Stromatolites at Little Darby Island
For much of Earth’s history, stromatolites dominated the ancient landscape (Awramik, 1984). With fossils dating back over three billion years (Byerly et al., 1986; Grotzinger and Knoll, 1999), the geologic record of stromatolites clearly shows the extensive role microbes have played in the evolution of Earth’s environment (Kasting, 2001). Stromatolite morphology and structure are the direct result of microbial activities and provide key insight into the nature of these complex ecosystems (Burne and Moore,1987).
Living stromatolites on the margins of Exuma Sound, Bahamas, are the only examples of modern stromatolites forming in open marine conditions similar to those that may have existed on Precambrian platforms. Six microbial mat types have previously been documented on the surfaces of stromatolites along the eastern side of Highborne Cay (Schizothrix, Solentia, heterotrophic biofilm, stalked diatom, tube diatom and Phormidium mats). Cycling of these communities create laminae with distinct microstructures. Subsurface laminae thus represent a chronology of former surface mats.
The present study documents the effects of environmental factors on surface microbial communities of modern marine stromatolites and identifies potential causes of microbial mat cycling. Mat type and burial state at 43 markers along a stromatolitic reef on the margin of Highborne Cay were monitored over a two-year period (2005–2006). Key environmental parameters (i.e., temperature, light, wind, water chemistry) were also monitored.
Results indicated that the composition of stromatolite surface mats and transitions from one mat type to another are controlled by both seasonal and stochastic events. All six stromatolite mat communities at Highborne Cay showed significant correlations with water temperature. Heterotrophic biofilms, Solentia, stalked diatom and Phormidium mats showed positive correlations with temperature, whereas Schizothrix and tube diatom communities showed negative correlations. A significant correlation with light (photosynthetically active radiation, PAR) was detected only for the heterotrophic biofilm community. No significant correlations were found between mat type and the monitored wind intensity data, but field observations indicated that wind related events such as storms and sand abrasion play important roles in the transitions from one mat type to another.
An integrated model of stromatolite mat community cycling is developed that includes both predictable seasonal environmental variation and stochastic events. The long-term monitoring of mat communities on Highborne Cay stromatolites and the resulting model are an important step in understanding morphogenesis of modern marine stromatolites, with implications for interpreting patterns of stromatolite lamination in the geologic record.
The attached photos are typical of stromatolites in The Exumas. They are located at Little Darby Island, where ongoing studies are based at the Darby Island Research Station.
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