A relict landscape in the centre of Fennoscandian glaciation:: cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles

The presence of well-developed tors, boulder fields, and weathering mantles in the Parkajoki area of northeastern Sweden, near the centre of Fennoscandian glaciation, has been used to suggest that these landscapes were preserved during all glacial cycles since ice-sheet initiation in the late Cainozoic. This implies that all successive large-scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation. Cosmogenic Be-10 and 26 At data from three tors and a meltwater channel in the Parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles. Apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at similar to 11ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion. Single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles. This estimate is based on (i) the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the DSDP 607 marine benthic foraminifer oxygen isotope record, in conjunction with (ii) a record of Fermoscandian ice sheet flow traces (and hence, ice sheet extent), and (iii) noting that cosmogenic nuclides are accumulated only during ice free periods. In addition, mean cosmogenic Al-26/Be-10 concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1.6 in Ma(-1). Thus, the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles. (C) 2002 Elsevier Science B.V. All rights reserved.