Life on a Young Planet (7 page)

Figure 3.1.
Proterozoic rocks of the Akademikerbreen Group exposed in the glaciated highlands of northeastern Spitsbergen. Each band of light or dark gray rock is about 1,000 feet thick.

The top of the sedimentary pile in this region contains Early Cambrian fossils, much like those discovered along the Kotuikan River. Below extends a vast geological tome, more than 20,000 feet thick, written in a tropical seaway 800 to 600 million years ago. Consistent with their stratigraphic position beneath the oldest Cambrian strata, these Spitsbergen rocks contain no skeletons, no compressed carcasses, no
tracks and trails—no evidence of animal life whatsoever. This doesn’t necessarily mean that animals were absent when these beds were deposited, but any animals that did exist must have been tiny creatures that made little impact on accumulating sediments. Of course, the Tree of Life tells us that animals were preceded in time by other organisms, so we might search these rocks not for clams and brachiopods but instead for the fossils of algae, protozoans, or even bacteria. Northeastern Spitsbergen is a nearly ideal place to ask paleontological questions about this earlier biology. That’s what we were doing there, seeking insights into life and environments before the Cambrian Explosion.

Spitsbergen isn’t the first place where Proterozoic fossils were found, and I certainly wasn’t the first person to find them. That honor goes Elso Barghoorn, the father of Precambrian paleontology and my mentor at Harvard. In 1954, Barghoorn and geologist Stanley Tyler reported the discovery of bacterial cells in nearly 2-billion-year-old rocks of the Gunflint Formation, western Ontario—I was three years old and unaware that the seeds of my professional life were being planted. Spitsbergen
is
, however, where my own odyssey in Precambrian research took wing. In 1978, a freshly minted Ph.D. and vividly green assistant professor, I was casting about for a project that would establish my scientific independence from the great Barghoorn. I had read about this inhospitable but potentially rewarding island in geological reports by Brian Harland of Cambridge University, and a hopeful letter to Harland (“Can you give me some pointers on working in Spitsbergen?”) drew the best possible response: “Can you join us next summer?” Sure I could, little dreaming that this would redefine my research future.

The following summer found me on a small Norwegian fishing boat, sailing with three Cambridge shipmates around the northern tip of the island. Some mornings the sea was like glass, mirroring the bright sun as well the ice-covered peaks that lined the coast. Other days brought storms, and with them gray-green waves that tossed our boat (and my stomach) about like a toy. I knew little about sailing, less about wielding a pike to nudge ice out of our path, and absolutely nothing about what to do when whales approached our boat. (Slow down and steer
very
carefully.) I did know a little about paleontology, and, fortunately, promising rocks were abundant in the outcrops we studied.

A minor frustration of Proterozoic paleontology is that the quarry is usually too small to be seen by eye. We can only collect rocks that experience tells us might yield fossils, ship them optimistically back to the lab, and then, months later, examine prepared specimens to learn of success or failure. I was lucky. The first sample I looked at turned out to be brimming with exceptionally preserved microfossils. Peering down my microscope, I felt like Howard Carter beaming his lamp into King Tut’s tomb, privileged to see “things, beautiful things” in the paper-thin slice of rock beneath my lens. I had a project, and over the next seven years I would return repeatedly to this island, traveling by helicopter and snowshoe in a protracted effort to learn the paleobiological secrets of this remarkable place.

I began by writing that the Spitsbergen rocks were deposited in a tropical seaway. How do we know this, and if we are correct, what are they doing on a mountaintop near the North Pole? Sedimentary geology is a vast repository of anecdote, lent structure by theory and experiment. A field geologist may notice the pattern of ripples that forms in sands along a lakeshore. Another will ask questions about these ripples in the laboratory, using a flow tank to determine the range of physical conditions under which they can form. Through repeated iterations of observation and experiment, an association grows between sediment pattern and the processes that govern its formation. In consequence, an experienced geologist can examine the pattern of bedding, structure, and texture in an ancient sandstone and from it infer the set of environments and sedimentary processes in play when it formed.

Interpreting the Spitsbergen rocks requires that we log the composition, thickness, and bedding features of successive layers. We must put hand lens to rock and eye to hand lens, scrunching our faces against the cliff to gain a magnified view of the ancient sediments. Every now and again, we wield a “persuader,” the steel-shafted hammer tucked into every geologist’s belt, to gather fist-size samples that can be shipped home. Back in the lab, field observations are supplemented by studies of thin sections (those paper-thin slices of rock) that reveal micron-scale features under the microscope. In the end, we associate each bed with a suite of characters that can be compared against features seen by other geologists in other rocks and in sediments accumulating today. Dialing
into that collective pool of experience, we reconstruct the history of a small part of the world as it existed long ago.

The backbone of the Spitsbergen succession is the Akademikerbreen Group, a thick (seven thousand feet) stack of limestones and related rocks deposited near the edge of an ancient ocean (
figure 3.1
). Irregularly laminated dolomites
¹
mark the shoreward edge of deposition. The millimeter-thick layers, or laminae, in these rocks (
figure 3.2
) closely resemble structures formed today where microbial mats spread across tidal flats, trapping and binding fine sedimentary particles to form thin layers, like delicate leaves of puff pastry. Prism-shaped cracks form a network of polygons in some beds. Most of us have seen similar features forming today where exposure to the sun causes wet muds to dry and crack; ancient mud cracks formed the same way.

