Cyclicity in ridge patterns on the prograding coasts of Estonia

Abstract

Recent LiDAR surveys have revealed that on postglacially uplifting coasts of Estonia rhythmic coastal landforms (beach ridge sequences and foredune plains) occur to a considerable extent. We studied four of them to reveal age and periodicity in these multiple ridge systems and discussed their genesis in the Subatlantic (semi‐continental) conditions of the Baltic Sea area. Using recent models of Fennoscandian uplift due to glacial isostatic adjustment (GIA), we constructed Holocene apparent sea level curves for the study sites at Õngu, Mänspe, Haldi and Keibu; converted distance–height shore profiles into time series (including corrections on shore profile non‐linearity and variations in GIA‐eustasy balance); and analysed the patterns using spectral analysis. It was suggested that due to non‐tidal conditions, relatively low‐energy hydrodynamic forcing and small aeolian contribution, the ridges mark ancient shorelines. They are relatively modest in height (mostly 0.2–1 m), form regular and extensive (up to 150 ridges) patterns, and date back to ~9000 years before present. We studied <5‐ka‐old sections. The mean ridge spacing varied, depending also on coastal slope, between 21 and 39 m. Both simple counting and spectral analysis involved some specific limitations, yet the estimates for typical spacing were alike, at 32 (±5) years. The regular nature of the low‐ridge patterns originated from relative sea level lowering and gradual sediment accretion/erosion. However, the progradation was rather uplift‐ than accretion‐driven and the stepwise process in ridge formation was probably not autocyclic. It was governed or modulated by quasi‐periodic 25–40 year cyclicity in local wave forcing, relative sea level variations and wind conditions. Being most likely connected to the North Atlantic Oscillation, the quasi‐regular, decadal‐scale, similarly phased variations may magnify each other's effect on the westerly exposed coasts of Estonia. Additionally, some other (e.g. event‐driven) mechanisms may also be present.

Holocene relative shore level changes and Stone Age hunter‐gatherers in Hiiumaa Island, eastern Baltic Sea

“…Absolute average uplift rate due to GIA is estimated to be 3.7 mm a −1 in the Kõpu area for the last 6000 years (Suursaar et al . 2019), thus the 0.4 mm a −1 difference between absolute rates and RSL rates can be attributed to concurrent ocean rise at about 2–3 m (Lambeck et al . 2014).…”

“…The location of the island, relatively fast glacial rebound (currently ˜ 3.2–3.3 mm a −1 ; Suursaar et al . 2019; Vestøl et al . 2019), strong winds and stormy waves have favoured the development and preservation of a mosaic topography of coastal landforms.…”

Late-Holocene relative sea-level changes and palaeoenvironment of the Pre-Viking Age ship burials in Salme, Saaremaa Island, eastern Baltic Sea

“…The island’s main landforms are inherited from the last glaciation and have been later reworked by the Baltic Sea, which is almost non-tidal (Leppäranta and Myrberg, 2009). The area is characterised by a slow postglacial isostatic rebound with absolute uplift relative to the geoid of about 2.3 mm/year (Suursaar et al, 2019; Vestøl et al, 2019).…”