Radiocarbon dating and absolute chronologies

The development of radiocarbon dating by Willard F. Libby (1908-1980) in the early 1950s provided a means of deriving an absolute chronology (in theory at least!) for events in the Holocene. Harry Godwin (1901-1985) was one of the first Holocene paleoecologists, along with Eric Willis, Donald Walker, and others, to take advantage of radiocarbon dating to provide an absolute chronology for pollenzone boundaries, peat-stratigraphic changes, and other events in the Holocene (Godwin 1960) and to discuss the implications of radiocarbon dating for Holocene climate research (Godwin 1966). A fascinating account of its early applications in Holocene research is given by Godwin (1981). As a result of a concerted dating program, Smith and Pilcher (1973) showed that major pollen-stratigraphic changes and pollen-zone boundaries were not synchronous, even within an area as small as the British Isles. A similar picture emerges at broader scales such as Europe (Huntley and Birks 1983; Berglund et al. 1996) or eastern North America (Webb 1988), raising serious doubts about the assumed synchroneity implicit in the von Post paradigm of Holocene climate history.

As more and more radiocarbon-dated pollen-stratigraphic data became available in Europe and eastern North America, attempts were made to display the spatial and temporal patterns of variation in the abundance of major pollen types, along the lines pioneered by Szafer (1935) in Poland. By mapping pollen values at selected time intervals from as many localities as possible, so-called iso-pollen maps (Szafer 1935) could be constructed. Such maps have been compiled at a range of spatial scales, ranging from the European continental scale (Huntley and Birks 1983) to single countries (e.g. Ralska-Jasiewiczowa et al. 2004). An alternative mapping approach, so-called iso-chrone maps, has also been developed (e.g. Moe 1970; Davis 1976; Birks 1989) to display the spatial patterns of the ages at which different pollen types have their first consistent occurrence or expansion, thereby illustrating the patterns of range expansion and contraction. Results from these early studies and from more recent, more detailed syntheses (e.g. Giesecke and Bennett 2004) suggest unexpectedly complex patterns of tree spreading, with trees apparently spreading at amazingly fast rates from a range of presumed source areas. The results raise many questions about the interpretation of pollen-analytic data, about what factors may have caused major pollen-stratigraphic changes (Giesecke 2005; Tinner and Lotter 2006), about how trees spread over large areas so quickly (McLachlan et al. 2005), and whether tree assemblages existed in the past that appear to have no modern analog today (Jackson and Williams 2004).

An important development in Holocene climate research involved a combination of pollen analysis and radiocarbon dating to study pollen-stratigraphic changes in space and time. This development took advantage of natural climate gradients and major vegetational ecotones (e.g. the prairie-forest ecotone, arctic tundra-forest ecotone) to study Holocene palynologic and hence vegetational changes in areas of potentially high sensitivity to climate change. A classic example of such a study was McAndrews' (1966) transect of sites near Itasca in north-west Minnesota. The transect ran from mixed coniferous-deciduous forest in the east, through deciduous forest and oak savanna, to prairie in the west and paralleled a major climatic gradient in precipitation today. McAndrews (1966) was able to show major changes in vegetation and infer major shifts in precipitation and soil-moisture deficit during the mid-Holocene "prairie period" after major vegetational and temperature changes at the onset of the Holocene. This research design of sites along major climate gradients has also been elegantly applied in studies in northern Fennoscandia using not only pollen percentages but also pollen concentrations and accumulation rates to detect Holocene changes in the northern extent of pine and birch, presumably in response to regional climate changes (Hyvarinen 1975; Seppa 1996).

By the early 1970s, Holocene researchers had accumulated large amounts of pollen-stratigraphic and other biologic data, often with radiocarbon dates providing an independent chronology. Climate reconstructions based on these data were primarily qualitative and based on indicator species or a comparative approach where modern and fossil assemblages were compared visually. Climate reconstructions were often molded into the paradigms of Holocene climate change derived from von Post (1946) or from the original Blytt and Sernander scheme. The next major paradigm shift in Holocene climate research was the development of transfer functions and the quantitative reconstruction of past climate.

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