Life on a Young Planet (8 page)

Chert is common in tidal-flat deposits of Precambrian age, often occurring as black nodules within carbonate beds (
figure 3.2
). The nodules formed within the sediments, not on the seafloor, as demonstrated by lamination and other features of bedding that run unbroken through the silica and surrounding carbonate. What’s more, the cherts display textural features normally associated with lime deposits—they contain the same ooids, microbial mats, and crystalline cement textures found in adjacent carbonate rocks. In many cases the nodules formed soon after deposition, before burial initiated the compaction that bent encompassing sediments around them. Silica has no color of its own; the blackness of the nodules comes from included organic matter.

Spitsbergen cherts contain abundant and remarkably well-preserved microfossils—exquisite tiny gems locked in a tomb of silica. Cherts in carbonates that formed above the high-tide mark usually contain only a single type of microfossil, thick-walled tubes about 10 microns across that form a tightly woven fabric in the rock (
plate 2a
). (A micron is extremely short—one-thousandth of a millimeter, or forty-millionths of an
inch. An eyelash is more than ten times as wide as these fossils.) The tubes are interpreted as the extracellular sheaths of filamentous cyanobacteria (
plate 2b
), those hardy bacterial practitioners of “green plant” photosynthesis. The microbial weave indicates that these minute organisms formed the microbial mats whose signature is written in the wavy lamination of encompassing carbonates. Low-diversity cyanobacterial mats occur today along the shoreward edge of restricted embayments from the Florida Keys and the Bahamas to the Persian Gulf and the arid coast of Western Australia.

On present-day tidal flats, microbial diversity increases toward the ocean, and Spitsbergen rocks show the same pattern. A series of mat-building cyanobacteria-like populations subdivided the ancient tidal gradient, forming discrete communities of mat builders and dwellers (organisms that lived in but did not contribute to the formation of the mat—like clams that nestle among frame-building corals in modern reefs).

Proterozoic microfossils have long been compared with living cyanobacteria, but how close is the comparison? Most “blue-greens” have simple shapes, and the morphological similarity between ancient and modern forms might mask deep physiological differences. Do we really mean to imply that cyanobacteria found today evolved before trilobites graced the oceans? One beautiful Spitsbergen population provides unusual insight into this issue.
Polybessurus bipartitus
consists of spheroidal cells 10 to 30 microns in diameter atop stalks made of extracellular secretions (
plate 2c
). Along the seaward edge of the ancient tidal flat,
Polybessurus
fossils are found as isolated individuals, but in more frequently exposed areas, they occur in dense populations that formed patchy crusts on the sediment surface. As my then graduate student Julian Green (now at the University of South Carolina) first recognized, morphological variations in the preserved fossils allow us to reconstruct their life history. Cells settled on the tidal-flat surface and, as they grew, began to secrete a series of extracellular envelopes. The stalks formed by successive envelopes enabled cells to maintain their position at the sediment-water interface despite an influx of lime mud. Once individuals reached a certain size, they divided repeatedly without intervening growth to form small cells that dispersed and settled again on the sediment surface, beginning the cycle again.

That’s a lot to know about a Precambrian microfossil, enough for us to seek meaningful comparisons with living organisms. Frustratingly, published compendiums of cyanobacterial biology do not describe living populations with the suite of characters observed in the Spitsbergen fossils. But we knew something else about our microfossils; they lived along a tidal flat that bordered a subtropical to tropical seaway where carbonate sediments accumulated.

Armed with this knowledge, I traveled with my friend, academic neighbor (at Boston University), and cyanobacteria guru Steve Golubic to the closest modern environmental analogue we could identify—the Bahama Banks. (Science occasionally compensates those who summer in Spitsbergen.) There, on the lonely western edge of Andros Island, we found small black crusts dotting a tidal flat of lime mud laced with cyanobacterial mats. The crusts formed in the upper part of the intertidal zone, built by small spheroidal cyanobacteria that secreted extracellular sheaths elongated in the downward direction (
plate 2d
). That’s right. Here, in a place predicted by Proterozoic rocks, we found the modern counterpart we sought—living but hitherto undescribed cyanobacteria whose morphology, life cycle, and environmental distribution match the ancient
Polybessurus bipartitus
.

The Spitsbergen example isn’t an isolated instance of good luck. Steve’s former graduate student Assad Al-Thukair (now at King Faisal University in Saudi Arabia) has discovered a half dozen new species of cyanobacteria that bore into and live within ooid grains; nearly all have exact fossil counterparts in silicified Proterozoic ooids of Spitsbergen and eastern Greenland. Because these cyanobacteria display stereotyped patterns of boring, even behavior can be included in the list of features shared by living and fossil populations. Steve and Montreal University’s Hans Hofmann have drawn equally fine-scale ancient-modern comparisons between mat-building cyanobacteria found today on arid tidal flats and fossils that lived in comparable environments 2 billion years ago.

Collectively, these discoveries put some teeth into the old saw that many Proterozoic fossils look like cyanobacteria. Because habitat range is a direct function of physiology, the close environmental similarity between ancient and living cyanobacteria suggests that the microorganisms distributed across Spitsbergen (and other Proterozoic) tidal flats
were essentially modern in morphology, life cycle,
and
physiology. Many of the cyanobacteria we see today are indeed survivors from the ancient Earth.

