Quartz-grain microweathering amid Pleistocene-aged deep-seated relict permafrost in Central Europe (NE Poland)

Abstract

Deep-seated relict permafrost was identified in borehole Udryn PIG-1 (north-eastern Poland) at depths from +450 to 357 m. In potential its extant provides an opportunity to expand the known effective realm of grain weathering. Specifically, uncertainty exists as to whether or not glacial-period frost weathering could even occur at such considerable depths, which is well outside an active-layer range (i.e., beyond the currently acknowledged domain of grain weathering). Sand deposits collected at a 410.70–396.75 m depth were subjected to grain-size analysis, then 0.5–0.8 mm quartz-grain surfaces were analysed under stereoscopic and scanning electron microscopy. The obtained results indicate that these relict-permafrost grains were both mechanically and chemically weathered. Mechanical weathering of quartz grains is identified as P- & F-type weathering. The former is associated with an increase in stress generated both by overlying ice-sheet masses and permafrost degradation. Here mechanical weathering was facilitated by the presence of well-sorted sediment, rounded or well-rounded grains, and high porosity. Its effects are microtextures such as breakage blocks and conchoidal fractures. F-type weathering is/was generated by freezing water in various sorts of cavities and cracks. Weathering is directly associated with the presence of permafrost at this depth. Additionally, chemical weathering is recorded on quartz-grain surfaces in the form of a ragged discontinuous coating. It has been proven here that weathering of quartz grains is not just a near-surface phenomenon, but also occurs deep below Earth's surface.