Stromatolites (literally meaning "layered or bedded rock" in Greek) have been commonly and continuously observed, mainly in carbonate rocks, from 3.5 giga years ago to today [1]. This is one of the oldest, most diverse, and highest yielding fossil classes on Earth. On the basis of their unique macroscopic morphologies (domal, columnar, and branching) and fine-laminated textures, since their first description over 100 years ago [2], they have been considered to be trace fossils formed in association with benthic microbial mats. In the 1970s, discoveries of modern stromatolites in both shallow marine and freshwater environments revealed that most are formed in the presence of cyanobacteria [e.g. 3], primordial oxygenic photosynthetic organisms. From these findings, it was deduced that "stromatolites are fossil records of cyanobacteria". However, this is only wishful speculation at this time because of several enigmas, namely: 1) secular changes in morphology and texture over time, 2) unsynchronized yields in relation to great oxidation events, and 3) existing counter-examples that form in dark environments and by abiotic processes. To better define stromatolites, I studied modern analogs in some hot springs in Japan, Indonesia, and the USA.

Stromatolites in hot springs consist of authigenic minerals precipitated in situ from hot spring water saturated with CaCO3. The textures of hot spring stromatolites are especially similar to those of most early stromatolites from the Archean to mid-Proterozoic, and consistent with some modern marine stromatolites composed of allochthonous coarse grained sands, with micritic crystals cementing the inter spaces of the sands. To confirm lamination-forming processes, water and sediment samples were sequentially collected over 24 hours and analyzed by chemical, sedimentological, and microbiological methods. Results from five hot springs showed that daily lamination resulted from daytime growth of biofilms that temporarily inhibited inorganic mineral precipitation [4, 5, 6, 7, 8]. In most cases, cyanobacteria dominated in the biofilm, while heterotrophs or chemolithoautotrophs also dominated at some sampling points where cyanobacteria growth was confined by physicochemical conditions. Although the daily rhythm of biofilm growth was driven by cyanobacterial production in the latter cases, it suggests the possibility of stromatolite formation by microbes other than cyanobacteria, as long as these are periodically supplied with fuels. To verify this possibility, I am now researching a brucite-carbonate chimney at a serpentinite-hosted vent field in the southern Mariana forearc (Shinkai Seep Field, 4). Processes here are distinct from those of early modern marine stromatolites and might be appropriate for interpretation of most early stromatolites. Recently, stromatolite-like structures have been identified in an outcrop image from Robar Mars exploration [e.g. 9]. Reinterpretation of fossil stromatolites will lead to better understanding of the evolution of early life on Earth (and Mars).


References: [1] Riding R (2011) Encyclopedia of Geobiology, pp. 635-654. [2] Kalkowsky E (1908) Zeitschrift der Deutsch geologischen Gesellschaft, Monatsberichte, 60, 68-125. [3] Monty CLV (1976) In Stromatolites. Developments in Sedimentology, 20, 193-249. [4] Okumura T et al. (2011) Geomicrobiology Journal, 28, 135-148. [5] Okumura T et al. (2012) Sedimentary Geology, 265-266, 195-209. [6] Okumura T et al. (2013a) Geomicrobiology Journal, 30, 910-927. [7] Okumura T et al. (2013) Island Arc, 22, 410-426. [8] Sugihara C, Yanagawa K, Okumura T et al. (2016) Sedimentary Geology, 343, 85-98. [9] Rizzo V., Cantasano N. (2009) Int. J. Astrobio, 8, 267-280.