Stromatolites as Inorganic Structures

Modern Stromatolites -- Shark Bay, Australia

Fossils and Pseudofossils: Lessons from the Hunt for Early Life on Earth
Silurian Stromatolite Reefs from Alaska
Stromatolites in the Hakatai Shale
Petrified Sea Gardens
Stromatolites from the Pilbara Region
Fossil record of Cyanobacteria
Cyanobacteria Images
Bitter Springs Formation

Stromatolites are defined as "accretionary organosedimentary structure[s], commonly thinly layered, megascopic, and calcareous, produced by the activities of mat-building communities of mucilage-secreting microorganisms, mainly filamentous photoautotrophic prokaryotes such as cyanobacteria" (Schopf, 1999, p. 184). These structures are present in sedimentary rocks from at least the early Proterozoic, and are present in the moden world as well, for instance Shark Bay Australia and Baja California.

Studies of extant stromatolites show that the laminae are produced by a combination of bacterial and sedimentary processes. The outer surface of stromatolites are covered by a mat of cyanobacteria, beneath which are photosynthetic green sulpher and purple bacteria. These can live beneath the cyanobacterial mat because they are sensitive to light wave lengths which pass through the overlying cyanobacteria (cyanobacteria use chlorophyll-a, whereas the others use bacteriochlorophyll, which are sensitive to different wavelengths). Beneath the green sulpher and purple bacteria are anerobic bacteria.

Occasionally, for a variety of reasons, grains of sediment come into contact with the outer surface of the stromatolite, and stick to it. In response, the bacteria migrate upwards to the new surface, and form a new bacterial mat. Over time, repetition of this process produces the numerous thin laminae, as seen in cross sections of stromatolites.

How do stromatolites fit into YEC earth history? I found two creationist web pages claiming that Rothman and Grotzinger (1996) have in some fashion demonstrated that stromatolites are inorganic structures. For instance, David Tyler states:

"We are developing an understanding of the Precambrian which involves devastation experienced during the early stages of the Flood: very high energies and extreme violence. The in situ growth of stromatolites does not readily fit into this model - and the abiotic interpretation could be regarded as one predicted by the larger scale model of Precambrian catastrophism."

Hugh Ross says similarly:

"Two MIT geophysicists, John Grotzinger and Daniel Rothman have just discovered four additional abiotic ways that the "life" chemicals in ALH84001 could have been generated. Specifically, they demonstrated that what was once termed indisputable evidence for life, namely the presence of stromatolites, can result from any one of four (or more) different inorganic processes."

The statement by Ross, by the way, which is taken verbatim from this page, is incoherent and confused. The Grotzinger and Rothman paper cited by Ross does not say anything about how "the 'life' chemicals in ALH84001 could have been generated."

Neither of these authors seem to appreciate the evidence showing that many stromatolites, from the early Proterozoic and younger sediments, are indeed biogenic structures. This is supported by the preserved fossils of photosynthetic bacteria in cherts associated with stromatolites, by the presence of biochemical "markers" in stromatolite associated sediments, by the isotopic signatures (C and S) in stromatolite associated sediments which strongly implies biogenic isotope fractionation, and, perhaps most importantly, by the almost perfect morphological correspondance between many ancient, buried, stromatolites and those which exist in the modern world. Discussing the very early Archaean stromatolites, Schopf (1999, p. 196) writes:

"Like the rock record itself, the abundance of stromatolite bearing beds peters out in increasingly older terrains: more than a thousands date from the more recent part of the Proterozoic, hundreds from the older Proterozoic, and fewer than 3 dozen from the Archaean. Only four or five stromatolitic horizons older than 3,200Ma have been discovered, a scarcity which has prompted some workers to suggest to question whether the stromatolites which have been described from these exceptionally ancient deposits might owe their origin to physical, geological processes . . . rather than to stromatolite-building microbes.

"Though a purely nonbiologic origin cannot be ruled out for some of these structures (particularly, sharp-tipped forms that occur as rare, isolated, or scattered fold-like bodies), there seems to be no reason to doubt the microbial genesis of the flat-layered, gently domical, or columnar stromatolites of this age, most of which are practically identical to younger stromatolites unquestionably formed by microbes. Even certain of the conical varieties defy a nonbiologic explanation"

Microfossil Evidence

William Schopf and other researchers have meticulously documented fossil cyanobacterial cells in stromatolitic cherts from the Archaean and Proterozoic, including the 3.45By Apex Chert (see below), the 2By Gunflint Chert, the 850My Bitter Springs chert, and several others. See Earliest Life at University of Münster. Below are thin section photos of the Bitter Springs and Apex Chert microfossils.

Biomarker Evidence

According to Summons et al (1999), cyanobacteria (which are the variety of bacteria which construct stromatolites) produce specific biochemical markers (2-methyl- bacteriohopanepolyols), and that these very same biochemical markers can be found in stromatolites in the precambrian geologic record:

"Biomarkers are potentially useful because the three domains of extant life-Bacteria, Archaea and Eukarya-have signature membrane lipids with recalcitrant carbon skeletons. These lipids turn into hydrocarbons in sediments and can be found wherever the record is sufficiently well preserved. Here we show that 2-methyl- bacteriohopanepolyols occur in a high proportion of cultured cyanobacteria and cyanobacterial mats. Their 2-methylhopane hydrocarbon derivatives are abundant in organic-rich sediments as old as 2,500 Myr. These biomarkers may help constrain the age of the oldest cyanobacteria and the advent of oxygenic photosynthesis. They could also be used to quantify the ecological importance of cyanobacteria through geological time. "

Isotopic Evidence

Biological processes "fractionate" carbon isotopes because they preferentially use the light C12 isotope; thus biogenic deposits are enriched in C12 relative to the atmosperic C12/C13 ratio. This relationship can be verified in the modern world, and can also be traced back deep into the fossil record. Schidlowski explains:

Manfred Schidlowski. Carbon Isotope Clues to the History of Life: Evolution of a Concept. Geophysical and Geochemical Constraints on Molecular Evolution. Session C01:2A. Symposium C01.

