Peat, by definition, is an accumulation of partially decayed plant matter. It is usually found in acidic wetlands such as bogs and peat swamp forests, but can also be found in wet areas of moorland habitats. The key characteristics of the area are permanent waterlogging and the continuous upward growth of the surface through the formation and storage of peat (Charman 2002). 2% of the planet's land mass is covered in peat; totalling 3 million km2 or 4 trillion m3 by volume (World Energy Council 2007). The dead remains of the vegetation growing on these wetlands falls to the surface and is trapped in the surface water. These remains are not alone; they are accompanied by tiny single and multi-celled organisms, insects, unlucky creatures, and microscopic particles carried in on the wind. Year after year, this material is slowly covered, buried by the next layer of fallen material. The acidic, waterlogged and anaerobic environment helps prevent decay; preserving a record of Earth's climate over time. This paper will review the study of peat deposits as archives of past climate change, examining the history and development of the field. Furthermore, it will introduce the core techniques and some of the many proxies used in European peat-based paleoenvironmental reconstructions in order to gain a better understanding of environmental change.
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The History of the Foundation of Peat Climate Studies
Human interest in peat as anything more than fuel for our fires was not sparked until 1829, when the Danish geologist Heinrich Dau first noticed the presence of light and dark layers while observing the side of an eroded peat deposit. These mysterious layers soon prompted the Royal Danish Academy of Sciences and Letters to offer a prize to the first person who could explain them scientifically (Birks 2009). Dau certainly suspected that those layers of peat had a tale to tell; a tale that would turn out to be a vast and complex archive of climate change, a crucial database dating back over ten thousand years.
Early work was inherently limited by the technology and methodology of the time, but was nonetheless quite accurate and laid the groundwork for more complex modern study. It was the Norwegian botanist and geologist Axel Blytt who first proposed that the darker layers represented the deposit of plant matter under drier conditions, while the lighter layers were buried within a wetter environment. Blytt introduced the terms 'Boreal' and 'Atlantic' (cool and dry, warm and moist) in his 1876 Essay on the Immigration of Norwegian Flora (Birks & Seppä 2010).
Blytt's work went on to directly inspire Rutger Sernander; a Swedish botanist, geologist and archaeologist. Sernander noted several sharp transitions between layers of peat; most notably one specific change showing a rapid drop in temperature and large increase in precipitation. He coined the term 'klimasturz' (climate plunge) to describe this event. Sernander went on to identify four distinct post-glacial climatic periods. Expanding on Blytt's terminology, Sernander named these periods Boreal (dry), Atlantic (warm and wet), Subboreal (warm and dry), and Subatlantic (cool and wet) (Mewhinney 1996). The 1908 Blytt-Sernander Sequence is significant as the first climatostratigraphic division of the Holocene; and, in an expanded form, is still used as a reference to postglacial climate stages. Sernander was supported by the work of Carl Weber, whose 1926 paper Grenzhorizont und Klimaschwankungen (cross horizon and climate fluctuations) describes a sharp 'boundary horizon' occurring in an identical position across a number of peat bogs in northern Germany. Weber dated this layer to about 500 B.C., aligning perfectly with Sernander's 'klimasturz' event (Mewhinney 1996). This was an early example of how multi-site comparisons are fundamental to correlating and substantiating observations from multiple peat studies.
Sernander's apprentice, Lennart von Post, worked with the Swedish Geological Survey for 21 years, closely studying peat. As one of the pioneers of palynology (the study of pollen), von Post developed the technique of studying the concentration of microscopic pollen grains to establish correlative stratigraphies to link local peat deposits. In 1909, von Post introduced the 'spruce-pollen boundary' as an improved tool for local correlations. He continued to use this method for several years, until realizing the focus on pine alone was insufficient and limiting his work. Fortunately, Swedish botanist Nils Gustaf Lagerheim was already hard at work; closely counting the arboreal pollen of pine, birch, alder, elm, ash, oak and lime trees in a sample from a Swedish swamp. His painstaking comparison of the percentage representations in pollen counts between two successive layers revealed distinct changes. Lagerheim's botanical study inspired von Post to expand and revise his earlier stratigraphic peat work, and he went on to famously present a very important tool, the pollen diagram, in his 1916 lecture, Skogsträidspollen i sydsvenska torvmosselagerföljder (Arboreal pollen in successions of peat-swamp layers of southern Sweden) (Manten 1967). Pollen analysis and the pollen diagram greatly expanded the physical region of visible plant life communicated within peat deposits, and also created an entirely new level of scientific accuracy in the quantification and identification of this life. This helped researchers draw much clearer reconstructions of local and regional plant life at each given point in time.
