conditions, however, does nonbiological fractionation approach the levels recorded in Warrawoona rocks. For this reason, the consistently large fractionation measured in North Pole samples suggests the presence of an early biosphere.
Carbon isotopes in organic matter from the chert veins that cut through Warrawoona sediments and lavas could record chemosynthetic bacteria that lived in hydrothermal waters. But the widespread distribution of organic matter in sedimentary rocks deposited on the seafloor supports the hypothesis that photosynthesis fueled microbial life in the Warrawoona ocean. Whether primary producers were mostly cyanobacteria or other types of photosynthetic bacteria with a similar isotopic signature remains uncertain. Sulfur isotopes in sedimentary pyrite and barite likewise suggest that sulfate-reducing bacteria lived the Warrawoona lagoon, although this, too, has been questioned by geologists grown skeptical about early Archean biosignatures.
At present, that’s about all we can say. The heat-scorched hills of North Pole suggest that life existed 3.5 billion years ago, and that, by itself, is remarkable. Warrawoona communities may have included photosynthetic microorganisms and other microbes with metabolisms still seen today. But many uncertainties persist. Warrawoona paleobiology remains a shadow play whose apparently familiar themes may be deceptive.
Northwestern Australia is one of two places in the world that contain well-preserved sedimentary rocks as old as 3.5 billion years. The other is the rugged Barberton Mountain Land, near Kruger Park in South Africa. The two areas are similar, so much so that some geologists believe that they form parts of a single ancient terrain, severed by plate tectonicmovements long after the Archean. Their paleontological inventories bear comparison, as well. Both include stromatolites of uncertain origin, along with organic matter whose carbon isotopic composition and sedimentary distribution suggest some type of photosynthesis. Both have been heated to temperatures that destroy biomarker molecules. And like those of Warrawoona, Barberton cherts contain spherical and filamentous microstructures reminiscent of fossils.
As a graduate student, I had a go at Archean paleontology, traveling to Africa as Elso Barghoorn’s field assistant. Having grown up on Tarzan books, I was excited as the plane touched down in Johannesburg late at night. I couldn’t wait to catch my first glimpse of Africa the next morning, and was only a little disappointed that the view from my hotel window looked a lot like Chicago. Within hours, we were on the road, and as the cityscape receded in the rearview mirror, the great South African veldt opened before us. Culturally, ecologically, and geologically, the Barberton Mountain Land was new to me. In each clump of thorn trees I sensed menace, and in each chert I espied fame. Neither fame nor menace materialized, but the cherts did turn out to contain microstructures that are probably, if not unambiguously, biological.
In one particular sample marked by centimeter-scale stromatolite-like precipitates, I discovered a large population of spherical microstructures 2 to 4 microns in diameter—the size and shape of small cyanobacteria ( figure 4.6a ). The structures occur in individual laminae. Moreover, they are made of organic matter, and some preserve both an outer wall and a raisinlike interior body, also organic. The microstructures are compressed along the bedding surface, much like younger microfossils—indeed, this slight flattening tells us that the structures formed before enclosing sediments were compacted by burial. The distribution of sizes in the population matches that of modern cyanobacteria, and the structures also show evidence of binary division, again much like living blue-greens.
So are these fossil cyanobacteria? Not necessarily. Many different bacteria are small and spherical. More sobering, nonbiological processes can, in
Carbon isotopes in organic matter from the chert veins that cut through Warrawoona sediments and lavas could record chemosynthetic bacteria that lived in hydrothermal waters. But the widespread distribution of organic matter in sedimentary rocks deposited on the seafloor supports the hypothesis that photosynthesis fueled microbial life in the Warrawoona ocean. Whether primary producers were mostly cyanobacteria or other types of photosynthetic bacteria with a similar isotopic signature remains uncertain. Sulfur isotopes in sedimentary pyrite and barite likewise suggest that sulfate-reducing bacteria lived the Warrawoona lagoon, although this, too, has been questioned by geologists grown skeptical about early Archean biosignatures.
At present, that’s about all we can say. The heat-scorched hills of North Pole suggest that life existed 3.5 billion years ago, and that, by itself, is remarkable. Warrawoona communities may have included photosynthetic microorganisms and other microbes with metabolisms still seen today. But many uncertainties persist. Warrawoona paleobiology remains a shadow play whose apparently familiar themes may be deceptive.
Northwestern Australia is one of two places in the world that contain well-preserved sedimentary rocks as old as 3.5 billion years. The other is the rugged Barberton Mountain Land, near Kruger Park in South Africa. The two areas are similar, so much so that some geologists believe that they form parts of a single ancient terrain, severed by plate tectonicmovements long after the Archean. Their paleontological inventories bear comparison, as well. Both include stromatolites of uncertain origin, along with organic matter whose carbon isotopic composition and sedimentary distribution suggest some type of photosynthesis. Both have been heated to temperatures that destroy biomarker molecules. And like those of Warrawoona, Barberton cherts contain spherical and filamentous microstructures reminiscent of fossils.
As a graduate student, I had a go at Archean paleontology, traveling to Africa as Elso Barghoorn’s field assistant. Having grown up on Tarzan books, I was excited as the plane touched down in Johannesburg late at night. I couldn’t wait to catch my first glimpse of Africa the next morning, and was only a little disappointed that the view from my hotel window looked a lot like Chicago. Within hours, we were on the road, and as the cityscape receded in the rearview mirror, the great South African veldt opened before us. Culturally, ecologically, and geologically, the Barberton Mountain Land was new to me. In each clump of thorn trees I sensed menace, and in each chert I espied fame. Neither fame nor menace materialized, but the cherts did turn out to contain microstructures that are probably, if not unambiguously, biological.
In one particular sample marked by centimeter-scale stromatolite-like precipitates, I discovered a large population of spherical microstructures 2 to 4 microns in diameter—the size and shape of small cyanobacteria ( figure 4.6a ). The structures occur in individual laminae. Moreover, they are made of organic matter, and some preserve both an outer wall and a raisinlike interior body, also organic. The microstructures are compressed along the bedding surface, much like younger microfossils—indeed, this slight flattening tells us that the structures formed before enclosing sediments were compacted by burial. The distribution of sizes in the population matches that of modern cyanobacteria, and the structures also show evidence of binary division, again much like living blue-greens.
So are these fossil cyanobacteria? Not necessarily. Many different bacteria are small and spherical. More sobering, nonbiological processes can, in
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