Settling velocity and grain shape of maërl biogenic gravel

Abstract

The importance of a grain-size-dependent shape metric, convexity, for determining the unusual settling velocity characteristics of maërl, a variety of unattached coralline algae, has been quantified by modeling of settling-tube data. A modification of the general settling-velocity equation of Ferguson and Church (2004), involving a dependence of the drag coefficient-related constant, C2, on grain size, produces a satisfactory fit to the experimental observations. For a given grain size and at Reynolds numbers greater than , 220, maërl grains experience greater drag than is predicted for natural quartz grains by Ferguson and Church (2004) because of this grain-size-dependent roughness. Subsequent detailed measurements of maërl grain shape using microscopic image analysis confirm a strong positive linear relationship between grain roughness, quantified by the reciprocal of convexity, and grain size. This departure from the ideal settling characteristics of siliciclastic gravel is hypothesized to explain the observed propensity of maërl, under suitable hydrodynamic conditions, to form beach deposits with a low percentage of sand. Maërl samples from three different sedimentary environments (open marine, intertidal, and beach) exhibit different linear relationships between roughness and grain size, probably resulting from different degrees of abrasion due to a combination of different wave climates and transport histories. This spatial variability in grain texture suggests that a general equation for maërl settling velocity is not possible. However, for maërl, and other branched sediment types, it may only be necessary to measure the convexity of the middle and largest size fractions to estimate the linear variation of C2 with grain size, resulting in an accurate estimate of the settling-velocity curve. In the broader context of physical sedimentology, our results indicate that, over a range of bottom current conditions between 200 and 250 mm s21, where the settling curve of maërl is flat and grain-size invariant relative to siliciclastic sediment, a larger part of the maërl grain-size distribution can remain in suspension compared to the siliciclastic sediment. This contrast in physical properties may be an effective process for the spatial separation of coarse siliciclastic and biogenic sediment. Copyright © 2014, SEPM (Society for Sedimentary Geology).