Dating & Correlating Peat: The Importance of "When" on the "What How & Why" of Climate Change
Always on Time
Marked to Standard
It was now time itself that was limiting the use of peat deposits as verifiable and datable archives of changes to the climate and environment. Up until the 1950s, the dating of any given section of a peat deposit was based on estimations factoring general depth, comparisons to other archives, placement along the Blytt-Sernander Sequence and a variety of other approximations that were (by today's standards) quite imprecise (Smith 1981). These issues were answered by the ground-breaking invention of radiocarbon dating in 1949 by Willard Libby and his team at the University of Chicago. Radiocarbon dating uses the naturally-occurring radioisotope carbon-14 (14C) to gauge the age of carbon-rich materials, effectively as far back as 62,000 years (Plastino & Kaihola 2001; Arnold & Libby 1949). Radiocarbon dating sensitivity is further improved through the use of accelerator mass spectrometry (AMS). This method also allows smaller samples to be dated, requiring only a few milligrams of carbon (Brown 1989). Radiocarbon dating does still present serious limitations. In the time scales applicable to peat studies, age ranges from radiocarbon dating are only accurate within two standard deviations of between 200 and 500 years (Pilcher 1993).
In order to focus the accuracy of 14C measurements to an acceptable level, 'wiggle-matching' is now used across virtually every modern study of peat deposits. The method requires careful matching of dense series of 14C dates to the observable wiggles in the date calibration curve (Killian 1995). The accuracy of wiggle-matched dates varies widely, but in most Holocene studies, falls within one standard deviation of 30 years (Blackford 2000). Researchers must be very selective when applying high-precision dates due to the intense preparation process and high cost of the many 14C dates required for just one wiggle-matched date (Bard 1998; RadiocarbonWebInfo). Because of these challenges, researchers often apply what is called 'age-depth modelling' to peat cores in order to determine date estimations across the different layers under observation.
Age-depth modelling basically attempts to build a linear or curved model of accumulation rates within the peat core, to determine how many years of accumulation is represented by each centimetre of peat.
The difficulty here is that peat does not accumulate at a constant rate. There are a number of variables affecting the rate of accumulation, including the composition of the material and climatic factors such as temperature and precipitation. In order to approximate accumulation rates, a sample is divided into phases. These phases are considered to have relatively constant accumulation rates based upon visual examination and the comparison of their plant macrofossil content. The horizons between phases are dated and the relationship between time and depth is plotted into a model. For example, a peat core from Tore Hill Moss (a raised bog in Strathspey, Scotland) was subjected to age-depth modelling. This revealed three distinct phases, with accumulation rates of 17.6 years/cm (248-172cm), 5.14 years/cm (172-40cm) and 19.6 years/cm (40-0cm). (See Figure 1; Age-depth model for Tore Hill Moss). These three distinct linear regressions formed an age-depth model, enabling the research team to conduct their climate reconstructions at 4cm intervals along the length of the core, dating consistently and accurately (Blundell & Barber 2004). As mentioned, this is only one type of age-depth model. The type of model used will depend on many variables, but each model will bear a distinctive age-depth relationship (Bennett & Fuller 2002). Dating the Tore Hill Moss study was not limited to just AMS 14C and a linear age-depth model, the researchers had also located a fine layer of ash in the peat core that would allow them to confirm their age-depth model and irrefutably link their palaeoclimatic study with those from other peat cores all across the region.
//////////////This tiny but indispensible layer of ash allowed the researchers to apply a widely-used method of multi-site correlation and date marking called 'tephra analysis.' When a volcanic eruption occurs, massive quantities of solid material are blasted upwards from deep beneath the surface of the Earth. Although the heavier debris falls around the base of the volcano; the fine, dust-like ash is carried over great distances by thermal currents and the wind (Thorarinsson 1944). This ash all contains a unique geochemical composition that allows uncovered samples to be easily matched with the single source eruption. The pioneer of this field was Sigurdur Thorarinsson, an Icelandic geologist, volcanologist, glaciologist, professor and lyricist. His discovery of the correlative potential of tephra deposits added a new and unquestionable method of correlating regional palaeoclimatic peat studies by using tephra from specific eruptions as a common marker, fixed across time.
