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

reactions. Notably, when photosynthetic organisms take up carbon dioxide to form organic molecules, CO 2 containing the lighter variety, 12 C, is incorporated more readily than CO 2 that contains 13 C. In consequence, the ratio of 13 C to 12 C in organic matter made by photosynthesis will be distinctly different from that of carbonate minerals formed in the same environment, a quantitative difference known as fractionation ( figure 3.4 ). The difference is not large—about 25 to 30 parts per thousand—but it can easily be detected by geochemists armed with mass spectrometers. And this fractionation is preserved in sediments, providing us with a geochemical probe for ancient photosynthesis. (Organisms like ourselves that eat plants, algae, cyanobacteria, or other photosynthetic bacteria do not impart much additional fractionation in the process.) In the Spitsbergen rocks, the ratios of carbon isotopes in carbonates and organic matter consistently differ by about 28 parts per thousand—so photosynthesis fueled ecosystems in late Proterozoic oceans, just as it does today.
    Chemistry also provides a paleobiological probe for sulfate-reducing bacteria. As noted in chapter 2 , sulfate reducers play a key role in completing the marine carbon cycle, using sulfate ions (SO 4 2 −) to respire organic molecules. The sulfate is converted to hydrogen sulfide (H 2 S) that may combine with iron to enter the sedimentary record as pyrite (FeS 2 )—the fool’s gold sold in rock shops. Biological sulfate reduction shows a chemical preference for 32 S (16 protons and 16 neutrons) over the heavier isotope 34 S (two extra neutrons), resulting in sedimentary pyrite that is enriched in 32 S relative to gypsum formed from the same water body. Spitsbergen rocks indicate that the essential biologicalcomponents of the sulfur cycle, like those of the carbon cycle, were in place when the gray rocks of this arctic island accumulated.

    Figure 3.4. Diagram illustrating how photosynthetic organisms fractionate carbon isotopes. Black dots on left side of the diagram depict carbon dioxide molecules that contain 12 C (smaller) or 13 C (larger). Photosynthetic organisms fix 12 CO 2 preferentially, with the result that the organic matter in photosynthetic organisms (and the organisms that eat them) is depleted in 13 C relative to its surroundings; biochemists speak of this as a kinetic isotope effect—hence, the label “KIE” in the figure. The isotopic fractionation imparted by organisms will be preserved in sediments as the difference in the ratio of 12 C to 13 C between limestone and organic matter in the same sample.
    As observed on first sighting, the frozen Proterozoic rocks of Spitsbergen contain no bones, shells, or fossil trackways—nothing that would reward the casual collector (or Darwin!) on a weekend fossil hunt. But the apparent lack of fossils is deceptive—shells simply provide the wrong search image for Precambrian paleontology.
    All of the carbonate minerals and organic carbon in the thick Spitsbergen succession bear the isotopic imprint of photosynthesis, and sulfur-containing minerals similarly preserve a metabolic signature of sulfate-reducing bacteria. Stromatolites document the ubiquity of microbial communities on the seafloor, while microfossils record aspects of biological diversity on the seafloor as well as in the water column.
    When we look carefully, then, the fingerprints of biology are all over the Proterozoic rocks of Spitsbergen. The geological record does contain a record of early evolution that can be used to trim the Tree of Life. Our experience in Spitsbergen tells us how to approach ancient rocks and what to look for. But at 800 million years, the oldest beds on this desolate island are still relatively young. What happens when we apply these lessons to the bottom of the pile?
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    1 Limestone consists of calcium carbonate (CaCO 3 ) particles cemented together to form a rock; the closely related mineral