Read Online Life on a Young Planet by Andrew H. Knoll - Free Book Online

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, reachingits 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.
    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