Timing and style of deglaciation of northeastern Poland from cosmogenic ³⁶Cl dating of glacial and glaciofluvial deposits

Determining the age of glacial and glaciofluvial deposits is necessary to better understand the deglaciation of northeastern Poland. Cosmogenic ³⁶Cl, accumulated in boulders and matrix, suggests that inheritance and erosion affect cosmogenic inventories and must be taken into account when calculating exposure ages of landforms. A simple approach to detect and distinguish between the two effects, by comparing the inventories of ³⁶Cl in boulders and matrix, allows us to compute appropriate corrections to apparent (uncorrected) ages and to determine model (corrected) exposure ages of the deposits. Apparent cosmogenic ³⁶Cl ages fall in the range between 11 ky and 28 ky (1 ky = 1000 calibrated ³⁶Cl years), pointing, correctly, to the end of the last glaciation, and correlate with oxygen isotope stage 2 (OIS 2). Model ages of glacial and glaciofluvial deposits fall into one of three time intervals. The oldest erratics, 27-28 ky (Kruszki and Bachanowo 1), date the advance of the Weichselian ice sheet. They are older than the surrounding surfaces, which confirms the existence of nunataks during later phases of the last glaciation. The main belts of recessional moraines formed 19.7 ± 1.0 ky ago (Gremzdy Polskie) and 17.9 ± 1.3 ky ago (Łopuchowo 2 and Gulbieniszki), and the last ice melted 14.4 ± 1.0 ky ago (Łopuchowo 1). Erosional terraces in the Czarna Hańcza valley were formed 14.7 ± 0.9 ky ago (Bachanowo 2) and 14.4 ± 1.0 ky ago (Bachanowo 3), probably by the melt waters from the last ice in the area. Our results suggest that different parts of the southern margin of the Fennoscandian Ice Sheet advanced and retreated independently, supporting the idea that the deposits at the southern margin of the Scandinavian Ice Sheet are not synchronous.