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.