Sediment sizes and sources in the cool-water, coastal environment of Adelaide, South Australia

Abstract

Coastal environment sediments in temperate, cool-water systems were assumed to be siliciclastics until the latter third of last century. This is not surprising as quartz grains dominate most beach sands. Scientists, delving below the sea-surface since ~1980, discovered carbonate grains increase dramatically with water depth. Physical parameters such as tides, storms and currents cause mixing, transport and weathering of the entire package of grains. Sea-floor morphology further alters the ill-conceived perception of a flat surface veneered with even sized, similar grains. Chemical weathering and minor biogenic predation cause further disruption, with anthropogenic activity impinging on the natural cycle in an alarming manner and rate. This project studied 295 samples from the entire beach to 20 m water depth from 22 transects orthogonal to the beach, along the 100 km coastline adjacent to the city of Adelaide, South Australia. Results showed sediment grain-size heterogeneity is widespread, as a result of differences in mineralogy and source, particularly of the biogenic carbonate grains. This has implications for successful beach management strategies. Modern society expects access to pristine beaches during its leisure time, yet industry expects to continue using the sea as a “rubbish dump”. Education concerning the fragility of the shallow sea-floor environment and the sedimentary cycle is urgently needed.