BUBBLE-SUPPORTED MICROBIAL MAT LIFTOFF STRUCTURES IN ICE-COVERED ANTARCTIC LAKES

Metabolically produced gas bubbles form in marine, freshwater, and hot spring microbial mats. Where bubble molds are preserved they form a suite of microbially influenced sedimentary microstructures. Supersaturated dissolved gasses can also lead to buoyant disruption of microbial mats creating macroscopic liftoff structures. In this contribution, we characterize modern mats with a range of liftoff structures forming in Antarctic perennially ice-covered lakes of the McMurdo Dry Valleys (MDV) and assess their utility as records of dissolved gas concentrations and regional climate change.

Benthic mats were imaged by divers from the shallow regions of Lakes Fryxell (2006), Hoare (2010), and Joyce (2010 and 2014) in the MDV. The video footage and photographs were used to catalog the extent and morphology of microbial mat liftoff at each lake. We identified three liftoff morphologies: tent, finger, and sheet with vertical relief ranging from 2 cm to 1.5 m, all of which are prone to delamination and transport. The distribution, size, and mat characteristics vary amongst the lakes and from historical observations. The maximum depth distribution of liftoff mats in this study is shallower than published observations in the early 1980s (8 m versus 11 m depth). During this time, the lakes have experienced long-term increases in lake level and fluctuations in ice-cover thickness.

Dissolved gasses responsible for microbial mat liftoff are due to in situ production by seasonal oxygenic photosynthesis and delivery from meltwater streams. Freezing at the base of the perennial ice cover over winter further concentrates dissolved gasses in the water column. Future work will constrain the relative influence of biological and physical processes on the formation of liftoff structures. The observed variability in liftoff mat morphology and depth distribution through time indicates that these structures may serve as indicators of ecological shifts that have affected the MDV lakes with regional climate change, offering a window into the relationship between microbial mats and dissolved gasses with higher preservation potential than individual bubble molds. In addition, recognition of these deposits in the sedimentary record may inform reconstruction of similarly gas supersaturated environments in Earth’s past.