Stanley M. Awramik

Precambrian Columnar Stromatolite Diversity: Reflection of Metazoan Appearance

Columnar stromatolites (organosedimentary structures built by bluegreen algae) show a marked decrease in diversity in the Late Precambrian; this decrease in diversity occurs at approximately the same time as the appearance of metazoans, 600 to 700 million years ago.

THE GUNFLINT MICROBIOTA

The microbiota of the Gunflint Iron Formation (∼2 Ga old) is sufficiently great in diversity as to represent a “benchmark” in the level of evolution at a time only somewhat less than intermediate between the origin of the earth and the present. To date, thirty entities from these ∼2 Ga old microfossiliferous cherts have been described and all but two systematically categorized. From our continuing detailed study of the Gunflint microbiota (ESB for over 20 years) and, in light of our recent investigations on blue-green algal cell degradation, we conclude that: (1) A considerable number of the taxa systematically described are either of doubtful biological origin, doubtful taxonomic assignment, and/or morphologically indistinguishable from previously described Gunflint microorganisms, (2) The microbiota is wholly prokaryotic. At present, we recognize sixteen taxa falling within three categories: (1) blue-green algae (6 taxa; e.g. Gunflintia minuta); (2) budding bacteria (4 taxa; e.g. Eoastrion simplex); and (3) unknown affinities (6 taxa; e.g. Eosphaera tyleri). Organisms of undoubted eukaryotic affinities have yet to be found in the Gunflint. The Gunflint assemblage includes a high percentage of morphologic entities of obscure taxonomic position. Recently, Walter (1975) and Knoll and Barghoorn (1975) reported Gunflint-type microbiotas of approximately the same age as the Gunflint from two localities in Australia. The dominant morphotypes of the Gunflint microbiota appear to be cosmopolitan and the striking similarity of the three assemblages may strengthen the potential of ancient microbiotas for use in Precambrian biostratigraphy.

The oldest records of photosynthesis

There is diverse, yet controversial fossil evidence for the existence of photosynthesis 3500 million years ago. Among the most persuasive evidence is the stromatolites described from low grade metasedimentary rocks in Western Australia and South Africa. Based on the understanding of the paleobiology of stromatolites and using pertinent fossil and Recent analogs, these Early Archean stromatolites suggest that phototrophs evolved by 3500 million years ago. The evidence allows further interpretation that cyanobacteria were involved. Besides stromatolites, microbial and chemical fossils are also known from the same rock units. Some microfossils morphologically resemble cyanobacteria and thus complement the adduced cyanobacterial involvement in stromatolite construction. If cyanobacteria had evolved by 3500 million years ago, this would indicate that nearly all prokaryotic phyla had already evolved and that prokaryotes diversified rapidly on the early Earth.There is diverse, yet controversial fossil evidence for the existence of photosynthesis 3500 million years ago. Among the most persuasive evidence is the stromatolites described from low grade metasedimentary rocks in Western Australia and South Africa. Based on the understanding of the paleobiology of stromatolites and using pertinent fossil and Recent analogs, these Early Archean stromatolites suggest that phototrophs evolved by 3500 million years ago. The evidence allows further interpretation that cyanobacteria were involved. Besides stromatolites, microbial and chemical fossils are also known from the same rock units. Some microfossils morphologically resemble cyanobacteria and thus complement the adduced cyanobacterial involvement in stromatolite construction. If cyanobacteria had evolved by 3500 million years ago, this would indicate that nearly all prokaryotic phyla had already evolved and that prokaryotes diversified rapidly on the early Earth.

Biostratigraphic and chemostratigraphic correlation of Neoproterozoic sedimentary successions: upper Tindir Group, northwestern Canada, as a test case.

