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Using Micro X-Ray Computed Tomography Scanning to Characterize Carbonate Precipitation in Lake Fryxell, Antarctica

Session: New Voices in Geobiology

Presenting Author:

Lauren Judge

Authors:

Judge, Lauren Elena¹, Mackey, Tyler James², Sumner, Dawn Y.³, Kohl, Jove H.⁴

(1) Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, USA, (2) Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA, (3) Univ California - Davis, Earth and Planetary Sciences, Davis, CA, USA, (4) Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA,

Abstract:

The McMurdo Dry Valleys of Antarctica are the continent’s largest ice-free region, containing several perennially ice-covered lakes (PICLs). Carbonate deposits in these lakes provide a record of lake paleoenvironments and past terrestrial climate in Antarctica. Lake Fryxell has a 3-6 m thick ice cover and is fed by seasonal glacial meltwater. The water column is both physically and chemically stratified, with a sharp oxycline from 8.5-10 m depth that transitions from O2-supersaturated to anoxic waters (Green & Lyons, 2009). Calcium carbonate is present as cements within benthic microbial mats and in the underlying sediment. Recent work has determined that these cements precipitate episodically, with seasonal variation in pore water oxygenation (Clance, 2022). Understanding the timing, fluid sources, and location of carbonate precipitation in these lakes is important for paleoenvironmental interpretations. paleoenvironments. Differences between carbonates present in the uppermost sediment layers and microbial mats versus carbonates in down-core sediments are important to understand, as the difference in location might be difficult or impossible to determine when looking at paleolake records.

This work expands our understanding of carbonate precipitation in Lake Fryxell by using micro X-ray computed tomography (XCT) scanning of frozen cores to create high-quality, three-dimensional images of sediments, carbonates, and microbial mats present within the core (Levett, 2024). These scans are done at resolutions (48-14 μm) that allow meaningful observations to be made without dissecting and disrupting the internal core structure. XCT scans of cores highlight the heterogeneity of subsurface layers. Sands and muds are present in all cores, with layers of microbial mat, mat with carbonate, and sediments with carbonate crusts appearing at various depths within the core. Using XCT scans in addition to geochemical observational techniques like cathodoluminescence (CL) allows us to examine cross-cutting relationships between chemical zoning of the carbonate and sediment layers. Within surface crusts, carbonates precipitated after mat accumulation and mud sedimentation, but prior to compaction of macroscopic microbial structures. Combining these observations with geochemical analyses like ICP-OES and stable isotopes can help constrain the timing of carbonate precipitation in the lake, as well as provide insight into the role of microbial metabolisms in precipitation.