Geochemical proxies in Lake Fryxell
Manganese and iron are useful biogeochemical proxies in carbonates, largely due to their redox sensitivity; reduced (divalent) cations of both Mn and Fe are able to occupy Ca sites in calcite, while their more oxidized (trivalent and higher) cations cannot (e.g. Barnaby and Rimstidt 1989, Braithwaite 2016). Additionally, the ratio of Mn to Fe in carbonates allows for more detailed investigation, as Fe oxidizes more readily and is more thermodynamically stable in oxidized phases than Mn in aqueous environments (Davison 1993). Lake Fryxell contains both manganese and iron throughout the water column, with very low (<50 μg/l) concentrations above the oxycline increasing to hundreds of μg/l around the oxic-anoxic transition (Harnish et al. 1991).
Water column Mn and Fe concentrations may not directly correspond to carbonate concentrations in Lake Fryxell, as carbonates are present within metabolically active microbial mats. Benthic mat metabolisms modulate mat pore water oxygenation; photosynthesis further supersaturates pore water oxygen within the hyperoxic DO maximum (Sumner et al. 2015), contributing to expected perennial oxygenation of surface mats that persists through the polar winter based on extended season observations of nearby Lake Hoare (Wharton et al. 1994). The depth limit for photosynthesis extends below the oxycline in Lake Fryxell, leading to mm-scale seasonal oxygen oases within mats performing oxygenic photosynthesis (Sumner et al. 2015). Thus, seasonal changes to pore water redox vary with depth due to both the water column oxycline and local metabolic effects in the benthic microbial mats. In this study, Mn and Fe concentrations in carbonate are examined as a redox-sensitive proxy for seasonal changes to local pore water redox conditions at the time of carbonate precipitation.
Stable isotopes of carbon and oxygen in carbonates record a wealth of biogeochemical, hydrological, and climatic information, such as metabolic processes and changes in fluid sources. In the Lake Fryxell water column, δ13CDIC follows a similar trend to the DO profile; δ13CDIC increases from ~0‰ VPDB below the ice cover to a maximum of 3-4‰ around 8 m, then decreases to approximately -4‰ near the lake bottom (Knoepfle et al. 2009, Neumann et al. 2004). This is interpreted as a result of preferential uptake of 12C by photosynthesizing microbes and a lack of water column mixing, resulting in the largest13CDIC enrichment at the same depth where maximum primary productivity occurs (Neumann et al. 1998, Neumann et al. 2004). δ18Owater is nearly constant throughout the water column, varying over a narrow range of -31.9 to -31.3‰ VSMOW (Dowling and Lyons 2014). Existing isotopic data for the benthic carbonates are limited to bulk values of 10-20 mg samples from sediment cores (Lawrence 1982, Lawrence and Hendy 1989).
The stable stratification of the Lake Fryxell water column and the lake’s depth profiles of oxygenation, redox-sensitive metal concentrations, and stable isotope compositions offer a robust suite of geochemical proxies which can be used to construct a facies model of carbonate precipitation in this perennially ice-covered Antarctic lake. The petrography and geochemistry of Lake Fryxell carbonates presented in this study help to describe the timing of carbonate precipitation and the changes in redox and isotope geochemistry during precipitation.