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This fixed time marker helps verify and substantiate traditional dating techniques within individual studies. In the case of the Tore Hill Moss peat core, a layer of tephra was located between 208 and 216cm, with the peak concentration of ash occurring at 212cm. The researchers compared the geochemical signatures from several other studies, and discovered that their tephra came from a well-known source called Glen Garry (Blundell &Barber 2004). Glen Garry tephra has been located in eight different peat deposits (See Figure 2; Glen Garry Sites) across Scotland and the north of England. This commonality effectively links the Tore Hill Moss study with Walton Moss, Langlands Moss, Temple Hill Moss, Shirgarton Moss, Mallachie Moss, Craigmaud Moss and Ben Gorm Moss. The actual physical location of the eruption site is still a mystery, but that has no real impact on the efficacy of using the tephra to help build a regional reconstruction of past climate in the area. Several different samples of the Glen Garry tephra were 14C radiocarbon dated at all eight sites, and given the inherent margin for error, a total of 37 different dates were revealed for the eruption. By wiggle-matching and averaging these dates, the most accurate date estimate for the event was established as 2176 cal. BP, with 95% confidence intervals are 2210-1966 cal. BP (Barber, Langdon, et al. 2008). The Glen Garry tephra layer was located at distinctly different depths across all eight peat studies (see appendix A), a further reminder of the wide range of peat accumulation rates that occur even in geographically adjacent study sites.
Tephra analysis does not always have to rely on correlation and 14C dating; sometimes an exact calendar date can be applied just by identifying the tephra's source. Glen Garry is not the only volcano to leave its mark on the peats of Europe. A famous volcano from the south of Iceland, Hekla, has been scattering its ashes around the region for thousands of years. Two distinct layers of tephra from two of Hekla's eruptions are present in many peat sites in northern Britain and Ireland. The first was dated to the autumn of 1104, the next on July 25, 1510. These are actual calendar dates, simply revealed through well-documented historical archives (Blackford 2000). The benefits of being able to date and correlate a single point across multiple cores down to a single day versus a 244-year confidence range (as with Glen Garry) should be obvious. This type of dating, however, is limited to the availability of historic records. There are numerous other dating and correlating techniques; but the use of 14C radiocarbon dating, wiggle-matching, age-depth modelling and tephra analysis are among the most common. Dating and correlating is crucial to interpreting past climates and understanding their changes, but only as far as it assists in the interpretation of the data packed away within layers of peat.
Reconstructing Climate Change: It Takes More Than One Proxy
The scientists introduced in this paper laid the foundations for modern palaeoclimatic studies of peat deposits; and although the past century has brought many innovations to help better interpret and understand findings from peat cores, the fundamental principles of the research have not changed. These principles are: (1) Surface vegetation responds to changes in the water table, which is in turn responsive to changing climate. (2) The macrofossil remains of the vegetation preserved as peat are an accurate record of original surface vegetation. (3) More decomposition occurs when the surface is dry; resulting in more humified, darker-coloured peat. (4) Less decomposition occurs when the surface is wet; resulting in lighter-coloured peat whose composition is more easily identifiable (Blackford 2000).
In order to derive any meaningful climate data from the modern study of peat, a multi-proxy approach is required. Some of the most common proxies analysed in peat-based climate reconstructions are plant macrofossils, humification, testate amoebae, and pollen. Most of these proxies help define the surface wetness of the peat bog; which must only be influenced by precipitation and evapotranspiration if climate observations are to be made. Peat bogs that experience runoff from other water sources or that have any form of drainage, are useless for this type of study. Runoff can also introduce foreign materials to the environment, contaminating observations. For these reasons, peat-based palaeoclimatic studies are therefore only conducted on ombrotrophic (rain-fed) sites (Blundell & Barber 2004).
The analysis of plant macrofossils is conducted through any number of techniques; with the basic goal of identifying vegetation species present at key horizons and quantifying them against the total composition. This is much easier to accomplish within less humified, well preserved peat horizons. This proxy data is combined with established knowledge of plant biology to generate an estimation of surface conditions at each analysed point in time. A macrofossil diagram is often created, tracking species representations through time (see appendix B). Sphagnum moss is often a main focus, as it is the most abundant peat-forming vegetation. Its presence and quantity in relation to other species is a basic tenant of this proxy, but challenges are present. Decayed samples render macrofossil analysis difficult or impossible, non-climatic factors such as competition from other species can push out sphagnum, and sphagnum's various subspecies prefer different surface conditions (Blackford 2000).
