peatlands 1 taitto.indd - International Peat Society

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Recording the Past: the Power of Peat Peat bogs are powerful archives containing records on the environment and culture of the past, such as climate change - as indicated by botanical remains - and bog finds such as archaeological materials, including human remains (bog bodies). Some recent research highlights from the Netherlands are presented. Peat bogs are growing continuously under certain climatic conditions such as those that prevailed during the Holocene - the present geological period that, following the last ice age, has lasted for approximately 11,500 years. Peat bogs and palaeoclimate By digging into terrestrial deposits, one travels, in general, back in time from the present to the past - the deeper, the older. The relative occurrence of plant species at a certain depth (and thus at a certain time in the past) indicates climatic conditions, like tem- Figure 2: The variations in the natural 14 C content, as measured in tree rings dated by dendro-chronology. The 14 C content is plotted as the deviation from the normal level, and is corrected for radioactive decay. 46 PEATLANDS International 1/2006 perature and humidity, under which the plants at this depth were growing. In general terms, dominance of tree pollen is indicative of warmth; the relative abundance of different species gives fi ner detail. For example, there usually is a dominance of birch and pine during the initial phase of an interglacial like the Holocene, followed later by mixed forest (oak, elm, hazel) in the temperate regions. Thus, records of preserved botanical remains like pollen and macrofossils (branches, leaves) form a proxy for the palaeoclimate. Peat bogs consist of bog mosses (species of Sphagnum). They are rain-fed and grow continuously. After death, the mosses are well preserved. Continual growth of the peat bog surface and burial of dead organic matter leads to peat sequences that can exceed 10 m in depth. Radiocarbon Dating The chronology of such deposits can be obtained by 14 C (or Radiocarbon) dating of organic remains like peat, macrofossils, or pollen. Radiocarbon is a natural radioactive isotope (with a half-life of 5730 years) of the element Carbon. This isotope is continuously produced by cosmic radiation in the upper atmosphere, and is taken up by the plants by photosynthesis. From there it fi nds its way into all living organisms via the food chain. There is an equilibrium Text and photos: J. van der Plicht J. van der Plicht. between uptake and decay of 14 C . After death of the organism, there is no longer uptake of 14 C; radioactive decay causes the content of 14 C to decrease. Hence, by measuring the remaining 14 C content of for example the botanical remains from within a peat bog, the age of these remains (more precisely: the moment of death) can be established. The isotope 14 C provides in fact a clock, built into the organic residues. Although this sounds like a simple and straightforward principle, in practice there are many complications of which only one is mentioned here. For the simple dating model to work, one has to assume that the natural atmospheric 14 C concentration is constant throughout time. It appears that this is not the case: the cosmic ray fl ux impinging on earth, and thus the amount of 14 C produced in the atmosphere, depends on factors like the strength of the earth’s magnetic fi eld, and solar activity. In addition, the 14 C concentrations are extremely small (1:1012-1015) so that measuring them requires highlyspecialized equipment, such as shown in Figure 1.
Trees as calendars The varying 14 C content in nature is known by a calibration curve, which is obtained by 14 C dating of tree rings from wood, that, in addition, are dated absolutely by dendro-chronology. This calibration curve makes dating possible because it allows the calculation of “historical ages” from “ 14 Cages”, thus taking the natural variations into account. The tree ring calendar extends back to more than 14,000 years ago. The calibration data are shown in Figure 2 as the relative 14 C content (corrected for radioactive decay) with respect to present day values. The long-term (millennium scale) trend of the data can be explained by the infl uence of the earth’s magnetic fi eld on the 14 C production rate. In general, this caused a higher 14 C content in nature millennia ago compared with the present. The sharp excursions on this long-term trend (like the one indicated at 800 BC) are caused by solar fl uctuations. At 800 BC, a climate change is registered in archives like peat bogs, known as the Subboreal-Subatlantic transition. This is illustrated in Figure 3, the peat bog Bargerveen in the Netherlands. The darker, more decomposed Subboreal peat was deposited under relatively warm and dry climatic conditions. The lighter coloured Subatlantic peat was formed under cooler, wetter climatic conditions. The change is marked in the fi gure, and is observed in many archives worldwide. Plant remains reveal past Botanical analysis of peat-forming plants from several peat bogs, in combination with precise 14 C dating, shows a remarkable phenomenon, as illustrated for the Engbertsdijksveen (the Netherlands) - see fi gure 4. The sharp rise of 14 C at 800 BC coincides with the shift from “older, darker” to “younger” Sphagnum peat at which point (see Figure 3) Sphagnum acutifolia shows a decline. Thereafter, following a dominance by highly hygrophilous moss species, Sphagnum cuspidatum and Sphagnum papillosum, the peat is formed mainly by remains of Sphagnum imbrica- Figure 1: The Accelerator Mass Spectrometer (AMS) for measuring 14 C concentrations at Groningen University, the Netherlands. tum, which has a preference for oceanic climatic conditions with a high air humidity. The wetter conditions around 800 BC also coincide in signifi cantly rising water tables in the Netherlands, as evidenced by archaeological investiga- tions. The climate shift is observed on a global scale, and seems to be connected with migrations of prehistoric peoples as well. It is also remarkable that this time corresponds with the Bronze Age / Iron Age transition in Northwestern Europe. 47
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