Biogeochemistry of Benthic Microbial Mats and Laminated Sapropelic Sediments in Perennially Ice-Covered Lake Untersee, Queen Maud Land, Antarctica

Abstract

Perennially ice-covered Lake Untersee (Queen Maud Land, Antarctica) is an isolated freshwater ecosystem with thick, year-round ice cover preventing direct gas exchange with atmosphere and strongly limits light and external nutrient inputs. It is hypothesized that: (1) Lake Untersee is recharged primarily by subaqueous melting and subglacial meltwater of Anuchin Glacier that releases occluded air bubbles directly to the water column, together with air saturated water and that Lake Untersee receives gases in atmospheric origin and, (2) microbial mat morphology and depth-dependent light availability regulate photosynthetic carbon sequestration, such that flat mats store less organic carbon than conical microbialites and carbon storage decreases with depth. To evaluate these hypotheses, dissolved noble gases (He, Ne and Ar), dissolved O₂/Ar ratios and δ¹⁸O-O₂ were used to assess gas sources and the contribution of photosynthetic oxygen, while cores from the north and south basins (~10-100 m) were analyzed for stratigraphy, organic carbon abundance, C:N, organic carbon content and radiocarbon of microbial mats and laminated sapropelic sediments. Noble gas isotope ratios (³He/⁴He, ²²Ne/²⁰Ne, ⁴⁰Ar/³⁶Ar) were near atmospheric values throughout the oxic water column, supporting recharge by mixture of occluded air and air saturated water released during subaqueous melting of Anuchin Glacier and subglacial meltwater. O₂/Ar and δ¹⁸O-O₂ indicate a measurable photosynthetic oxygen contribution to the oxic water column, although gross photosynthetic rates remain low relative to other perennially ice-covered lakes of Antarctica. In the well mixed and oxygenated north basin, flat microbial mats store ~1-2.5 kg C m⁻² at shallow depths, decreasing to <1 kg C m⁻² near 100 m in parallel with declining photosynthetically available radiation (PAR). In contrast, conical microbialites at comparable depths store substantially more organic carbon and shows differences in δ¹³C values. In the south basin, benthic microbial mats are absent and laminated sapropelic sediments accumulate under persistent anoxic conditions, reaching high organic carbon contents. Radiocarbon and stable carbon isotope measurements indicate that this organic matter is produced near the chemocline within a deep chlorophyll rich layer and subsequently preserved on the lakebed. Overall, the results emphasize the importance of gas sources, microbial mat morphology and basin-specific biogeochemical conditions in regulating carbon cycling and microbial functioning in one of the most extreme aquatic environments on Earth.