Stromatolites - Paleoenvironments and Present Environments in the Bahamas
Stromatolites are one of the longest living forms of life on this planet. They can be traced back 3.5 billion years to the early Archean eon (Awaramik and Sprinkle, 1999). Stromatolites are sedimentary structures produced by the sediment trapping, binding, and precipitating activity of phototrophic microbes. In the case of stromatolites, these phototrophic microbes include cyanobacteria/blue-green algae (McNamara and Awaramik, 1992). The marine stromatolite communities occur mostly in hypersaline subtidal to supertidal settings (Awaramik and Sprinkle, 1999). Nonmairine stromatolites have been found in streams, lakes, thermal springs, and even frozen lakes (McNamara and Awaramik, 1992).
Cyanobacteria is key organism in the formation of microbial carbonates (Riding, 2000). Bacteria are prokaryotes; they lack a nucleus and other membrane bound cell organelles. All other organisms are eukaryotes; their cells have a distinct nucleus (Riding, 2000). The communities of cyanobacteria, which form stromatolites, can include up to ten different species of bacteria, and can grow in population densities in excess of 3000 million individuals per square meter (McNamara and Awaramik, 1992). Cyanobacteria are usually found in areas where there is reduced grazing and borrowing by other organism, and a reduced occurrence of macro-algae and plants. These areas usually include hypersaline conditions, but also include habitats of increased alkalinity, low nutrient levels, elevated or decreased temperatures, precipitation of mineral material during growth, and strong wave or current actions (McNamara and Awaramik, 1992).
Modern stromatolites were first discovered in Shark Bay, Australia in 1956, and through out western Australia in both marine and non-marine environments (Steneck, Miller, Reid and Macintyre, 1998). Stromatolites continued to be discovered in places, such as the thermal springs of Yellowstone National Park, USA, lakes in Antarctica, marine environment off the Bahamas, and in land-locked pools supersaturated with calcite on Aldabra, in the western Indian Ocean (McNamara and Awaramik, 1992).
These discoveries lead to further study of stromatolites. Through this research scientists revealed there are at least two ways stromatolites form. During the first method each cyanobacteria cell produces and secretes a sticky film of mucus that traps the sediment. The sediment is then bound together with mucus, and the cyanobacteria grows over the grains, towards the sun (McNamara and Awaramik, 1992). The bacteria are photosynthetic and mobile and therefore are able to move towards light. This mobility allows them to keep up with the accumulating sediment. Finally, calcium carbonate, precipitated from the water, cements the grains to the structure (McNamara and Awaramik, 1992). Without this final stage of precipitation and cementation, the structure would not have been preserved in the fossil record, and there would be no record of them existing. The second method of stromatolite construction occurs through its own precipitation of calcium carbonate framework, with little incorporation of sediment into the structure (McNamara and Awaramik, 1992). Marine stromatolites form primarily from the first method. Nonmarine stromatolites, in places such as lakes, form mainly from the second method (McNamara and Awaramik, 1992).
Through study of present-day stromatolites, and the fossil remains of ancient stromatolites, scientists have been able to infer how ancient stromatolites lived. The first record of stromatolites begins around the early Archean, about 3.5 billion years ago, one billion years after scientist have placed the beginning of earth’s geologic history. The presence of stromatolites at such an early age in geologic history is significant to paleobiology. Their presence indicates that even at such an early age, advanced prokaryotes were present, indicating that life on Earth could have began as early as four billion years (McNamara and Awaramik, 1992).
From the Archean, stromatolites continued to flourish and increase in diversity through the Proterozic eon (Figure 1). This first period of increase lead to 176 different known forms of stromatolites. This diversification considered the most significant radiation that affected stromatolites, and the bacteria that constructed them. During this period, the atmosphere went from lacking oxygen to plenty of oxygen, anoxic to oxic (McNamara and Awaramik, 1992). The change from anoxic to an oxic atmosphere enabled cyanobacteria to diversify and disperse far and wide. As well geologically during this period, there was a transition to marine environments with expansive continental shelves, which provided more habitat for stromatolites populate (McNamara and Awaramik, 1992).
Stromatolites continued to increase through the middle of the Paleoporterozoic era (early Proterozic), and then declined for a short time. By the beginning of the Riphean, stromatolites were again on the rise, and diversified. During this time, stromatolites reached their maximum level of diversity, with up to 340 different types having been identified (McNamara and Awaramik, 1992). The cause of the second period of diversification of stromatolites is presently unknown. This period was also when eukaryots became more common, and would have been in competition with the stromatolites (McNamara and Awaramik, 1992).
The stromatolite continued to increase until the middle of the Riphean when they began a sharp decline (Awaramik and Sprinkle, 1999). The decline in stromatolites, during the Proterozoic, continued into the Phanerozoic eon. Scientists have attributed this drop in diversity and numbers to the increase in grazing metazoans, multicellular animals, and sediment disturbance by metazoans (McNamara and Awaramik, 1992). Some scientists dismiss this claim because there is no evidence in the fossil record of an increase in metazoans. Other theories regrading the decline in stromatolite diversity include an increase in the occurrences of larger sediment accumulation, which would have been less suitable for stromatolite construction. As well, there could have been an increase in nutrient levels. Stromatolites prefer habitats with low nutrient levels. In a case at Lake Clifton in Western Australia, scientists are witnessing algae out competing the cyanobacteria because of an increase in nutrient levels (McNamara and Awaramik, 1992).
