Holocene 17 (4)

Stable carbon isotope ratios from the latewood cellulose of twelve trees from two sites in northern Finland are used to construct an isotope chronology covering AD 1640 to 2002. By measuring isotopic ratios of every sample independently it is possible to identify and remove the juvenile portion of each 13C series, correct the individual series for anthropogenic changes in atmospheric carbon dioxide isotopic ratios and concentrations, and to quantify changes in signal strength through time. Most importantly, it is possible to demonstrate that there are no long-term trends in the carbon isotope series that are related to tree age. This means that it is not necessary to detrend the series and so they have the potential to retain climate information at all temporal frequencies. The correlation between the non-detrended carbon isotope series and July/August mean temperature is high (r=0.72) and comparison with meteorological records suggests that the dominant control over tree ring 13C at these high latitude, moist sites is photosynthetic rate rather than stomatal conductance. Summer temperature reconstructions based on three different calibrations are presented, with verification based on a mixture of jacknife and split period designs, providing robust and near identical results. Reconstructed late summer temperatures in the early 1900s are very low but the years centred around AD 1660 and 1760 appear to have experienced warmer summers than the late-twentieth century, thus our late summer reconstruction does not show a recent warming trend. These results are in agreement with other palaeoclimate reconstructions for northern Fennoscandia, which show late-twentieth-century warming occurring predominantly in the winter. The results suggest that, where replication and common signal strength are sufficiently high, stable carbon isotope dendroclimatology may provide high resolution proxy time series that also record climate information at lower temporal frequencies, thus avoiding the `segment length curse' that can apply to palaeoclimate reconstructions based on other tree ring parameters such as ring widths and density.

Pollen analyses of a peat profile collected from Butterburn Flow, northern England have been used to reconstruct vegetation and land-use change from the late Neolithic (c. 3230 cal. BC) to the present day. `Wiggle-matching' of 42 AMS 14C dates has enabled a precise (decadal scale) chronology to be established, and the results are interpreted within the context of previous studies of vegetation change in the area. Three Late Neolithic--Bronze Age woodland clearance phases occurred, with the first beginning c. 2290 cal. BC. Cleared areas were utilized for pastoral and limited arable agriculture. Late Iron Age clearance and agricultural intensification began at Butterburn Flow c. 300 cal. BC; the peak period of clearance c. 90 cal. BC--AD 50 cal. was comparable in intensity with that in the fourteenth century AD. Farmland in the area was abandoned during the period c. AD 90--450 cal., contemporaneous with the Roman occupation of the region; following Roman withdrawal, a resurgence of agriculture occurred. The timing of later periods of agricultural decline relate to climatic deterioration, political instability and disease in the region.

Lakes created by retreating ice at the end of the last glaciation underwent rapid acidification during the first few thousand years of their existence, a phenomenon attributed in part to progressive leaching of soil bases since it was discovered more than eighty years ago. Though a role for leaching is still acknowledged, the most recent studies see this as subordinate to the effects of biological and climatic changes initiated by deglaciation, chiefly primary vegetation succession and species immigration. Until now there has been no quantitative test of the relative importance of these different mechanisms. The author uses geochemical modelling of runoff acidity to show that the extent and timing of early Holocene lake acidification in eight published palaeoecological records can be explained by leaching of the calcium phosphate mineral apatite from the granitic till soils of their catchments, at a rate controlled by simple dissolution kinetic factors. This shows that a single progressive abiotic process is capable of explaining a natural phenomenon that is currently attributed to complex biological and climatic interactions. It is argued that the finding has important implications for our understanding of long-term lake ecosystem development because the mechanism is inherently irreversible, in contrast to the alternative ecological and climatic mechanisms which are not. Furthermore, the finding reinforces the view that long-term ecosystem modelling cannot safely neglect nutrient limitation, as is currently the practice in widely used global dynamic vegetation models.