34-2 Microbial Calcium Carbonate Columns in Lake Joyce, Antarctica: How Geochemistry Relates to Physical Variation

Session: Geobiology and Astrobiology in Modern and Ancient Environments: From Microbial Interactions to Planetary Exploration

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Abstract:

Most of the history of life on Earth is microbial, and microbialites preserve information about how those microbial communities functioned. However, both abiotic and biotic structures can resemble microbialites, and determining the biogenicity of fossilized structures can be challenging. To build a framework for distinguishing abiotic versus biotic structures, more work is needed to understand how microbes form macroscopic structures, preserve 3D morphology, and record chemical signatures.

Observations of modern microbialite development in natural environments can inform these criteria. In Pearse Valley, Antarctica, Lake Joyce serves as a natural laboratory to observe microbial communities and associated mineralization. The lake is perennially ice-covered and abuts Taylor Glacier on one side, with the opposite side containing a delta from local alpine glacier meltwater. Microbial mats cover the lake bed and serve as the primary location for carbonate precipitation. At ~20-22 m depth, the mats form columnar structures with a mm-thick layer of calcium carbonate at the center (Mackey et al. 2018), while smaller peaks and flat mats dominate shallower depths. Sediment cores containing varying morphologies were taken from different depths and locations. Thickness and symmetry of the carbonate columns vary with height, getting thicker with distance from the lake bed despite no change in conditions relevant to photosynthetic activity. This suggests that water mixing controls the carbonate thickness, which we aim to address with this work. Stable isotope analysis of 𝛅¹³C and 𝛅¹⁸O values of carbonate from inner, middle, and outer zones of the carbonate columns was performed, assuming outward growth. Location of the samples appeared to have some impact on 𝛅¹⁸O values, with carbonate near Taylor Glacier having a wider range of 𝛅¹⁸O values (-39.0‰ to -29.0‰), while carbonate from the Lake Joyce delta only ranges from -37.5‰ to -35.0‰. Trace element concentrations within the carbonate varied slightly with water depth rather than with the lateral variation of the samples.

Constraining the location of carbonate precipitation and the influence of metabolic versus physical controls on precipitation within Lake Joyce will help inform the interpretation of ancient microbialites locally. Continuing expansion of this dataset will also help build the framework for assessing the biogenicity of ancient samples from various locales.

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