Sediment in motion
Authors:   , van Hateren Johannes Albert 
Published: May 2023

This thesis is about creating a new decryption key for the sedimentary record and applying this key to unravel climatic events of the last glacial period. In chapter 2, we compare the currently available end-member modelling algorithms available for the unmixing of sediment grain size distributions. Some algorithms clearly outperform others. Furthermore, it is proven that the statistical approaches used to determine the number of subpopulations in a dataset may be misleading.
To (partially) overcome ambiguity of current methods for determination of the transport process, chapter 3 applies end-member modelling to combined grain size and grain shape data as a new tool to decipher sedimentary records. For each sediment sample, the size and shapes of all grains are compiled into one volumetric grain size-shape distribution (SSD). End-member modelling can be performed on a group of these SSDs. Application of the method to a modern beach-dune environment in the Western Netherlands results in three primary components: bedload on the beach and in notches that were dug by man through the shore-parallel foredune ridge, modified saltation on the windward and leeward slope of the intact foredune, and suspension in the vegetated hinterland. The three processes can be distinguished by their characteristic SSD. This “fingerprint” of the transport processes overcomes part of the ambiguity associated with prior methods.
The method is subsequently employed on aeolian deposits of the last part of the last Glacial period in the central and eastern Netherlands to reconstruct environmental and climatic conditions. The four resulting subpopulations are: coarse and fine bedload, modified saltation and suspension. Synchronous fluctuations in the proportion of the subpopulations at several study sites indicates that the severe glacial climate resulted in a polar desert environment during the Late Pleniglacial, shifting towards a wet sand sheet during the final phase which implies increased humidity and decreased wind speed. Loess was deposited during the Bølling due to increased vegetation density and humidity, followed by a re-opening of the landscape (return of bedload components) due to stadial conditions of the Younger Dryas. The Betula phase of the Allerød interstadial, is characterised by a humid sediment bed (large proportion of loess) reflecting moist climate conditions. The subsequent Pinus (pine) dominated phase was drier (more bedload and modified saltation), thought to relate to Pinus-induced drought and wildfires. The Younger Dryas stadial is similar to the Older Dryas. In the Middle Ages aeolian processes recommence due to intensive land use. Sediments from this period are set apart by a higher proportion of suspension and modified saltation, reflecting the wetter climate and denser vegetation of the Holocene.

The study area of chapter 5 is located in the southern Netherlands where the pan-European loess- and sandy aeolian belts meet. Mid-range loess deposition during the Middle Pleniglacial is a sign of humid, quiescent conditions and a northerly position of the loess belt, polar front and winter sea-ice limit. Development of a dry aeolian sand sheet in the Late Pleniglacial indicates southward displacement of the loess- and aeolian sand belts. Drier conditions and increased wind stress attributed to increased continentality, polar desert conditions and southward displacement of the polar front, enabled entrainment of sand by wind. The changed conditions were attributed to increased continentality and polar desert conditions due to a southward displacement of the polar front and sea-ice limit. The aeolian environments diversify during the stadial periods of the Late Glacial: dune complexes form on permeable substrates and sand sheet deposition continues on impermeable substrates. The tranquil conditions and increased vegetation density during the Late Glacial interstadials caused a decrease in bedload transport resulting in formation of loess deposits.