Sedimentology and geochemistry of an urban coastal lake system: Coombabah Lake Nature Reserve, Gold Coast, Queensland

摘要

A study was initiated to address environmental concerns associated with changes in land use in the catchment area of Coombabah Lake, a brackish coastal lake system located in southeast Queensland. Sedimentological and geochemical data derived from a series of cores that penetrate the ca. 0–6000 year-old lacustrine sequence indicate that throughout much of its history, Coombabah Lake has remained a quiet, shallow, water body fed by fine-grained sediment dropped from suspension. Discrete and laterally continuous, shelly horizons form the basis for the stratigraphy developed for the lake sequence. A lithological transition in the upper 50 cm of the sediment column, from mud to shell-rich, sandy mud, suggests that the depositional regime changed ~500 years ago as accommodation space was filled. At this time, the dominant sedimentary processes changed to include reworking, partial bypass of sediment, the intermittent concentration of shell debris by winnowing, and the development of a flood-tidal delta complex in the lower reaches of the lake. Sharp increases in total organic carbon, total nitrogen, and total phosphorus concentrations below the lithological transition at 50 cm depth suggest that the capacity of the system to preserve organic matter and trap nutrients decreased significantly after the transition in depositional regime. Results indicate that current conditions in Lake Coombabah are governed by natural processes associated with the long-term evolution of coastal lake systems, and reveal little to suggest that there were any adverse affects resulting from recent human activities in the surrounding catchment.