Settling Velocity and Grain Shape of Maerl Biogenic Gravel

Abstract

Abstract The importance of a grain-size-dependent shape metric, convexity, for determining the unusual settling velocity characteristics of maerl, 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, maerl 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 maerl 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 maerl, under suitable hydrodynamic conditions, to form beach deposits with a low percentage of sand. Maerl 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 maerl settling velocity is not possible. However, for maerl, 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 s−1, where the settling curve of maerl is flat and grain-size invariant relative to siliciclastic sediment, a larger part of the maerl 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.