Modern sedimentary analogues and integrated monitoring to understand varve formation in the Mediterranean Lake Montcortès (Central Pyrenees, Spain)

Abstract

Varved sediments provide unique opportunities to carry out high-resolution paleoclimatic and paleoenvironmental reconstructions with accurate time control. To better interpret the sediment record it is necessary to understand the physical, chemical and biological factors that influence varve formation and preservation. We explored the link between the annual limnological cycle and current varve deposition in the oligotrophic hardwater Lake Montcortès (Central Pyrenees). The varves of this lake consist of couplets of dark organic and light calcareous laminae. A two-year limnological monitoring (10/2013–10/2015) combined with a sediment trap study were conducted at monthly resolution. Limnological and sedimentological measurements were compared with meteorological data. Although the lake was considered meromictic in the first limnological studies, we documented total mixing of the water column both winters. In spite of this, long periods of stratification and hypolimnetic anoxia create suitable conditions for varve formation and preservation. Sediment deposition followed a clear seasonal pattern related to biological processes in the euphotic zone. During summer and fall, calcite precipitation was favored by high calcite saturation indices and enhanced primary production that promoted relatively high pH values as a result of CO2 uptaking. There was considerable variability in the amount of calcite deposition between years, which was linked to seasonal temperature differences. In addition, calcite crystal sizes and diatom fluxes showed seasonal patterns related to calcite saturation index and changes in water stratification, which in turn were also related to temperature variability. Seasonal sedimentation patterns were strongly linked to primary producers and especially sensitive to temperature shifts. It results in a clear seasonal signal and varve formation. We compared our results with previous sedimentological interpretations of the varved record of this lake. This study improves the interpretation of Lake Montcortès sediment record extending back several millennia.