Cenozoic microfossils in northern finland: Local reworking or distant wind transport?

Allochthonous Cenozoic microfossils have been reported from Late Pleistocene lake and mire host sediments across an area of >30,000km² in northern Finland. Two main groups of microfossils are recognised: Palaeogene marine diatoms, silicoflagellates and ebridians that include taxa from around the time of the Palaeocene-Eocene Thermal Maximum and Pliocene to early Pleistocene freshwater diatoms. The presence of these microfossils has been regarded as evidence that Eocene marine and late Neogene freshwater sediments formerly existed on the shield surface. Both groups have been referred to frequently in reconstructions of the sea level, tectonic and erosion history of the northern Fennoscandian shield. The questions raised by the presence of allochthonous Cenozoic microfossils in northern Finland are, however, strongly resonant of the debate over the biota, origin and age of the Pliocene Sirius Group in Antarctica where competing hypotheses have been put forward of local deposition and reworking versus distant wind transport of marine diatoms from the continental shelf.This review explores alternative origins for the allochthonous Cenozoic microfossils in northern Finland. Local reworking of Palaeogene marine sediments during Pleistocene glaciation is unlikely, as no source rocks of Palaeogene age are known from the shield surface or from surrounding sedimentary basins in the Baltic and White Sea. Moreover, at all sites except Akanvaara, the marine diatom taxa cover wide age ranges and occur only as minor components in diatom assemblages that are dominated by Quaternary freshwater taxa. Local reworking of Pliocene-Pleistocene freshwater diatoms is, however, compatible with the widespread survival of pre-Pleistocene deep weathering although no in situ or unmixed, ice-rafted Pliocene-Pleistocene lacustrine sediment has yet been found. An alternative origin for the marine Palaeogene microfossils by distant wind transport is proposed. In this hypothesis, Palaeogene diatomites on the Barents Sea shelf were exposed to deep glacial and fluvioglacial erosion during the Pliocene and Early Pleistocene and in low sea level stages of the Middle and Late Pleistocene. Intense wind action acting on comminuted mudstones on outwash plains carried dust including microfossils into northern Fennoscandia to be deposited by rain-out in lakes and wetlands. This material may have been later further recycled by glacial and meltwater transport and more localised wind action, processes that also may help to account for the distribution of Eemian marine diatoms well beyond Eemian shorelines. The distant wind transport hypothesis implies that the presence of marine Palaeogene diatoms on the shield surface in northern Finland cannot be regarded as vestiges of former marine sediments and so do not constrain the tectonic and geomorphic history of the northern Fennoscandian shield in the Cenozoic.