Holocene coastal lowlands in NW Europe: autocompaction and the uncertain ground

Abstract

Abstract The continuous, progressive, irreversible and asympototic processes of sediment autocompaction play a significant but so far largely neglected role in determining the coevolving character of Holocene landscapes and sedimentary sequences formed in coastal marshlands. Autocompaction ensures that actual total sediment deposition rates are always greater than the rate at which sea-level rise creates accommodation space. High rates of autocompaction help to foster resource-rich, mineralogenic tidal marshes that can be readily exploited by humans from neighbouring higher ground, for example, dryland areas, or locations within the marshland where peat formation had persisted sufficiently late that domal, raised bogs had not yet completely collapsed. Subsequent autocompaction may bring about a significant inversion of relief. The general effects of autocompaction are a high degree of vertical stratigraphic distortion and displacement amounting to as much as a few metres, depending on the depth to ‘basement’, the lithologies present, and their order of deposition. Autocompaction results in inaccurate sea-level curves when based on intercalated peats, inaccurate rates of sea-level change when calculated on a bed-averaged basis, and inconsistent age-altitude relationships among archaeological sites preserved in coastal deposits. The challenge is to predict/retrodict autocompaction in Holocene coastal lowlands, in order to better understand sequence development, the evolution of surface environments, topography and landscape, and the constraints placed upon (and opportunities presented to) human communities living in these contexts.