QUANTIFICATION AND DESCRIPTION OF SPRINGS EMERGING THROUGH THE THROMBOLITES OF LAKE CLIFTON, WESTERN AUSTRALIA

Microbialites form in the transition zone between surface water and groundwater in shallow seawater, freshwater and saltwater lakes, and streams. Though rare in nature, when present microbialites are commonly found associated with areas of groundwater input. Local patterns of groundwater flow in the transition zone between groundwater and surface water may therefore be critical to understanding the biogeochemistry and microbial ecology associated with thrombolite formation. Here, we document for the first time the local hydrologic and chemical interactions between groundwater and saline lake water associated with thrombolites in Lake Clifton, Western Australia.

Diurnal temperature patterns in the shallow subsurface (< 0.50 m depth), conductivity profiles (< 0.50 m depth), and underwater video were used to characterize local patterns of groundwater flow within thrombolites and sediments along a ~4 m long transect. The diurnal temperature patterns were monitored for 3 days during February 2014 and used to calculate vertical water fluxes. Fluxes were calculated in the profiles over 2 hour intervals and the flux determined from each vertical temperature profile was significantly different from the flux in every other profile (Welch’s t-test, alpha = 0.05). Groundwater flux through thrombolites was upward at 0.34 and 0.63 m/day. Vertical conductivity profiles measured beneath thrombolites suggested advection of groundwater upward through the thrombolites. Conductivity profiles also indicated preferential flow of comparatively fresh groundwater through thrombolites in relation to sediments. Sediments located adjacent (< 1 m) to thrombolites had either downward or very small upward fluxes measuring -0.27, -0.07, and 0.004 m/day. Sediment conductivity profiles were consistent with no flow or a small downward flux of lake water. Finally, we support vertical flux and conductivity data with underwater video of springs emerging through the tops of thrombolites.

We hypothesize that the local physical and chemical interactions described above are critical to the development of Lake Clifton thrombolites. Focused discharge of relatively fresh groundwater through thrombolite springs may result in microbial niches that enable thrombolite microbial communities to survive despite highly saline lake water.