A. Yu. Rozanov

Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the ‘Cambrian explosion’

We report 87Sr86Sr measurements on a suite of well preserved sedimentary carbonates from Lower Cambrian strata of the Lena River region of Siberia. Stable isotopes and major and trace element chemistry have been used to identify potentially unaltered samples for Sr isotopic measurements. The Sr data define a smooth curve of paleoseawater 87Sr86Sr values from the Tommotian through to the early Middle Cambrian. During the Tommotian-Atdabanian interval, 87Sr86Sr rose rapidly from 0.7081 to 0.7085. The rate of change in Sr ratios decreased during the Botomian but rose to 0.7088 in the late Toyonian to early Middle Cambrian. The rate of 87Sr86Sr increase during the Tommotian-Atdabanian was ca. 0.0001/m.y., comparable to the late Miocene change in seawater Sr. We infer that an interval of enhanced erosion during the ‘Cambrian explosion’ was responsible for this increase. An important source for radiogenic Sr to the oceans may have been erosion of the Pan-African orogenic belt of southern Africa. The rapid change in paleoseawater Sr corresponds with an interval of highly variable marine δ13C values. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the δ13C record is not a consequence of direct erosional forcing. However, our inference of high erosion rates during the Tommotian-Atdabanian implies enhanced fluxes of nutrient elements such as P to the oceans. Phosphorite deposits and black shale deposition in coeval strata suggest that periods of high marine productivity and anoxia may be in part related to enhanced river dissolved fluxes. Our results thus provide some insight into environmental conditions during the ‘Cambrian explosion.’

Precambrian-Cambrian boundary problem: carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco

At least three distinctive cycles are preserved in the carbon-isotope patterns for the Vendian/Tommotian interval of the Siberian platform and the Anti-Atlas Mountains of Morocco. In Siberia, an early Tommotian carbon cycle provides the first detailed test of correlations based on archaeocyathid biostratigraphy between classic sequences on the Aldan and Lena rivers, and demonstrates that the Early Cambrian zone boundaries are reasonably good time lines. The carbon signal is preserved in both limestone and dolomite and across regional faties variations. The δ^(13)C results from the late Vendian section in Morocco display a pattern similar to that found in Siberia, suggesting that carbon isotopes can be used to test stratigraphic correlations on an intercontinental scale.

A carbon isotope reference scale for the Lower Cambrian succession in Siberia: report of IGCP Project 303

Four sections from the mid-Atdabanian to lowest Toyonian (middle Cambrian) along the Lena River of Siberia were sampled for carbon isotope stratigraphy. These show a mainly heavy but highly oscillatory SC signature for the Atdabanian to mid-Botomian interval, coincident with the major phase of invertebrate innovation. A prolonged interval of negative <SC followed until late Toyonian times, coincident with Botomian-Toyonian mass extinctions. Eleven carbon isotope cycles are identified through the lower Cambrian, which should now be tested for their utility in global correlation and relationship to bioevents in the Cambrian explosion.

Magnetostratigraphy of Lower Cambrian strata from the Siberian Platform; a palaeomagnetic pole and a preliminary polarity time-scale

Four sedimentary sections seen in continuous exposures along the Lena River on the Siberian Platform in Yakutia contain a record of the geomagnetic field during the Tommotian and Atdabanian stages of Early Cambrian time. The stable remanent magnetization indicates that the Siberian platform was located on the equator, and the corresponding paleomagnetic pole provides a well-dated extension of the Siberian apparent polar wander path. A belt of archaeocyathid bioherms which separates two major facies zones in the Lower Cambrian was positioned on and aligned more or less parallel with the paleoequator. The geographical position of this belt appears to have tracked the southward motion of the Siberian Platform during post-Tommotian time. These palaeomagnetic results combined with the extensive biostratigraphy of the Siberian Platform provide a provisional geomagnetic polarity time scale for this part of Early Cambrian time. Comparison of these results with data of similar age from Central Australia suggests that strata of Tommotian and lower Atdabanian age are not present in the Amadeus Basin of Australia.--Modified journal abstract.

The stage scale of the Cambrian System

A stage subdivision of the Cambrian System is proposed. In the lower series four stages are distinguished (the Tommotian, Atdabanian, Botomian, Tojohnian); in the middle series, two (the Amga and Maya stages); in the upper series, three (the Ayusakkanian, Sakian, and Aksayan stages). The stratotypes of the stages of the lower and middle series are located on the Siberian Platform, while the stratotypes of the stages of the upper series are found in Kazakhstan (the Maly Karatau). The lower boundary of the Cambrian is drawn along the bottom of the Al. sunnaginicus Zone, the lower boundary of the middle series along the Schistocephalus Zone, and that of the upper series along the Cormagnostus simplex Zone. The Cambrian–Ordovician boundary is not quite clear yet, but it is most likely to be drawn along the bottom of the C. proavus Zone.

Upper Cambrian Archaeocyatha from Antarctica

Geological mapping in the Ellsworth Mountains of west Antarctica have established the existence of extensive Middle and Late Cambrian strata. Forms tentatively referred to Archaeocyatha have been examined by F.D. and A.R., who concluded that, despite the commonly held opinion that the phylum became extinct at the boundary of the Lower and Middle Cambrian, the Ellsworth Mountains forms represented Irregularian Archaeocyatha which survived in protected niches into the Late Cambrian. The age of the fauna containing the archaeocyaths is firmly dated by abundant trilobites including species of Homagnostus, Pseudagnostus, Kormagnostella, Erixanium and Onchopeltis.--Modified journal abstract.

Prasinophyceae (Green Algae) from the Lower Proterozoic of the Kola Peninsula

Eukaryotic organisms discovered from the earliest Lower Proterozoic phosphorites (2.04 Ga) of the Kola Peninsula are described. These are fossil forms Pechengia melezhiki gen. et sp. nov., which are tentatively assigned to green algae of the class Prasinophyceae.

The evolution of the early precambrian geobiological systems

Bacterial paleontology provides vast opportunities for the study of all types of sedimentary and volcanic-sedimentary rocks, at any stage of metamorphism and of any age. Bacteria are shown to play an important role in weathering, transfer, sedimentation, and diagenesis of the sediments and in the formation of many minerals that have previously been thought to be abiogenic.

Fossil Bacteria from the Permotriassic Trappean Strata of Siberia

The strata of the Permotriassic Trappean Complex of Siberia (Ilimpeya River and Kapchan locality) are studied. The water-lava and water-tuff boundaries are shown to be promising for bacterial paleontological studies. The analysis of fossilized microbial communities shows that they vary depending on sedimentation conditions. This example is important for a better understanding of the prospects for the study of similar situations in the Archean and Proterozoic.

Life conditions on the early Earth after 4.0 Ga

The organization level of Precambrian fossils is the most reliable indicator of the state and parameters of the biosphere, such as the atmosphere composition, average temperature of the earth’s surface, and others. At present, cyanobacteria, unicellular and multicellular eukaryotes, and coelomates are considered to appear in the geological history of the Earth much earlier than it was supposed previously. Our knowledge and ideas of the early Earth are very important for considering the problems of the origin of life. A key boundary of the earliest period was probably about 4 Ga. This boundary is between the periods documented and undocumented by the geological record. The Earth history and probable surface conditions before 4 Ga are considered by L.M. Mukhina, A.V. Vityazeva, G.V. Pechernikova, and L.V. Ksanfomaliti in this volume.