Middle and Upper Palaeolithic Environments and the Calibration of 14C Dates beyond 10,000 BP

Article excerpt

Advances in our understanding of the Quaternary history of the earth's magnetic field provide the means to correct the radiocarbon time-scale for long-term (millennia) deviations from the calendrical one beyond the upper limit of the tree-ring-based calibration. The conversion is essential when Middle and Upper Palaeolithic archaeological sites are to be placed within the context of the complex climatic history of the last glacial interval and following deglaciation.

Recent years have brought the recognition that glacial and interglacial climates are prone to rapid and often large oscillations lasting from a few hundred to several thousand years. For the Holocene period radiocarbon dates calibrated with tree-ring data allow us to take account of such events on a calendrical time-scale, but tree-ring calibration of 14C dates is not yet possible beyond the Holocene. Therefore, the Pleistocene climatic record, resting largely on ice and ocean sediment cores which use nearcalendrical time, could not be compared to the Middle and Upper Palaeolithic history which is based mainly on uncalibrated 14C dates. Now recent studies of the temporal variations of the earth magnetic field have furnished the means to adjust 14C dates for long-term deviations of the radiocarbon time-scale from the calendrical one, a procedure I call here correction to distinguish it from the familiar tree-ring based calibration of Neolithic and later dates.

The unstable climate of the last glacial

Between c. 50 and 25 kyr (kiloyear = 1000 years) BP, during oxygen isotope stage 3 (OIS-3), the Neanderthals in Europe were replaced by anatomically modern human beings, a momentous event that is often contemplated against a backdrop of vast ice sheets and barren tundra, a harsh landscape seen as poor in resources. But such conditions characterized only a minor part of the last glacial interval, and the middle pleniglacial (c. 60-25 kyr BP) especially, although bracketed by the initial and final glacial maxima [ILLUSTRATION FOR FIGURE 1: left OMITTED], was much less hostile than either. Recent publications (Miskovsky 1992; Gamble 1993; Stiner 1994; Kuhn 1995; Broglio 1996; Kozlovski 1996; Mellars 1996a; 1996b) display a growing awareness of the quite moderate climate of the mid-glacial, but the true nature of its palaeo-environments and their spatial and temporal variations is far from clear. Consequently, different authors have reached different conclusions regarding the impact of changing environments on later Palaeolithic human history (e.g. Gamble 1986; 1987; Roebroeks et al. 1992).

That the middle pleniglacial was mild, representing a state somewhere between a full glacial and an interglacial, has been obvious for some time from the oceanic oxygen isotope record, but until recently it was regarded as having also enjoyed a fairly stable climate. Now detailed studies of Greenland ice and North Atlantic sediment cores have proved this view to be seriously in error [ILLUSTRATION FOR FIGURE 1: right OMITTED]. Instead, 15-20 sharp climatic events, as much as 7 [degrees] C warmer than the brief intervening cold spells, define a bipolar climatic sequence that was unstable on a scale of centuries to a millennium for cold events and from one to three millennia for warm ones (Bond et al. 1993; Dansgaard et al. 1993; GRIP Members 1993; McManus et al. 1993). Quantitative temperature and precipitation estimates based on pollen and coleoptera data (Guiot 1990; Ponel 1995) from central France imply mild, wet periods lasting several millennia that reached near-present temperatures (local annual mean 7 [degrees] C compared to 11 [degrees] C) and precipitation levels around 500-600 mm [ILLUSTRATION FOR FIGURE 2 OMITTED]. During the intervening colder, much drier events which usually lasted only a few centuries, mean annual temperatures were around 0-2 [degrees] C, close to values for the preceding glacial advance (Guiot et al. 1989).

Coleoptera from warm mid-glacial deposits in London dated at c. …