TEXTURAL CONTRIBUTION OF BIOGENIC AND INORGANIC COMPONENTS TO SAND FRACTION WITHIN A MIXED-SEDIMENT LAGOONAL SEQUENCE, CAPE MAY, NEW JERSEY

The sedimentological analysis of particles exhibiting varying shapes often produces different results based on the type of the instrument (sieving, settling tube, or a laser particle-size analyzer). Two sediment cores (5.7-7.0-m-long) recovered from a backbarrier wetland at Cape May, New Jersey, were analyzed on a Camsizer particle analyzer to assess the contribution of various sediment fractions to the sand-sized population. In a total of 55 subsamples, the sand content ranged from 3 to 84%. The mean grain size of the sand fraction varied from 2.28 to 2.97 φ and was negatively correlated with particle sorting. Skewness values of -0.61 to -0.05 indicate the depleted fine fraction (mostly quartz) in nearly all samples. In addition to granulometric statistics, the sphericity values were used to examine the origin of particles exhibiting different shapes. In the coarsest fraction (>1 mm), the elongated components (sphericity < 0.50) correspond to biogenic fragments (rhizome and root fragments). Because vegetation remains are not likely related to the deposition of the sand (e.g., storm surge horizon), statistical removal of this fraction allows for a more accurate representation of the original grain-size distribution. Sea urchin spines, gastropod shells, and saltmarsh rhizomes were added to several previously characterized samples in order to isolate the textural contribution of known particles. In all cases, the biological components were clearly distinguishable from the background sand based on their low (0.2-0.6) sphericity value. The compositional analysis of the finest fraction (very fine sand) is currently underway, but it is likely that any substantial decrease in sphericity will reflect the enrichment in heavy minerals. In contrast, micaceous minerals, if captured in plan view by dual-camera analyzers, will cause an overestimation of hydrologically equivalent fraction. This study demonstrates that an initial assessment of particle origin (biogenic vs. minerogenic) can be achieved through the examination of statistical parameters generated by particle-size analyzers.