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Mudcracks along the shoreline of Storr's Lake (San Salvador Island, Bahamas) 2 | by James St. John
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Mudcracks along the shoreline of Storr's Lake (San Salvador Island, Bahamas) 2

Mudcracks along the western shoreline of Storr's Lake, eastern San Salvador Island, eastern Bahamas.

 

San Salvador Island has numerous inland bodies of water (see map - newton.newhaven.edu/sansalvador/ssmap_11x17.PDF). Christopher Columbus remarked upon them during his visit in October 1492. These ponds and lakes can have freshwater, brackish water, hyposaline water, normal marine-salinity water, or hypersaline water. Many of these lakes have aquatic biotas quite distinctive from adjacent lakes.

 

Storr's Lake is a moderately large, elongated body of water that represents a cutoff lagoon/estuary. This depression was formerly connected to the ocean, essentially identical to modern day North Pigeon Creek, a tidal estuary in the southeastern part of the island. Storr's Lake does have a few conduits (connections with the modern ocean), but they have little impact on the lake (little seawater enters). Before it was even a lagoon, before the Holocene highstand, this feature was a terrestrial depression.

 

Storr's Lake is shallow (less than 2 meters deep) and has very salty water (60 to over 80 ppt, or 6 to over 8%, cf. normal marine salinity of 35 ppt, or 3.5%). The high salinity is the result of dry seasonal conditions and high evaporation rates. The water is frequently turbid, with a brownish or light greenish or greenish-brown color. The turbidity is due to suspended organic matter - algae, halophilic bacteria, dinoflagellate cysts, diatoms, etc. The high turbidity allows very little light to reach the lakefloor.

 

Storr's Lake is famous for being a stromatolite locality. Mineralized microbial buildups are common in the lake - they form by bacteria inducing local precipitation of calcium carbonate minerals, not by trapping or binding of sediments. The general term for mineralized microbial buildups is "microbialites". If microbialites are layered, they are stromatolites. If they are massive (non-layered), with a clotted fabric, they are thrombolites. If they are non-layered, and have meso-scale bundled branching structures, they are dendrolites. Storr's Lake has stromatolites and thrombolites (see above photo). The dominant mineral in these microbial buildups is high-magnesian calcite, plus minor aragonite. Five microbialite morphologies are present in the lake, and have been characterized as: 1) calcareous knobs; 2) plateau-shaped structures; 3) pinnacle mound structures; 4) "sharpy"-shaped structures; and 5) mushroom-shaped structures.

 

Traditional stromatolites are constructed by photosynthesizing cyanobacteria. They are common in the Proterozoic fossil record, but are uncommon to scarce in the Phanerozoic. Living stromatolites occur at few localities - reported examples include Shark Bay, Australia; the Gulf of California; and the Exuma Islands in the Bahamas. The water of Storr's Lake is frequently turbid, resulting in little light reaching even shallow depths (light penetration here is 10 to 20 cm deep). It's been speculated that some or many of Storr's Lake's microbialites were constructed by non-photosynthesizing microbes, such as sulfate-reducing bacteria (the lake is stinky - there's lots of sulfur activity & the water there has 3.3 times more sulfate than seawater). Light measurements taken at the bottom of the lake show that small levels of light do reach the substrate, so photosynthesizing cyanobacteria could be responsible for the microbialites. Suspended cyanobacteria occur in the lake, but stromatolites at deeper depths (>10 cm) may be constructed, at least in part, by heterotrophic bacteria (aphotic microbial activity). Five genera of sulfate-reducing bacteria have been identified in Storr's Lake microbialites.

 

Other organisms in Storr's Lake include >20 species of ostracods, known from modern lakefloor sediments and cores of Holocene, shallow subsurface sediments (see list & photos in Corwin, 1985). Gastropods at Storr's Lake include Cerithidea costata (costate horn snail) and Cerithium eburneum aliceae.

 

The mudcracks shown above are some of the most spectacular examples I've ever seen. Mudcracks are sedimentary structures that form under alternating wet and dry conditions - they typically develop in fine-grained sediments (clay and mud). They are often preserved in the nonmarine sedimentary rock record. The examples shown above have especially wide cracks and thick polygon crusts.

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Much of the above is synthesized from info. provided by Lisa Park and Varun Paul and Russel Shapiro and:

 

Corwin, B.N. 1985. Paleoenvironments, using Holocene Ostracoda, in Storr's Lake, San Salvador, Bahamas. M.S. thesis. University of Akron.

 

Paul, V. 2012. Characterization of modern microbialiates and the Storr's Lake ecosystem. The 16th Symposium on the Geology of the Bahamas and Other Carbonate Regions, June 14-June 18, 2012, Abstracts with Program: 39-40.

 

Paul, V., D.J. Wronkiewicz, M.R. Mormile & C. Sanchez Botero. 2012. A biogeochemical investigation of the ecosystem and the microbialites in Storr's Lake, San Salvador Island, Bahamas. Geological Society of America Abstracts with Programs 44(7): 74.

 

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Taken on March 14, 2013