Comparison of radiocarbon dating of buried paleosols using arbuscular mycorrhizae spores and bulk soil samples

Abstract

Ten paleosols from four separate soil pits located in Kärkevagge, a glaciated trough in Swedish Lapland, were dated using radiocarbon. Each soil was dated using both conventional bulk soil organic material (SOM) and a pure sample of arbuscular mycorrhizae (AM) fungal spores. The latter are produced by ubiquitous mycorrhizal fungi associated with the roots of many plant genera and may be viewed as a fossil material that has not interacted with any soil constituent subsequent to its emplacement in the soil — at a time presumed to mark the cessation of a favorable soil-forming environment. Regional deglaciation is presumed to have been about 10 000 BP, while a cosmogenic exposure date obtained from the valley floor in Kärkevagge dated at 13 100 ± 1638 BP. The youngest paleosol, buried at ~6 cm in soil pit M3, produced a spore date of 0—281 cal. yr BP (1σ). However, bulk SOM dates of the same paleosol A horizon gave widely divergent dates and varied with the sample pretreatment, ie, the combustion temperature and the acid-base treatment. For example, the bulk SOM dates for that paleosol ranged from a post-bomb date of 0—314 cal. yr BP (1σ) to 2366—2710 cal. yr BP (1σ) when subjected to different pretreatments (acid only, acid-base-acid) and ignition temperatures (400, 800, or 900°C). The oldest paleosol in the set, buried at ~61 cm in soil pit M6, dated at 5479—5698 cal. yr BP (1σ) using spores, but beyond calibration using bulk SOM. The spore dates were all within the range to be expected of postglacial paleosols, but the bulk SOM dates were frequently beyond the generally accepted time of deglaciation. In addition, all of the spore dates followed a conventional age/depth pattern while the bulk SOM dates did not. There are known possible sources of geogenic carbon contamination in Kärkevagge which may well account for the obviously invalid older bulk SOM dates. An additional complication is that the spore dates vary somewhat with their density and diameter. However, where other types of fossil or charcoal are unavailable it appears that the enormously broad distribution of spores and their lack of interaction within the soil and persistence may well offer the prospect of an unusually useful radiocarbon dating medium within paleosols.