The Sand-Mud Transition: Observations and Environmental Controls on two Italian Mudlines

The textural and geochemical properties of surficial sediment were characterized on nearshore cross-shelf transects spanning the sand-mud transition (SMT) off the Tronto and Pescara rivers, Apennine Margin (Adriatic Sea), Italy. Measured parameters were geometric mean diameter (GMD), specific surface area (SSA), mud fraction (<63μ m), heavy metals concentrations (Cu, Pb, Zn), calcium carbonate content and clay mineralogy. The SMT on the Tronto River is identified between 15 20 m and parallels the coastline while the SMT associated with the Pescara River undulates between 20 25 m. GMD, SSA, and mud fraction were found to be highly correlated with depth. Depositional dynamics were examined with a recently developed parameter to describe the amount of material in sediments deposited as flocs, floc fraction in sediments, f_s, which also correlated significantly with depth and the sediment properties. A spatial analysis of the Pescara SMT during 35 years of reduced suspended sediment supply showed only slight changes to the depth of the SMT. This observation and the strength of the correlations suggest that wave-generated shear stress is the primary control on the depth of the SMT. The three metals clearly indicate sharp increases across the Apennine SMTs but do not show anthropogenic enrichment. The metals also correlated significantly with f_s, which confirms deposition of contaminants by flocs and suggests energy levels influence seabed metal distributions. Investigation of the relationship between wave energy and SMT depth in 10 diverse coastal environments showed the depth of the SMT scales linearly with significant wave height.