Controls on Whiting Event Triggering at the Interface between Lake Geneva and the Rhône River

Abstract:

Whiting events are transient phenomena commonly occurring in alkaline and hardwater lakes that manifest as a chalky blue coloration of surface waters due to massive calcium carbonate precipitation. Although the bio-physical drivers of carbonate precipitation are theoretically known, their relative contributions in controlling the spatial and temporal extent of whiting events remain poorly understood. In Lake Geneva, Switzerland, conditions for calcite precipitation are usually met during summer while whiting events are only reported for restricted time-periods at the interface between lake waters and the Rhône River. In this study, we aim at identifying the mechanisms responsible for a specific whiting event that started in June 2019 at the river inflow and spread along the northern lake shore during four weeks, as observed from satellite images. Based on spatially-resolved data collected during two sampling surveys along with isotopic analyses and geochemical modeling, we show that authigenic calcite precipitation during whiting conditions is triggered by the mixing of cold snowmelt-diluted sediment-rich river water with warmer lake surface layers. Moreover, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy analyses confirm the advection of the fine-grained sediment fraction by river interflow throughout the lake metalimnion, and an enrichment in authigenic carbonates due to settling of heavier detrital particles and potential biologically-induced precipitation. Altogether, these results provide a refined mechanistic understanding on the conditions controlling the dynamics of whiting events and outline their recurrent and substantial contribution to carbonate cycling in Lake Geneva.