Stromatolite
Long ago when the earth was young and life was just beginning to evolve, shallow water environments were hosts to millions of bacteria and algae. These organisms lived together in colonies that were formed by trapping sediments and secreting mucus to bind particles together. As the colony grew, so did the structure that they made - layer upon layer of sediments bound by the organisms and the secretions they produced. In order to capture as much sunlight as possible, the shape of the top of the structure tended to be curved so that there was maximum exposure to the sun. These curved structures with multi-layered interiors are known as stromatolites.
When a stromatolite becomes preserved, the dome-like top of the structure can be used to show which direction is stratigraphically younger. The internal layers will all be parallel and bowed towards the top, because the rounded surface of the stromatolite was oriented towards the sun. In the photo to the right, you can clearly see the parallel internal laminations that were formed as the organisms built their colony.
The term stromatolite was coined by the German geologist Ernst Kalkowsky in 1908, based on Triassic lacustrine stromatolites in Central Germany. However, stromatolites had already been described in 1825, from upper Cambrian successions of New York State, United States. Stromatolite research has therefore lasted for almost 200 years, and it is still ongoing. Google Ngram Viewer (http: //books.google.com/ngrams) shows a trend that stromatolite research initiated in 1970s, in the middle of the Cold War, and continues to be important to the scientific community.
Definition
What Is a Stromatolite?
The term “stromatolite” came from a combination of two Greek words: στρῶμα (strōma), meaning “layer” and λίθος (lithos), meaning “rock”. As one can infer from its etymology, a stromatolite is typically a layered, mostly with convex-up layers, sedimentary rock formed by microbial organisms. However, there are many other sedimentary rocks with convex-up layered structures. Some animals (e.g., sponges, stromatoporoids, bryozoans) as well as algae, form thin layers and produce structures similar to that of stromatolite. They may form reefs by themselves or together with microbial layers, thereby confusing the definition of stromatolites. If we go to an earlier part of geologic history, such as the Archean and Proterozoic, it becomes more complicated. In these ancient stromatolites, abiogenic sparry crusts are common components of stromatolites. Some stromatolites are formed by alternation of these sparry crusts and biogenic layers. Thus, what is a stromatolite, and how can we define it?
Definition of Stromatolite
Three major definitions of stromatolites have been proposed: (1) microbial and laminated, (2) microbial but not necessarily laminated, and (3) laminated but not necessarily microbial. The first definition is perhaps the most widely accepted one. It fits with Kalkowsky’s original proposal-microbially formed benthic structure. But, this definition requires that formative processes must be studied in detail to prove a microbial origin, and it is not an easy question to answer as we go back into the geologic past (see “How to Identify Stromatolite” section).
How to Identify a Stromatolite
How can you identify a stromatolite? The simple answer is to look at its cross-section. If you look at cross-sections of stromatolites, you will easily recognize convex-upward laminae. It is not easy to generate such structures by physical sedimentological processes, thereby evincing
a biological origin of stromatolites. In some cases, however, chemical precipitation of minerals from water can form convex-upward laminae similar to stromatolites. Stalagmites in limestone caves can be a good example. Structurally folded strata may also produce similar,convex-upward structures.
It is thus important to correctly describe stromatolites to identify whether they are true stromatolites formed by microbes or pseudo-stromatolites formed by abiotic processes. There are eight criteria to identify the biogenicity of stromatolites, which are:
1. The structures must occur in sedimentary (or metasedimentary) rocks.
2. The structures must have formed together with the deposition of surrounding sediments.
3. Convex-upward layers should occur dominantly within the structures.
4. Laminae are thicker over crests and thinner over troughs.
5. Laminae are not flat, but rather wavy or wrinkled.
6. Microfossils may be present within the structures.
7. Microfossil assemblages change together with stromatolite morphologies.
8. Microfossils would show evidence of trapping/binding of detrital sediments or organically induced precipitation.
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