Field and Laboratory Investigations of Bio-induced Calcium-carbonate Precipitation Mechanisms at the Origin of Whiting Events

Whiting events - the episodic precipitation of fine-grained CaCO₃minerals suspended in the water column - have been documented across a variety of environments, including both marine and lacustrine settings. Whitings could be a predominant source of carbonate muds, especially in the Precambrian, before the rise of green algae, a major source in the Phanerozoic, and are important archives for geochemical proxies of Earth history. While several biological hypotheses have been proposed to explain the onset of these precipitations, no consensus has been reached so far, and it is still unclear which process dominates in which environments. Our understanding of these mechanisms is however crucial to our understanding of the geological record.

We are performing both field and laboratory tests of different proposed models for biological CaCO₃precipitation involving cyanobacteria, diatoms, and viruses, as possible mechanisms for whiting events. Time-series field studies were initiated this year at Green Lake in Fayetteville (NY), which experiences a whiting every year initiated in the spring and persisting through the summer. Relevant geochemical parameters were analyzed in the water column, while suspended minerals, cells, and extracellular materials were collected and visualized using scanning electron microscopy and scanning transmission X-ray microscopy. Laboratory experiments aiming at reproducing different proposed (bio)mineralization mechanisms at the lake, including the novel hypothesis of viral lysis involvement, as well as investigations into the impact of interactions with biological materials on the Ca isotopic composition of carbonate minerals, are forthcoming.

Preliminary results suggest that two groups of plankton are involved in whiting events at Green Lake. As previously reported, Synechococcuscells are associated with large calcite grains. A new finding is the role of diatoms: small carbonate grains are ornamenting their siliceous tests, and larger grains are associated with diatom-exuded EPS. Monthly sampling reveals the evolution of the whiting in terms of water depth, water chemistry, biotic mediation, and crystal growth.