Sedimentation and time-of-transition techniques for measuring grain-size distributions in lagoonal flats: comparability of results

ABSTRACT A comparative study was performed of three instruments used to measure thegrain-size distribution of thirty sediment samples from shallow lagoonal flats:the hydrometer, the Sedigraph 5100 and the CIS-1. The hydrometer andSedigraph are based on sedimentation whereas the CIS-1 uses the time oftransition. The percentage of the samples accounted for by the <8 lm fractionwas not affected by the technique used, but this was not the case with the clayfraction (<2 lm). Due to its relative independence from the analytical methodapplied, the <8 lm fraction can be used in ternary diagram classifications. Thisfraction also has an environmental significance in coastal lagoons in terms ofhydrodynamics, organic enrichment and macrozoobenthos assemblages. Thelinear relationships obtained in this study may provide useful operationalindications for similar studies.Keywords Grain-size analysis, lagoon, mud, non-sortable silt, sortable silt,time of transition.INTRODUCTIONAlthough common standards of analysis have notyet been established by the scientific community,grain size and grain-size distribution are keyfactors in sedimentology and landscape evolution(Goossens, 2008). Sediment grain-size data arecommonly used for textural classification, but thedetails remain unresolved for lagoon sediments.Lagoons may display a large variety of sedimentpatterns depending on the relative strength ofwaves and tides (Nichols & Boon, 1994). Grain-size distribution is certainly one of the morehelpful tools for describing the environmentalconditions in lagoon systems, because fine-grained material correlates strongly withpollutants. In addition, variability in chemical,physical and hydrographical parameters is alwaysrelatedtovariationinsedimentgrain size(Kjerfve,1994). Particle dimensions therefore describeenvironmental conditions and provide informa-tion about processes acting on the ecosystem; theymay thus be regarded as an ‘environmental tracer’.For these reasons, it is very important to charac-terize the grain size of bottom sediments. Indeed,grain-size data are essential for the modelling andmanagement of lagoon environments.Flemming (2000) proposed an updated versionof the texture ternary diagram (Reineck & Siefert,1980; Pejrup, 1988), which increases the range ofapplication and the environmental sensitivity oftextural sediment classification. The classifica-tion incorporates a genetic element by distin-guishing between different hydrodynamicregimes. Although Molinaroli et al. (2009a,b)showed that the <8 lmandca 20 lm fractionscould be used to classify lagoonal sediments interms of their hydrodynamics, Chang et al. (2006,2007) showed that 8 lm is an important size limitin the Wadden Sea sediments, delimiting thetransition between cohesive flocs and aggregatesand non-cohesive single grains. The debateSedimentology (2011) 58, 1407–1413