Integrating field and laboratory approaches for ripple development in mixed sand–clay–EPS

Abstract

The shape and size of sedimentary bedforms play a key role in the reconstruction of sedimentary processes in modern and ancient environments. Recent laboratory experiments have shown that bedforms in mixed sand–clay develop at a slower rate and often have smaller heights and wavelengths than equivalent bedforms in pure sand. This effect is generally attributed to cohesive forces that can be of physical origin, caused by electrostatic forces of attraction between clay minerals, and of biological origin, caused by ‘sticky’ extracellular polymeric substances (EPS) produced by micro‐organisms, such as microalgae (microphytobenthos) and bacteria. The present study demonstrates, for the first time, that these laboratory experiments are a suitable analogue for current ripples formed by tidal currents on a natural mixed sand–mud–EPS intertidal flat in a macrotidal estuary. Integrated hydrodynamic and bed morphological measurements, collected during a spring tide under weak wave conditions near Hilbre Island (Dee Estuary, north‐west England, UK), reveal a statistically significant decrease in current ripple wavelength for progressively higher bed mud and EPS contents, and a concurrent change from three‐dimensional linguoid to two‐dimensional straight‐crested ripple planform morphology. These results agree well with observations in laboratory flumes, but the rate of decrease of ripple wavelength as mud content increased was found to be substantially greater for the field than the laboratory. Since the formation of ripples under natural conditions is inherently more complex than in the laboratory, four additional factors that might affect current ripple development in estuaries, but which were not accounted for in laboratory experiments, were explored. These were current forcing, clay type, pore water salinity and bed EPS content. These data illustrate that clay type alone cannot explain the difference in the rate of decrease in ripple wavelength, because the bed clay contents were too low for clay type to have had a measurable effect on bedform development. Accounting for the difference in current forcing between the field and experiments, and therefore the relative stage of development with respect to equilibrium ripples, increases the difference between the ripple wavelengths. The presence of strongly cohesive EPS in the current ripples on the natural intertidal flat might explain the majority of the difference in the rate of decrease in ripple wavelength between the field and the laboratory. The effect of pore water salinity on the rate of bedform development cannot be quantified at present, but salinity is postulated herein to have had a smaller influence on the ripple wavelength than bed EPS content. The common presence of clay and EPS in many aqueous sedimentary environments implies that a re‐assessment of the role of current ripples and their primary current lamination in predicting and reconstructing flow regimes is necessary, and that models that are valid for pure sand are an inappropriate descriptor for m...

The role of microphytobenthos in soft‐sediment ecological networks and their contribution to the delivery of multiple ecosystem services

“…The cohesive nature of EPS reduces both bedload transport and bedform development such as ripple formation and size (Baas et al, ; Lichtman et al, ; Parsons et al, ), which can significantly affect large‐scale geomorphological features of sediment. Biostabilization, by altering sedimentary properties and processes, can lead to the formation of large ridge and runnel systems (Weerman et al, ) and state changes in sedimentary habitats (Van De Koppel et al, ).…”

Discontinuity in Equilibrium Wave-Current Ripple Size and Shape Caused by a Winnowing Threshold in Cohesive Sand-Clay Beds

“…Additionally, clay winnowing, a hydrodynamic sorting process which suspends the finer clay but leaves the coarser sand in the bed (e.g., Cizeau et al, 1999), played a significant role in the transformation of ripples in mixed sand-clay to an increasingly sandy composition in the experiments of Baas et al (2013) and Wu et al (2018). Importantly, Baas et al (2019) has recently highlighted the role of bed cohesion in decreasing current ripple dimensions in the Dee Estuary, U.K., demonstrating that previous laboratory findings are applicable in natural environments. There has, however, been very little research on the dynamics of ripples in mixed sand-clay beds under combined wave-current flows, which are crucial to the sediment dynamics in the majority of estuaries and coastal seas.…”

Interactions between sediment microbial ecology and physical dynamics drive heterogeneity in contextually similar depositional systems

“…7B). Laboratory (Malarkey et al 2015; Parsons et al 2016) and field investigations (Lichtman et al 2018; Baas et al 2019) have recently illustrated the influence of low EPS contents distributed deeper into the sediment bed. In these investigations, microbially produced EPS hampered sediment transport, bedform development, and bedform migration without the presence of a visible biofilm on the surface (see also Chen et al 2017).…”