CONTROLS ON GROWTH RATE OF MODERN STROMATOLITES IN A HYPERSALINE BAHAMIAN LAKE

Storrs Lake, located on the eastern edge of San Salvador Island, the Bahamas, is home to a modern, unique stromatolite community. Stromatolites on the western shore of Storrs Lake are distinctly larger in size than those on the eastern shore. XRD analysis has shown that western shore stromatolites consist of high-magnesian calcite and aragonite, while an eastern shore stromatolite also contains amorphous calcium carbonate. Although a few freshwater conduits enter along the western shore there was no large difference in conductivity or major ionic concentrations between eastern and western waters observed and sampled in January 2016. This research explores reasons for differences in size and mineralogy observed across the 1.5 km transect of Storrs Lake. Growth rate of stromatolites along the western shore is ~16 cm/ky (Paull et al., 1992). New radiocarbon dates from an eastern shore stromatolite show that this stromatolite, representative of the eastern shore, is both younger and growing at a slower rate than reported for the western shore (this study). The difference in age for these two studies is reinforced by the difference in mineralogy; amorphous calcium carbonate is common in newly precipitated biomineralized carbonates. Iron (Fe) was selected as a representative micronutrient; previous work has shown that Fe is critical for electron transport during photosynthesis by cyanobacteria. Freshwater draining into Storrs Lake at the conduit may contain Fe leached from soils or bedrock, which was initially supplied from Fe-bearing illite blown in from the Sahara Desert. Fe concentration in lake water samples along the east-west transect was determined by ICP-MS. Water samples from the eastern shore are 3 to 4 times higher in Fe than of samples from the conduits and western shore. Preliminary data suggests conduit waters are not a source of Fe. Other hydrochemical parameters will be explored as controls on growth rate in this field site.

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