Another curious feature helps to focus environmental interpretation. Here and there, sets of laminae several centimeters thick turn upward to form ridges, commonly fractured at their crests (
figure 3.2
). In cross section, these features resemble wigwams, hence, their easily remembered name: tepee structures. Tepees form today along warm lime-rich coastlines in the zone just above high tide, where surface sediments are baked by the sun and only occasionally inundated by the sea. In this regime, the growth of carbonate and gypsum crystals builds up pressure in surface beds. Eventually, the beds buckle, forming the cracked ridges.

The stacked beds of the Akademikerbreen Group, thus, provide a record of time
and
environment. We just learned that some of the Akademikerbreen carbonates formed at the very edge of a coastal carbonate platform. Not surprisingly, these rocks occur in close association with beds deposited just seaward, in the zone between high and low tides. Intertidal rocks are characterized by alternating sandy and muddy layers that record variations in wave and current energy, microbial mat layers without tepees, thicker beds formed by flowing tides whose angled, sandy laminae impart a herringbone texture to the rock, and shallow channels cut by tides into underlying sediments. A comparable suite of features can be seen today in places like the Bahama Banks, where they form a clear sign of tidal-flat sedimentation.

Figure 3.2.
Akademikerbreen carbonates deposited near the high-tide mark show wavy laminations characteristic of cyanobacterial mats, as well as tepee structures that provide a key to environmental interpretation. Nodules of black chert within the carbonate beds contain abundant fossils of filamentous microorganisms. Scale is 6 inches long.

Intertidal rocks, in turn, mingle with beds formed below the low-tide mark, in a coastal lagoon filled by lime muds, sands, and flakes stripped from the tidal flat during storms. The ancient Spitsbergen lagoon was protected by a shoal of ooids, tiny spheres of concentrically laminated carbonate whose modern counterparts form where coastal waves repeatedly suspend particles in warm, lime-rich waters. Laminated domes and candelabrum-like structures up to several feet thick also punctuate the succession—patch reefs built by microbial communities.

The first lesson learned by aspiring geologists is “the present is the key to the past.” Thus, modern sediments on the Bahama Banks help us to make sense of the Akademikerbreen Group. The ability to link processes observable today with patterns that can be recognized in ancient rocks makes the geological elucidation of our planet’s history possible. We
must be careful, however, not to get carried away by the beauty of this premise. Uniformity of process does not boil down to
plus ça change, plus c’est la même chose
. The tectonic, sedimentary, and geochemical
processes
at work today may have been in force throughout Earth history, but that doesn’t mean that the
state
of our planet’s surface has been invariant through time. Ocean chemistry, geography, and climate have all changed through time in ways that have been decisive for the history of environments—and life.

Mies van der Rohe, the great Bauhaus architect, reputedly said that “God is in the details.” That’s also where we’ll find the keys to ancient states of the ocean and atmosphere. Take, for example, the ooid shoals mentioned earlier. Today, marine ooids have a maximum diameter of about one millimeter—the size of sand grains. The Akademikerbreen ooids, in contrast, reach the size of garden peas. Evidently, the chemistry of the Spitsbergen seaway wasn’t quite like that of its closest modern analogues; it was more highly charged with calcium and carbonate ions, causing ooids to accrete faster and attain larger sizes than can be accomplished today. The giant ooids of Spitsbergen provide a first hint that the Precambrian Earth was not simply our own world with the plants and animals stripped away, an observation that will be developed in later chapters into a principal theme of deep Earth history. For now it is sufficient to remember that the
uniformitarian principle—
”the present is the key to the past”—is a statement about process, and one that should be viewed more as working hypothesis than universal truth in studies of the early Earth.

Having outlined our reasons for interpreting the Spitsbergen rocks as products of tropical sedimentation, we should consider, at least briefly, why they sit today in refrigerated cliffs north of the Arctic Circle. The explanation is that plate tectonic processes transported them to their current position. The hypothesis that continents have drifted through time was proposed early in the twentieth century by the German meteorologist Alfred Wegener, but it gained wide acceptance only in the 1960s and 1970s, when geophysical observations revealed how a seafloor conveyor belt, formed at oceanic ridges and destroyed beneath deep trenches, moves continents from place to place. Northeastern Spitsbergen moved poleward through the Paleozoic and Mesozoic eras, reaching
its current latitude more than 100 million years ago. Then, with the opening of the Atlantic Ocean, this piece of real estate broke away from its closest geological relatives (now in Greenland), eventually to enter a deep freeze as the great Pleistocene Ice Age began. The geographic repositioning of Spitsbergen is a bit of good fortune, leaving us with rocks that are beautifully exposed and little altered by surface weathering—an unlikely prospect had this landmass remained at low latitudes.

By now, we know that the Spitsbergen rocks formed before the dawn of the Cambrian, in coastal environments at the edge of a tropical ocean. But was there life in that ocean, and did it leave a record in the Akademikerbreen sediments? That’s what we really want to know, and to find out, we must search for rocks likely to preserve delicate biological remains. Chert (also known as flint) is one such rock, an extraordinarily hard substance made up of tiny interlocking crystals of quartz (crystalline silica, or SiO
₂
). Chert is hard enough to withstand the mechanical ravages of tectonic deformation and impermeable enough to shield its contents from corroding fluids. Encased in chert, then, sedimentary features—including
biological
features—can be preserved for the ages.