Cyanobacteria are common today in coastal habitats where very salty water or other environmental challenges restrict invasion by animals. By coincidence, the chert nodules in Proterozoic carbonates also center on coastal environments where silica was precipitated much like salt from evaporating seawater. Thus, chert’s paleontological lantern shines most brightly on just those environments where cyanobacteria have always thrived. Cyanobacteria do not live alone on modern tidal flats, however; mat communities contain a host of other organisms, especially bacteria. Why don’t we see this greater microbial diversity in chert nodules?

Tidal flats are harsh environments. At low tide, their inhabitants must tolerate the desiccating glare of a blazing sun; salty water provides an osmotic trial when the weather is dry; fresh water does the same during storms. Cyanobacteria respond to these challenges by secreting an extracellular envelope that protects the cells inside. That envelope is of particular importance to paleontologists because, unlike the cells within, it resists bacterial decay after death. Cyanobacteria, then, have the microbial equivalent of a clamshell, and in tidal-flat cyanobacteria this feature is especially well developed. While other bacteria live on tidal flats, most lack preservable walls or envelopes. And to make matters worse, they are tiny and have simple shapes that frustrate biological interpretation. The very fact that preserved fossils show evidence of postmortem decay means that heterotrophic bacteria must have lived in tidal-flat environments. As discussed below, geochemical signatures enable us to identify at least a few of these populations, but we must face the fact that what we see preserved in thin sections of chert, while extraordinary, represents only a limited sampling of the microorganisms that lived along the Proterozoic shoreline.

Fortunately, the sample preserved best is one well worth understanding. Cyanobacteria are the working-class heroes of the Precambrian Earth—the main primary producers in early oceans and the source of the oxygen that transformed terrestrial environments. We know a great deal about living cyanobacteria, including their phylogenetic relationships.
Add the good fortune that they are easily preserved and include species that can be recognized by form alone, and it becomes clear that cyanobacteria make an excellent flagship for paleontological studies of early life.

Although most fossils in the Spitsbergen cherts are undoubted or probable cyanobacteria, rare specimens of relatively large microfossils (greater than 100 microns), some shaped like miniature vases (
plate 2f
) and others studded with spines, provide tantalizing glimpses of a different biology. These fossils are limited to sediments that washed into tidal channels from offshore, suggesting that if we continue our environmental transect seaward, we might discover a diversity of Proterozoic life barely hinted at by the cherts.

During our field expeditions I collected many samples of black shales that accumulated in quiet subtidal environments beyond the ooid shoal. (Like the cherts, these shales are black because they contain organic matter.) Mesmerized by the chert biotas, I didn’t do much with our shale samples, but when Nick Butterfield (now, like Harland, at Cambridge University) came on board as a graduate student, I suggested that he take a look at them to get some firsthand experience with Precambrian rocks.

Microscopic fossils are common in shales of all ages, packed cheek by jowl with clay minerals that inhibit decay. The mineral fabric of these rocks can be dissolved in strong acid, leaving organic remains to be mounted on glass slides and studied by optical or electron microscope. Conventional preparations of Spitsbergen shales yield conventional fossils, but Nick developed a set of unconventional procedures that enabled him to identify and gently free fragile remains. His painstaking work revealed a paleontological treasure trove. Plenty of cyanobacteria occur in these shales; neither then nor now are these microbes limited to tidal flats. But the Spitsbergen shales also contain diverse fossils of eukaryotic organisms, recognizable by their distinctive shapes. The vaseshaped fossils that washed onto the tidal flat are there, as are the large cells decked with spines. More exciting, however, the shales contain multicellular algae, the remains of small seaweeds that formed lawns on the shallow seafloor. Some of these fossils closely resemble green algae
that can still be seen today (
plate 2e
). Others, however, have no close modern counterparts. Like trilobites and dinosaurs they are extinct, consigned by selection or catastrophe to the dustbin of (natural) history.

The Spitsbergen fossils are abundant and beautifully preserved, they are distributed over a range of sedimentary environments, and they include both prokaryotes and eukaryotes. On the other hand, they occur only in limited horizons of black chert and shale. The true ubiquity and diversity of late Proterozoic life is revealed by other biological indicators, most conspicuously
stromatolites
, the wavy-laminated, domed, and candelabrum-like structures seen in Akademikerbreen rocks (
figure 3.3
).

Stromatolites are the predominant features of carbonate rocks formed in Precambrian oceans. Stromatolitic buildups are uncommon today, but examples from places like the Bahama Banks and, especially, remote Shark Bay in Western Australia show how they form. Communities of microorganisms spread across sediment surfaces, weaving together to form coherent mats. Cyanobacteria (and, sometimes, algae) at the mat surface trap and bind fine particles supplied by waves and currents. As a veneer of mud or sand accumulates, these populations grow upward, reestablishing the mat at the sediment surface. Deeper in the mat, bacteria consume dead cells, changing local chemistry in a way that causes carbonate crystals to form. The processes of colonization, trapping and binding, and carbonate precipitation are discontinuous, but endlessly repetitive, with the result that fine layers of 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 the
association 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.