Diverse erosional indicators along a rapidly retreating Holocene strandplain margin, leeward Hiiumaa Island, Estonia

Abstract

A diverse suite of erosional features along the shoreline of Lehtma (Hiiumaa Island, Estonia) is used to assess their morphodynamic context and preservation potential. The Holocene strandplain along the east-facing (leeward) shoreline has experienced rapid retreat due to the anthropogenically induced sediment deficit. The study site is located just updrift of the erosional-depositional fulcrum segment, with southerly longshore transport resulting in the accumulation of eroded sand along a drift-aligned spit. The most prominent erosional indicators are mature pine trees in different stages of undercutting, toppling, fragmentation, and burial. Morphological features include scarps in paleo-beach/dune ridges (height: > 1 m), as well as modern berm scarps. Mineralogical indicators are exemplified by heavy-mineral concentrations (HMCs) of variable thickness (some > 2 cm) and concentration. Representative samples show a substantial increase in bulk low-field magnetic susceptibility ranging from < 10 µSI common to the nearby Holocene coastal lithosomes to > 8,000 µSI in second-cycle HMCs within berm enrichment zones. A conceptual morphosedimentary model describing the recent and current state of the system, as well as the preservation potential of specific structures and recognition in geological research (e.g., georadar signatures of buried trees and HMCs) is proposed. The approach presented in this study can be used to assess the distribution and preservation potential of erosional indicators along the Baltic Sea coast and mineralogically heterogeneous, forested sandy shorelines worldwide.