Recent advances in Proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of Neoproterozoic (1000-540 Ma) successions is possible. Because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available, the upper Tindir Group of northwestern Canada provides an opportunity to test this hypothesis. The age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc-shaped-scale microfossils that may provide insights into the early evolution of biomineralization. A reinterpretation of upper Tindir microfossil assemblages suggests a late Riphean age. Although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates, least altered samples indicate that delta 13C of contemporaneous seawater was at least +4.7%, typical of Neoproterozoic, but not Cambrian, carbonates. Strontium isotopic compositions of the least altered samples yield values of approximately 0.7065, which can be uniquely correlated with late Riphean seawater. Together, micropaleontology and the isotopic tracers of C and Sr constrain the upper Tindir carbonates and their unique fossils to be late Riphean, likely between 620 and 780 Ma.

Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites

Fossil and living stromatolites are abundant around the margins of Lake Tanganyika, Africa, and provide a wealth of paleolimnologic and paleoclimatic information for the late Holocene. Six lines of evidence show that stromatolites and cements are precipitating in the lake today: (1) carbonate saturation state calculations, (2) documentation of living stromatolites and their depth distribution, (3) new stable isotope data showing the lake’s present mixing state and ancient evaporation and inflow balance, (4) new radiocarbon data and a reevaluation of apparent 14 C ages derived from Lake Tanganyika carbonates, (5) the presence of modern Mg-calcite cements derived from lake waters, and (6) the presence of modern, biologically mediated Mg-calcite precipitates in the lake. Lake Tanganyika’s lake levels have been remarkably stable over the past 2800 yr, fluctuating around the marginally open to marginally closed level through most of this time period. Lake lowstands and high δ 18 O values from the ninth century B.C. to the early fifth century A.D. indicate that the lake basin was comparatively dry during this time. However, the period prior to the most recent opening of Lake Kivu into the Lake Tanganyika basin (ca. A.D. 550) was not marked by major lake lowstands, nor was this opening accompanied by a dramatic lakelevel rise. The Kivu opening was roughly coincident with a significant shift toward isotopically lighter (δ 18 O and δ 13 C) lake water, which persists today. The lake remained close to its outlet level between the sixth and thirteenth centuries A.D. Lake levels rose between the fourteenth and sixteenth centuries. At some time between the late sixteenth and early nineteenth centuries, lake level fell to perhaps its lowest level in the past 2800 yr. By the early nineteenth century, lake level had begun to rise to the overflow level, apparently the result of a regional increase in precipitation/evaporation ratios. Weak δ 18 O/δ 13 C covariance for late Holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level, with only occasional periods of closure. However, there is no simple relationship between solute input from Lake Kivu, isotope input from Lake Kivu, and lake levels in Lake Tanganyika. Lake Kivu waters are the primary source of major ions in Lake Tanganyika, but are much less important in controlling the δ 18 O and the lake level of Lake Tanganyika. Because the Ruzizi River’s discharge into Lake Tanganyika is largely derived from sources other than Lake Kivu, the overflow events in the two lakes have been uncoupled during the late Holocene.

A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA

A thin carbonate unit associated with a Sturtian-age (≈750–700 million years ago) glaciogenic diamictite of the Neoproterozoic Kingston Peak Formation, eastern California, contains microfossil evidence of a once-thriving prokaryotic and eukaryotic microbial community (preserved in chert and carbonate). Stratiform stromatolites, oncoids, and rare columnar stromatolites also occur. The microbial fossils, which include putative autotrophic and heterotrophic eukaryotes, are similar to those found in chert in the underlying preglacial units. They indicate that microbial life adapted to shallow-water carbonate environments did not suffer the significant extinction postulated for this phase of low-latitude glaciation and that trophic complexity survived through snowball Earth times.

