Abstract
The present study combines palaeoecological and isotope-geological analyses of Holocene peat sequences to yield detailed information on temperature and rainfall. Analyses were carried out on two peat sections from the Meerstalblok (NE of the Netherlands). The Meerstalblok is one of the last remnants of the formerly extensive Bourtangerveen, situated between the Hondsrug (west) and the river Ems (east). A map of the stratigraphy of the peat profile shows that this part of the bog was differentiated into a hummock and a hollow during almost the entire period of peat accumulation, and that levels of the same age are situated lower in the hollow than in the hummock. The palaeoecological part of the study consists of a micro- and a macrofossil analysis. From the microfossil (pollen) analysis the development of the surrounding forests was reconstructed. Pollen concentrations and the relation between 14C dates and cumulative pollen concentrations were used to derive a time-scale according to the method of pollen density dating. Because the fine-scale accumulation of peat fluctuates strongly, this time-scale is thought to be more accurate than interpolation through depth between the dates. Macro- and microfossil analyses together provided a detailed picture of vegetation development of the raised bog in the period between 7000 and 1000 B.P. The bog development showed alternating wet and dry phases, probably as a response of the raised bog to differences in the available amount of effective rainfall. A semi-quantitative curve for the moisture conditions of the bog was calculated from the palaeoecological data obtained. Several phases corresponded to similar ones of a neighbouring raised bog (the Engbertsdijksveen). In the case of wetter vegetation types, the average seepage of the bog increases and more water may be discharged, whereas in the case of drier vegetation types the average seepage decreases and more water may be stored. The ratio between the area and the boundary of the southern part of the Bourtangerveen limits the total discharge of effective precipitation in this part of the bog. In case the effective rainfall increased to a quantity above the upper limit, a better natural drainage should develop. This probably occured at 2500 B.P. when a bog-burst discharged a water surplus and eroded the bed of the bog rivulet Runde. The effect of this bog-burst seems to be reflected in the hollow sequence of the Meerstalblok. Comparison of the regional pollen curves from the hummock and the hollow revealed an erosion hiatus of roughly 400 yrs and the disturbance of the adjacent layers in the hollow sequence. The local development confirmed the disturbance in the form of charcoal and seeds of Drosera intermedia. After the bog-burst probably a new balance was established in equilibrium with an increased amount of precipitation. The isotope-geological part of the study consists of measurements of the 2H/ 1H ratio in cellulose derived from peat. The ratio of precipitation depends on the temperature difference between the region of evaporation and the region where the rain falls. Because during a climatic change the temperature differences at higher latitudes are greater than at lower latitudes, the 2H/ 1H ratio of precipitation is related to the temperature. However, the 2H/ 1H ratio of cellulose of bog plants is also taxon-dependent. Transpiration of water from leaves enriches the heavy isotope content in the plant. This transpiration together with a biochemical fractionation of isotopes during photosynthesis introduce the species-bound dependence of the 2H/ 1H ratio. By determining the relative species abundance of each sample, the 2H/ 1H ratio was corrected for its taxon-dependence and a temperature record could be derived. Periodic regression analysis showed that a period of 200 yrs is the dominant one in this temperature curve. These analyses were carried out in the two Meerstalblok sections and in one from the Engbertsdijksveen. The results of the three sequences were mutually compared and also compared with stable isotope data ( 18O/ 16O) from the Camp Century ice core from Greenland. It is concluded that the average annual temperature dropped from over 10° to about 9°C between 4000 and 3000 B.P., rose shortly after 3000 B.P. to almost 11°C and fell off again to about 9°C shortly after 2000 B.P.
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