical techniques enable quantitative assessment of the chronological reliability of radiocarbon-dated peatland profiles. The use of peatlands as datable archives A widely used basic age-depth model is of past climate and environmental change linear interpolation between the dated goes back for more than a century. Early levels (Bennett, 1994). This method as- Science Highlights: Peatlands Highlights: Peatlands Science age estimates were based on correla- sumes abrupt accumulation rate changes ting peat stratigraphy within and among between each dated level, which for peat sites, assuming that specific horizons re- bogs seems an unrealistic scenario. Age- presented regionally synchronous events depth models that are considered more (Fig. 1). Peat layers with different vegeta- ecologically plausible and that take into tion or degrees of decomposition were account likely modes of peat accumula- thought to reflect past changes in climate tion, include (1) linear accumulation (over (e.g., cooler, warmer, drier or wetter; von limited time intervals; Belyea and Clymo, Post, 1946). The boundaries between 2001; Blaauw and Christen, 2005), (2) con- these layers were then linked to the ages cave curves (through continuing decom- of environmental changes as inferred from position of fossil matter in the peat depos- independently dated archives (e.g., varve it; Yu et al., 2001), (3) convex curves (with chronologies, links between pollen and ar- deposits slowing down their accumula- cheological phases). These early peatland tion when close to a height limit; Belyea age-models were relative and imprecise, and Baird, 2006), and (4) Bayesian mod- owing to their dependence on inferred els that can include prior information on synchrony of layers with imprecisely dated Figure 1: Section of a Sphagnum magellanicum raised stratigraphy, accumulation rate and vari- events in other archives. peat bog in Tierra del Fuego. Note the 'layer-cake' stra- ability, and/or detect outlying dates (e.g., tigraphy where the changes from dark to humified peat can be traced for ~50 m. Blaauw and Christen, 2005; Bronk Ramsey, Radiocarbon dating 2008; Blaauw et al., in prep.). The development of radiocarbon dat- bomb-peak 14C dating for peat younger Confidence intervals of calibrated ra- ing in the 1950s led to estimation of ab- than c. AD 1960 (van der Linden et al., diocarbon ages (Reimer et al., 2009) are solute ages for organic material such as 2008), and lead dating for peat younger often considerably larger than the original peat. While deposits such as soils, oceans than c. AD 1850 (Turetsky et al., 2004)). 14C measurement uncertainties, especially or lakes often contain little and/or he- Moreover, well-dated and geochemically during wiggles or plateaux in the calibra- terogeneous carbon (e.g., blends of stable identifiable volcanic ash (tephra) layers tion curve. For example, a date of 2.45±20 and labile organic components, hard wa- can provide valuable dating points, and 14C ka BP (i.e., on the Hallstatt plateau) ter effects through carbonate seepage can be used to align proxy archives be- obtains a 95% range of 440 years on the from surrounding bedrock), carbon is tween regions (Pilcher et al., 1995). calendar scale. Therefore, many 14C-dated very abundant in peat and is mostly de- peat chronologies possess centennial- rived from the local, in situ, vegetation. Age-depth modeling scale uncertainties. However, through As the living, contemporary plant matter Even though most peat contains abundant matching dense series of 14C dates to wig- on the bog surface becomes overgrown datable organic matter, it can be difficult gles in the calibration curve (radiocarbon by new vegetation, cool, wet and acidic to select sufficient above-ground macro- wiggle-match dating; van Geel and Mook, conditions combined with high decay-re- remains from highly degraded peat. More- 1989), much higher precision chronolo- sistance of Sphagnum tissues in the accu- over, selection and cleaning of material for gies can often be obtained (Christen et al., mulating sub-fossil organic matter, ensure 14C dating can be time-consuming and 14C 1995; Kilian et al., 1995; 2000; Mauquoy et slow decomposition. Therefore, most of dates are expensive. Therefore, generally al., 2002; Blaauw et al., 2003; Blaauw and the carbon in a slice of peat will generally only a few sample depths in a peat profile Christen, 2005; Chambers et al., 2007; see have 14C ages corresponding to the time at are dated. Moreover, 14C dates themselves also van Geel and Mauquoy, this issue). which that slice was accumulating