Biogenic Magnetite in Stromatolites. II. Occurrence in Ancient Sedimentary Environments

Abstract In this paper we report the discovery of fossil bacterial, single-domain magnetite particles in ancient stromatolites. The biogenicity of the crystals was determined by the following criteria: (1) distinctive morphology and habit, (2) composition and (3) environment of deposition. Stromatolites ranging in age from the Middle Archean to Pleistocene, composed of both carbonate and chert, were analyzed for the presence of single-domain magnetite using rock magnetic methods. The granulometry and composition of the ultra-fine-grained magnetite crystals extracted were determined by transmission electron microscopy and electron diffraction. The oldest magnetofossils were extracted from stromatolitic chert of the Gunflint Iron Formation which is approximately 2000 Ma old. The implications of these findings and the potential uses of fossil bacterial magnetite in studies of the evolution of biomineralization and prokaryotic metabolic processes, paleomagnetism, and as an indicator of ancient oxygen levels are discussed. Bacterial magnetite represents the oldest evidence of biomineralization yet discovered in the fossil record.

Organic-walled microfossils from earliest Cambrian or latest proterozoic Tindir Group rocks, Northwest Canada

Abstract Uppermost Tindir Group limestones exposed in the headwaters of Tindir Creek, Yukon Territory, Canada, contain a diverse and abundant microbiota of mineralized and organic-walled microfossils. Preserved primarily in chert modules and chert beds, the organic-walled forms include mat-building cyanobacterial filaments and coccoids accompanied by less common bacteria, fungi and forms of uncertain affinity as well as planktonic coccoidal cyanobacteria and acritarchs. Several lines of evidence support interpretation of an Early Cambrian age whereas others suggest a Late Proterozoic age. Of the 54 organic-walled taxa recognized, 12 are newly described: Yukonosphaeridion interior Awramik, n. gen., n. sp., Microagglomeratus borealis Awramik n. gen., n. sp., Phacelogeminus lineatus Awramik n. gen., n. sp., Palaeoanacystis magna Awramik, n. sp., Cephalophytarion majesticum Allison n. sp., Heliconema bulbosa Allison n. sp., Trachyhystrichosphaera magna Allison n. sp., Fusilabellum brevistriatum Awramik n. gen., n. sp., Sphaeranasillos irregularis Allison n. gen., n. sp., Hyalocyrillium clardyi Allison n. gen., n. sp., Eophycomyces herkoides Allison n. gen., n. sp. and Archeomyces dimakeloides Allison n. gen., n. sp.

PLIO-PLEISTOCENE LACUSTRINE STROMATOLITES FROM LAKE TURKANA, KENYA: MORPHOLOGY, STRATIGRAPHY AND STABLE ISOTOPES

Abell, P.I., Awramik, S.M., Osborne, R.H. and Tomellini, S., 1982. Plio-Pleistocene lacustrine stromatolites from Lake Turkana, Kenya: morphology, stratigraphy and stable isotopes. Sediment. Geol., 32: 1--26. A sequence of fossil stromatolites from Lake Turkana in Kenya was examined for 51SO and 513C content. These stromatolites, ranging in age from Holocene (~10,000 yrs B.P.) to Middle Pliocene (-3 m.y.) showed a variety of growth forms from oncolitic, columnar layered to bulbous heads. The stromatolites used in our study contain filamentous blue-green algae of one morphological type and rare coccoids; thus the stromatolites are considered biogenic. The stable isotope ratios for oxygen and carbon indicate changing climatic conditions, ranging from a cool, wet climate prior to ca. 1.9 m.y. to much drier, warmer conditions around 1.4 m.y., followed in turn by a somewhat cooler and wetter climate at the end of the Pleistocene.

The History and Significance of Stromatolites

The fossil record of prokaryotes extends back 3500 million years and prokaryotes were only forms of life known for the first 2000 million years of Earth history. Understanding the fossil record of these organisms is important for the comprehension of the interaction of geological factors and the evolution of life. Prokaryo-tic microbial fossils, however, do not provide sufficient detailed evidence to help a great deal in this. Stromatolites, the biosedimentary products of microbe-sediment interactions, have the potential to provide significant information on the interaction of the biosphere, atmosphere, hydrosphere, and litho-sphere throughout the entire history of life on this planet.