34-3 Micro-Facies of McMurdo Dry Valleys Microbial Mats in Response to Sedimentation and Buoyant Disruption

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

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Abstract:

Perennially liquid fresh water habitats in the Antarctic McMurdo Dry Valleys (MDV) are restricted to perennially ice-covered lakes (PICLs). These lakes contain benthic microbial mats within the photic zone, with transmission of photosynthetically active radiation (PAR) through the ice cover varying from <1 to 20% depending on thickness and opacity of the ice cover. Different ice cover thicknesses are also associated with changes in sedimentation; thick ice accumulates sand lenses over years that episodically deposit as mounds on the lake bed, whereas disseminated sand grains pass through thin ice cover seasonally. The extent and character of ice covers are projected to decrease with a warming climate, making present microbial habitats particularly sensitive to minor climatic changes. Understanding of how these mats will respond to changing lake conditions drives both future predictions of ecological implications, as well as past interpretations of the lake environment. 

Some PICLs contain sufficient dissolved gasses that benthic mats nucleate gas bubbles, become buoyant, and delaminate from the lake bed. PAR modulates the extent of lift off, with expanded lift off zones during periods of higher PAR transmission. Lift off mat exposes sections of the lake bed which can subsequently be regrown. Here, we characterize the micro-facies of mats through episodes of regrowth and sand sedimentation in Lake Fryxell of the MDV. Two suites of samples were collected within the liftoff zone at 8.6 meters depth. One suite was collected in the 2024-25 season across the margin of a mm-cm thick sand mound, deposited during the 2022-23 austral summer. The second suit was collected across the transition from intact mat to recently regrown lift off scar. Samples were imaged using X-Ray Micro-CT, which allows for micron-scaled characterization of organic laminae, carbonate precipitation, and deposited sediment. Micro-sedimentary facies highlight changes in mat structure associated with sedimentation and lift off, which provide a framework for predicting future implications of changes to ice cover attributes on benthic mats.

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