Long-term frost weathering rates of limestone beach clasts, Fårö Island, Central Baltic Sea

Abstract

ABSTRACT The known post-glacial isostatic land rise rates at Fårö Island, Central Baltic Sea, allowed for the reliable estimation of frost weathering rates of limestone clasts along five beach profiles. Weathering rates measured by clast dimension change were quantified over periods ranging between 640 and 2090 years. While the deposited beach materials at each profile have varying lithostratigraphic compositions these attributes appear to have no direct influence on the long-term comminution rates. Mean clast size of initially deposited beach materials is suggested to be primarily a function of local wave energy and sourced from local rock platforms. Field observations, climate data and literature point to the beach materials in the upper 10–20 cm being fractured under an active diurnal to short-term frost environment that has persisted over the past 1000–2000 years. Pore water freezing by 9% volumetric expansion and the development of pore water expulsion and hydrofracture is suggested to increase pore volume and pore interconnectedness over time and facilitate ice segregation growth. Observed modes of clast fracture, bursting, flaking and granular disaggregation suggest different frost weathering mechanisms to operate on different limestone lithologies. Long term rates of break down appear however not differentiated by limestone composition.