at the can show considerable scatter (Scott, Wiggle-matching performs best where original bog surface. Above-ground plant 2007), and calibrated distributions can the calibration curve has pronounced wig- remains such as moss, seeds, stems, leaves be irregular and wide. In order to provide gles, often resulting in decadal-scale pre- or pollen are generally preferred over bulk calendar ages for each (dated or undated) cision during the Subboreal/Subatlantic material, because such selected remains depth of a core, some sort of modeling is transition (Kilian et al., 2000) or during the will be contemporaneous with the depth needed. Each model has its specific advan- Little Ice Age (Mauquoy et al., 2002; Fig. 2). of the peat, while bulk dates can contain tages and limitations, and choosing a par- a mix of contemporaneous and older or ticular type of age-depth model will yield Multi-site comparisons younger material (e.g., rootlets). a distinct age-depth relationship (Bennett Until recently, most comparisons of proxy For recent peat deposits, additional and Fuller, 2002; Telford et al., 2004). reconstructions between cores, sites and dating techniques are available (e.g., regions have been visual and rather sub- 9 PAGES news • Vol 18 • No 1 • April 2010 Charman et al. (2009), who quantify the synchroneity of climate events between a peat bog and an adjacent lake in northern England (Fig. 3). Importantly, the degree of synchroneity of proxy events between archives depends on the chronological resolution of these archives. If chronologi- cal uncertainties of the archives are, for example, at the centennial scale, these archives cannot be used to answer ques- tions at decadal resolution (Blaauw et al., 2007). A wealth of paleoecological data has Science Highlights: Peatlands Highlights: Peatlands Science now been obtained from peat archives across the globe, and one of the remain- ing challenges is how to interpret these data in terms of quantitative past climate and environmental changes. By apply- ing novel numerical methods to peatland chronologies, more realistic estimates can now be obtained for the spatio-temporal pattern of past environmental events (e.g., regional differences). Novel numeri- cal methods are also being developed to assess the reliability of proxy records as Figure 2: a) Bayesian age-depth model of a late Holocene peat core from Lille Vildmose in Denmark (Mauquoy et quantitative archives of past climate and al., 2002). The model consists of many small sections of linear accumulation, with a degree of memory between environmental changes. For example, neighboring sections (Blaauw et al., in prep.). Blue shapes are calibrated probability distributions of 17 radiocar- Blaauw et al. (2010b) suggest that we can- bon dates (together with the core's surface, which is set at an age of AD 2000±1). Gray shapes show the posterior probability distribution of a) the age-depth model and b) hummock macrofossil proxies (relative % abundance of not assume that every proxy change is Sphagnum capillifolium). Dotted lines in (a) indicate 95% confidence range. Abundant hummock proxies indicate necessarily forced by a single traceable ex- relatively dry conditions on the bog's surface. Precisely and reliably dated core sections are plotted dark and narrow, ternal factor such as climate change (e.g., while wide grey smudges indicate chronologically uncertain depths (Blaauw et al., 2007). Blaauw et al., 2010a). Hopefully, such nu- jective. Proxy curves from multiple sites tween sites. Recently developed statistical merical methods will eventually enhance are generally plotted on either indepen- alternatives however can quantitatively our understanding of the possibilities dent or tuned timescales, after which syn- assess leads and lags (Parnell et al., 2008) and limitations of peat archives for recon- chroneity, leads or lags between sites are or synchroneity (Blaauw et al., 2007, 2010a) structing past environmental change. inferred through “eye-balling” or by draw- between independently dated proxy ar- ing subjective lines connecting events be- chives. A recent example is provided by References Blaauw, M. and Christen, J.A., 2005: Radiocarbon peat chronologies and environmental change, Applied Statistics, 54: 805-816. Blaauw, M., Christen, J.A., Mauquoy, D., van der Plicht, J. and Bennett, K.D., 2007: Testing the timing of radiocarbon-dated events be- tween proxy archives, The Holocene, 17: 283-288. Blaauw, M., Bennett, K.D. and Christen, J.A., 2010b: Random walk simu- lations of fossil proxy data, The Holocene. Charman, D., Barber,