During the Ordovician, a second decline in diversity and numbers occurred. This decline corresponds to the radiation of benthic marine invertebrates. As eukaryotic life radiated in the continental shelf areas, stromatolites became an uncommon occurrence in the rest of the Phanerozic fossil record (McNamara and Awaramik, 1992). They retreated to habitats where the pressures from these new evolving reef organisms were low (Steneck et al, 1998), which is where they can be found presently.
As previously stated, modern stromatolites can be found in many places throughout the world. Specifically, the stromatolites found in the Bahamas. It was once thought that they could not exist in an open marine setting (Reid, Visscher, Decho, Stolz, Bebout, Dupraz, Macintyre, Paerl, 2000), but with the discovery of stromatolites along Exuma Sound during the 1980’s, this notion has been refuted. Stromatolites in this area occur in three distinct settings: subtidal sites in tidal passes between islands, subtidal sites in sandy embayments, and intertidal sites along sandy beaches (Reid et al, 2000). Within each of these three settings, there are specific sites where stromatolite communities live. For example, in the subtidal setting in tidal passes, stromatolites can be found at Adderly Channel, Iquana Key, Bock Cay, and several other places within the sound (Figure 2, Table 1)(Reid et al, 2000).
The subtidal setting in tidal passes is the most common place to find Exuma stromatolites. The structures in this setting have been recorded at maximum depths of 10m and at maximum heights of 2.5m, but there are a variety if depths and heights at which different communities have been recorded (Reid et al, 2000). Their maximum height is usually correlated to the surrounding sand waves. In contrast to this setting, stromatolites are found at Little Darby Island in a sandy embayment that lacks the strong currents of the tidal passes (Reid et al, 2000). This type of setting for stromatolites was previous unknown. They occurr in water at depths of 0.5-2m, in a bare, rippled sand bottom (Reid et al, 2000). These stromatolites are an enigma to scientist because of the low current action and lack of sand waves.
Finally, stromatolites can be found in the intertidal sites along sandy beaches along Stocking Island (Reid et al, 2000), and Highborne Cay (Reid et al, 2000). The stromatolites along Stocking Island reef complex extend up to 1km along the beach and form back reef, and reef-flat faces of algae ridges (Reid et al, 2000). The stromatolites are up to a meter thick and began forming up to 1500 years ago. The stromatolites in Highborne Cay extend 2.5km along the eastern shore of the island facing Exuma Sound (Reid et al, 2000).
As the Highborne Cay complex was more thoroughly investigated, it was revealed that there were three mat types representing changes in the microbial community structure. The type 1 mats, which comprise 70% of the total mats, exhibit a sparse population of cyanobacteria and represent a pioneer community. Another 15% of the mats represent type 2 mats, a more mature surface community than that of the type 1 mats. The remaining 15% of the mats fall into the type 3 mats. These mats are more fully developed than the type 2 mats and include a more abundant population of cyanobacteria (Reid et al, 2000). These mats represent the climax community of the stromatolite community (Reid et al, 2000).
These three settings, subtidal sites in tidal passes, subtidal sites in sandy embayments, and intertidal sites along sandy beaches, are three very different types of habitats for stromatolites communities. None the less, stromatolites are complex communities of cyanobacteria that have found a way to adapt and change to their surroundings. They have lasted 3.5 billion years in the planets oceans and are still in existence today. They are a resilient and amazing organism.
Even with all the research that has been completed on stromatolites since their modern forms were discovered in 1956, more is needed. There are still many unanswered questions concerning these bacteria communities. More research is needed on the lithification process that occurs. The environment stromatolites live in, and what restricts their growth in other areas need further study, and there are still many questions concerning their ancient forms. These questions and more can not only lead to a better understand of modern stromatolites, but also toward developing new insights into ancient stromatolites, and Earth’s history.
Table 1. Stormatolite locations in the Exuma Cays, Bahamas.(Reid et al, 2000)
Figure 1. Graph of stromatolite diversity in the Proterozoic Marine Evolutionary Biota.
(Awramik and Sprinkle, 1999)
Figure 2. Distribution of stromatolites along Exuma Sound, Bahamas.
(Reid et al, 1995)
Awramik, S.M., J. Sprinkle. 1999. Proterozoic stromatolites: the first marine evolutionary
biota. Historical Biology. Volume 13. PP 241-253.
McNamara K.J., S.M. Awramik. 1992. Stromatolites: a key to understanding the early evolution
of life. Science Progress. Volume 76. PP 345-364.
Reid R.P., I.G. Macintyre, K.M. Browne, R.S. Steneck, and T. Miller. 1995. Modern marine
stromatolites in the Exuma Cays, Bahamas: uncommonly common. Facies. Volume 33. PP 1-18.
Reid R.P., P.T. Visscher, A.W. Decho, J.F. Stolz, B.M. Bebout, C. Dupraz, I.G. Macintyre, H.W. Paerl, H.L. Pinckney, L. Prufert-Bebout, T.F. Stepper, and D.J. MesMarais. 2000. The role of microbes in accretion, lamination and early lithification of modern marine stromatolites. Nature. Volume 406. PP 989-992.
Riding, R. 2000. Microbial carbonates: The geological record of calcified bacterial-algal mats and biofilms. Sedimentology. Volume 47. PP 179-214.
Steneck R.S., T.E. Miller; R.P. Reid; I.G. Macintyre. 1998. Ecological controls on stromatolite development in a modern reef environment: a test of ecological refuge.
Carbonates and Evaporates. Volume 13. PP 48-65.
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