Humification analysis is frequently used in conjunction with plant macrofossils. This method is especially effective in response to the challenge faced by macrofossil analysis on highly-humified peat layers. Traditionally, samples are subject to chemical extraction and colorimetric analyses to determine the degree of humification over time (see appendix C) but several recent technological advancements have greatly improved the speed and accuracy of testing (McTiernan, Garnett, et al. 1998). Both plant macrofossil and humification analysis are based on plant matter, a factor suggesting the use of another proxy to determine surface wetness.
This proxy is the analysis of testate amoebae. These tiny single-celled organisms are encased in a shell referred to as a test. They live in fresh water and wet soil, and are found in large quantities in peat cores. Their shell protects them from decomposition, and the preservative environment of peat bogs further maintains their integrity. There are numerous species and subspecies of testate amoebae, and they prefer different environments; varying in wetness, temperature and acidity. By carefully identifying and counting the testate amoebae present at each focus sample depth, researchers can compare the percentage populations over time against modern data sets and extrapolate past conditions (see appendix D) of the surface water (Bobrov, Charman, et al. 2002).
The three proxies discussed so far are all tied to life within the bog itself. They reveal little about the life outside the immediate boundaries of the peatlands. This is where von Post's field of palynology shines as a proxy. Pollen is carried in on the wind and deposited on the surface of the peat bog; storing a detailed and accurate record of regional plant species. Researchers can create a pollen diagram by identifying and counting the pollen in a peat sample and then tracing the change in representations over time. This also helps to verify surface vegetation estimates from plant macrofossil analysis, as pollen is released by vegetation within the peatlands as well as from the surrounding region.
Individually, these proxies each provide a series of snapshots of data pertaining to the climate conditions influencing the formation of peat at specific points through time. Each proxy, on its own, has different strengths and weaknesses. In order to smooth out the accuracy of each reconstructive peat study, multiple proxies are combined to create a more effective and scientifically sound data set. Together, the data from the proxies conducted in each study are worth far more than the sum of their parts. In much of the same way as the combination and correlation of proxies improves the resolution of each study, the connection and contribution of multiple studies draws together an immense wealth of climate data across regions.
The Big Picture
Much of the study on peat records, as discussed, is to reconstruct the hydrological conditions at the surface of peatlands at specific points in time. The focus on ombrotrophic peat systems allows researchers to obtain this hydrological data from an ecosystem that is entirely and solely connected to the atmosphere, and not to ground water infiltration. The immense and constantly growing database of moisture conditions at the surface of peat bogs throughout the Holocene provides a wealth of information pertinent to the reconstruction of not just precipitation, but climate itself.
Modern climatologists have a wealth of understanding on the complex interactions between the various quantifiable elements and influencers on climate patterns. Computer modelling and centuries of research have created a synthesis that allows researchers to reconstruct overarching climate conditions from just a few key variables. As one of the most important variables is precipitation, the abundance of Holocene hydrological data from peat studies can be concretely applied to established mathematical patterns in order to accurately reconstruct the climates of the past. Other proxy data, such as that on pollen, provides further contributing information from outside the peatlands. The growing presence of a certain species of tree that thrives in colder conditions and the decline of a grass that prefers dry soil indicates, quantifiably (base on changing proportions) that the climate is growing cold and wet. As discussed, these proxies thrive when applied together.
Credible regional models have been constructed through the intricate correlation of multiple studies, based on wide-ranging, relatively sudden and synchronous
changes in peat profiles. These abrupt climatic shifts to wet and cold conditions (Blackford 2000) have been attributed by many of today's scientists directly to fluctuations in solar activity
(Van Geel, Rensen, et al. 2001).
Our understanding of the true causes of past fluctuations in global climate will grow with time, and hopefully will help us predict, prepare for, or even influence the environmental impacts of the constantly changing climate. In order to grasp the complex nature of our planet's climate it is important to look into the past to understand the present and predict the future. Over the past two centuries, countless scientists have laboured tirelessly to develop and expand the field of peat-based paleoenvironmental reconstruction. They have uncovered many answers to questions on past climate change hidden deep within the archival layers of peat deposits, yet there are certainly countless more to discover. Whatever these answers may be, it can be certain that many will be found not by looking to the skies, but deep beneath the swampy peatlands at our feet.