185-7 Implications of Sedimentation Under Perennial Ice Cover for Microbial Mat Microenvironments

Session: New Advances and Voices in Geobiology (Posters)

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Abstract:

The McMurdo Dry Valleys (MDV) in Antarctica are the largest ice free region on the continent, and home to many perennially ice covered lakes. The ice cover is notable for creating distinctive patterns of sedimentation. While most lakes are dominated by mud and silt sedimentation, MDV lakes with >3 m thick ice cover contain dm- to m-scale sand mounds and ridges atop laminated muds despite a low energy environment. Sand is blown onto the ice cover,  partially melts through the ice and accumulates into sand pods. Sand episodically migrates through the ice cover along cracks or in periods of thinner ice cover leading to localized deposition. The bottom of these lakes also contain benthic microbial mats, which have been attributed to the ice cover preventing metazoan grazers from entering and disrupting mat formation. Here, we attempt to determine if these sand sedimentation events impact the microstructure of microbial mats and associated geochemical microenvironments. Two sets of microbial mat samples were collected from Lake Fryxell in the MVD at 8.6 meters depth. The two sets were collected within 30 cm of each other and under similar conditions, with the only difference being that one sample recently had a mm-scale layer of sand deposited during the 2022-2023 austral summer. This time range was determined via diving observations from previous field seasons and counting corresponding lamina above the sand layer. X-Ray Micro CT was used to image 1 cm wide cubes of freeze dried mats with 4 x 4 x 4 µm voxels. Scans demonstrate that the sample without sand deposition has formation of mm scaled bubble molds near the surface of the mat, but bubble molds were not present in the sand rich sample. These bubble molds are consistent with those observed by divers. The occurrence of bubble molds suggests that sand deposition events may act to influence O2 microenvironments present within these mats. In addition to bubble associated features, tying responses of mat growth and lamina development to sedimentation provides a potential framework for the response of microbial mats to projected thinning ice cover and resulting changes in sedimentation in the future.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7458

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