Free Life on a Young Planet by Andrew H. Knoll Page A

limestone accrete, one atop another. Stromatolites can be planar, domal, conical, or cylindrical; each one records a history of microbial growth on the ancient seafloor.
    Chert nodules in the wavy laminated carbonates of Spitsbergen tidal flats preserve a direct record of mat-building microorganisms, and in one locality, the cyanobacterial architects of an offshore stromatolitic reef were preserved by fine carbonate cements that encrusted individual filaments. Most Spitsbergen stromatolites, however, contain no microfossils and so must be interpreted as biological features by invoking theassociation between sedimentary pattern and microbiological process outlined in the preceding paragraph. That’s not such a bad practice in younger Proterozoic successions like Spitsbergen, but as we shall see, assumptions about stromatolite formation become more contentious as we descend deeper into the past.

    Figure 3.3. Stromatolites in the Akademikerbreen Group. (a) A microbial patch reef, some 15 feet thick, seen in a cliff face. (b) Close-up of a columnar stromatolite, showing the characteristic pattern of convex upward lamination. Note pocketknife for scale.
    In general, then, stromatolites provide a sedimentary proxy record of microbial communities, revealing, like Friday’s footprints in the sand, the presence but not the character of their makers. Still, this information is helpful, for it shows that 600–800 million years ago, microorganisms colonized nearly every available surface on the Spitsbergen seafloor, from tidal flats to the open ocean.
    Like stromatolites, organic matter is much more widely distributed than microfossils in Akademikerbreen rocks. Roger Summons, an Australian geochemist now at the Massachusetts Institute of Technology, has shown that the organic matter in Proterozoic sedimentary beds includes biomarkers , biological molecules of known origin that are preserved in and can be extracted from the rock. These molecules consist mainly of lipids that have survived a gauntlet of bacterial decay. (Sadly, nitrogen- and phosphorus-rich molecules like DNA have a vanishingly small likelihood of preservation in very old rocks.) To date, the search for biomarkers in Spitsbergen rocks has not been notably successful, but elsewhere, especially in shales of similar age exposed deep within the Grand Canyon, abundant and diverse biomarker molecules preserve molecular signatures of archaeans, bacteria, protozoans, and algae, most of which have left no recognizable microfossils.
    Biology is encrypted in the Spitsbergen rocks in one additional, even more generalized way. Individual microorganisms are tiny, but their collective physiological effect can be strong enough to influence the chemical composition of the ocean. As a prime example, photosynthetic organisms influence the isotopic composition of carbonate minerals and organic matter deposited beneath the sea.
    Isotopes provide our second set of Jacob Marley facts (the metabolic diversity of bacteria was the first). We have to come to grips with this bit of chemistry, because isotopes will allow us to track aspects of metabolic evolution through time. Moreover, as we’ll see in subsequent chapters,isotopes provide a key to understanding the interplay between life and environmental change through our planet’s history.
    Carbon atoms come in three varieties, distinguished by molecular weight. About 99 percent of all carbon is in the form of 12 C, meaning that it contains six protons and six neutrons, for a total molecular weight of twelve. (Electrons have negligible mass.) Most of the remaining 1 percent consists of 13 C, its extra neutron contributing to a molecular weight of thirteen. There is also a bit of 14 C (two extra neutrons), but this form is radioactive and decays to nitrogen on a timescale of millennia. Thus, 14 C doesn’t figure in discussions of very old rocks.
    Because of their differing molecular weights, these isotopes behave differently in some chemical