"Since the pioneering work by Nier and Gulbransen (1939) it is known that the incorporation of inorganic carbon into living systems entails sizeable fractionations of the stable carbon isotopes. Consequently, it became firmly established that the observed bias in favour of the light isotope (12C) characteristic of biogenic substances derives, for the most part, from the isotope discriminating properties of the principal carbon-fixing enzyme (ribulose-1,5-bisphosphate carboxylase) that is operative in the main chemosynthetic and specifically photosynthetic pathways, channeling most of the carbon transfer from the nonliving to the living world. With most biochemical processes enzymatically controlled, and all living entities representing stationary states undergoing rapid cycles of anabolism and catabolism, it is generally accepted that the 13C/12C fractionations observed are mostly due to kinetic rather than equilibrium effects.

"Most importantly, biological carbon isotope fractionations are basically retained when organic carbon is incorporated in sediments, the enzymatic isotope effect thus being propagated into the rock section of the carbon cycle. The carbon isotope archives preserved in sedimentary rocks have been shown to constitute important sources of information regarding the global state of the terrestrial carbon cycle and the biosphere through geologic time. This holds particularly for the extension of the isotope record to the early Precambrian, testifying that biologically mediated carbon isotope fractionations have persisted over 3.5, if not 3.8 Ga, of recorded Earth history. Since changes in delta 13C(org) and delta 13C(carb) observed in the oldest terrestrial sediments can be referred to a high-t isotopic reequilibration between the two carbon species in response to amphibolite-grade metamorphism, there is little doubt that the biological signature of the record had originally extended to the very beginning of the presently known record (Schidlowski et al., 1979, 1983), a conclusion recently confirmed by a novel approach utilizing advanced microanalytical (ion microprobe) techniques (Mojzsis et al., 1996).

Here's an example of such isotopic evidence for bacterial metabolism in 2.7ga sediments:

Carbon and Sulphur Isotopic Changes within the 2.7 Ga Ngesi Group, Belingwe Greenstone Belt, Zimbabwe. Geophysical and Geochemical Constraints on Molecular Evolution. Journal of Conference Abstracts. Volume 4 Number 1. Symposium C01

"The NERCMAR drillcore through the Belingwe greenstone belt revealed sequences containing organic-rich horizons and sulphide facies ironstones (the Manjeri Formation) between basement granite gneiss and overlying Reliance Formation komatiitic volcanics (Hunter et al., 1998), and which were poorly preserved in outcrop. Stable isotopic analysis of these horizons and additional analyses from shallow drill and outcrop samples in both Manjeri and Cheshire Formations (Abell et al., 1985), allows tentative reconstructions of the bacterial processes taking place.

"Comparison of stromatolitic and non-stromatolitic outcrop data and core samples reveal significant differences between the lower Manjeri Formation and the upper Cheshire Formation, the two sequences being separated by the thick volcanic pile comprising the Reliance and Zeederberg Formations. Cheshire formation stromatolites represent a cyanobacterial mat community with a narrow range of delta13C of carbonates close to 0‰. The associated Cheshire shales contain kerogen with most values within the range -37‰ to -44‰, suggesting the activity of methanogenic bacteria throughout the sequence. The older Manjeri stromatolites also have carbonate delta13C close to 0‰, though extracted kerogen is consistently around -22‰. Greater heterogeneity of carbon isotopic values is recorded in sulphide-rich horizons of Manjeri Formation drillcore. Some organic horizons intercalated with sulphide ironstones give evidence for methanogens (-37‰), but most kerogen values are around -30‰. Significant sulphur isotopic heterogeneities exist in the ironstone horizons (delta34S of -19‰ to +17‰, see Grassineau et al., 1997). Disseminated sulphides away from these horizons show more restricted values. The general trend is from +3‰ at the base of the NERCMAR core, to 0‰ in the middle, and -5‰ throughout one of the Cheshire shale horizons.

"Worldwide evidence suggests that methanotrophy is present in many sedimentary successions at the Archaean-Proterozoic transition, but earlier evidence of methane cycling is more sporadic (Hayes, 1994). The isotopic data from the 2.7 Ga Ngesi Group record a snapshot of oxygenic photosynthesis and methanotrophic activity, and record a simultaneous increase in the activity of sulphur-reducing bacteria.

References

Grotzinger and Rothman (1996) demonstrate that "stromatolite" morphology can be predicted with equations defining growth that is completely independent of biological processes.

Grotzinger, J.P. & Rothman, D.H. 1996. An abiotic model for stromatolite morphogenesis, Nature, 383(3 October), 423-425.

Grotzinger, J.P. & Knoll, A.H., 1999, Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks? Annu. Rev. Earth Planet. Sci., 27, 313-358.

Schopf, J.W. 1968. Microflora of the Bitter Springs Formation, Late Precambrian, central Australia. Journal of Paleontology 42(3):651-688.

Schopf, J.W. 1993. Microfossils of the Early Archean Apex chert: New evidence of the antiquity of life. Science 260:640-646.

Schopf, J.W. 1999. Cradle of Life: The Discovery of the Earth's Earliest Fossils. Princeton University Press.

Summons. R.E., Jahnke, L.L., Hope, J.M., and Logan, J.H.. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400(6744):554-7; 1999.