Seasonal and inter-annual sedimentation in meromictic Lake Shira (Siberia, Russia) during disturbance of meromixis

Abstract

Detailed studies of processes of sediment-record formation are necessary for accurate sediment-derived paleo environmental reconstructions. In order to understand the deposition of sediments in meromictic Lake Shira (southern Siberia, Russia), we conducted a multi-year seasonal sediment trap study of sediment fluxes of chemical elements, organic and inorganic carbon, total nitrogen and photosynthetic pigments for a period from 2012 to 2017. In 2015 and 2016, a rare event of transition from meromixis (i.e. long-term hypolimnetic anoxia) to holomixis was observed, which was accompanied by the temporary disappearance of hydrogen sulfide from the water column in spring and a decrease in hydrogen sulfide in other seasons compared to the meromictic state. In general, we did not reveal any noticeable differences between the meromictic and holomictic states in the composition of sediments, except for okenone. The content of okenone, a specific carotenoid of purple sulfur bacteria, decreased in traps during the holomictic state and did not considerably increase two years after the meromixis restoration. Using a freeze-corer, we observed that the content of okenone in the uppermost 3-mm layer of sediments also decreased markedly from 2013 to 2017, as well as in traps. The sedimentation flux of molybdenum reflected the seasonality of hydrogen sulfide content in the water column, and it was maximal in winter, in contrast to all other sediment components. Other components reflected the seasonality of organic production and terrigenous inputs, with the maximum in late summer and autumn. An increase in the sedimentation rate of terrigenous elements in the autumn of 2012 was supposedly caused by the anomalous amount of precipitation in August 2012. Since detailed inter-annual observations of sedimentation processes are not often found in limnology, we hope that our results will be useful for reconstructing regional climate and, further, the understanding of sediment dynamics of stratified temperate water bodies.