Julie K. Bartley

Global Events Across the Mesoproterozoic-Neoproterozoic Boundary: C and Sr Isotopic Evidence from Siberia

Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur–Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in 13 C from values near 0‰ in the early Mesoproterozoic to values near +3.5‰ after about 1300 Ma. Over this same interval, primary 87 Sr/ 86 Sr values decrease from 0.7060 to 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval. Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, 3.5‰ positive shift ca. 1250–1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in 87 Sr/ 86 Sr heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.

Marine carbon reservoir, Corg-Ccarb coupling, and the evolution of the Proterozoic carbon cycle

The post–2.0 Ga Proterozoic C isotope record reveals two distinct, yet interrelated trends: a stepwise increase in average δ13C from ∼0‰ (calculated with respect to the Peedee belemnite isotope standard) prior to ca. 1.3 Ga to >+5‰ in the Neoproterozoic, and a concomitant increase in the magnitude of isotopic excursions. Steady-state and non–steady-state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon (DIC) reservoir. The DIC reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation. Major rearrangements of carbon cycling during the Proterozoic, in part related to the evolving marine carbon reservoir, permit elevated δ13C values to be sustained for geologically long time spans. Recognition of this dependence on DIC reservoir size provides, for the first time, a direct link between changing carbonate precipitation styles and the marine C isotope record and may help constrain estimates of Proterozoic p CO2.

Actualistic taphonomy of cyanobacteria; implications for the Precambrian fossil record

Actualistic taphonomy can elucidate aspects of the fossil record that would otherwise be uninterpretable. Investigation of taphonomic alteration patterns in modern cyanobacteria permits evaluation of preservation biases and decomposition timescales in the Precambrian rock record. In this study, a method for assessing alteration of filamentous cyanobacteria was developed, and cyanobacterial decomposition was evaluated during short-term (days to months) experiments. Results indicate that extracellular sheaths are more likely to be preserved than are trichomes, and that rates of decomposition are variable among cyanobacterial taxa. The effects of oxygen, temperature, and desiccation on taphonomic alteration were also investigated. Desiccation of cyanobacteria produces cell shriveling, a feature that may be diagnostic of evaporitic environments. Short-term decomposition of cyanobacteria by heterotrophic bacteria under anaerobic and aerobic conditions did not produce systematically different degradation patterns; thus, alteration patterns are of limited utility for constraining the oxygen content of depositional environments. Rapid rates of decomposition suggest that in order to achieve excellent morphological preservation, heterotrophic activity must have ceased within days to weeks of deposition. These results provide constraints on the timing of lithification for comparable Precambrian lagerstdtten.

A Vendian-Cambrian Boundary Succession from the Northwestern Margin of the Siberian Platform: Stratigraphy, Palaeontology, Chemostratigraphy and Correlation

Siberia contains several key reference sections for studies of biological and environmental evolution across the Proterozoic-Phanerozoic transition. The Platonovskaya Formation, exposed in the Turukhansk region of western Siberia, is an uppermost Proterozoic to Cambrian succession whose trace and body fossils place broad limits on the age of deposition, but do not permit detailed correlation with boundary successions elsewhere. In contrast, a striking negative carbon isotopic excursion in the lower part of the Platonovskaya Formation permits precise chemostratigraphic correlation with upper-most Yudomian successions in Siberia, and possibly worldwide. In addition to providing a tool for correlation, the isotopic excursion preserved in the Platonovskaya and contemporaneous successions documents a major biogeochemical event, likely involving the world ocean. The excursion coincides with the palaeontological breakpoint between Ediacaran- and Cambrian-style assemblages, suggesting a role for biogeochemical change in evolutionary events near the Proterozoic Cambrian boundary.

Molar tooth structures in calcareous nodules, early Neoproterozoic Burovaya Formation, Turukhansk region, Siberia

Abstract Molar tooth structures are abundant in large (1–2 m diameter) carbonate nodules within fine-grained, subtidal carbonates of the early Neoproterozoic (lower Upper Riphean) Burovaya Formation along the Sukhaya Tunguska River, Turukhansk Uplift, northwestern Siberia. Although molar tooth structures are regionally abundant in this unit, here they occur only within the nodules. Stable isotopic compositions of molar-tooth-filling dolomicrospar cements and of thinly bedded dolomicrite within and surrounding the nodules are indistinguishable from one another. The carbon isotopic compositions (mean δ13C=+2.8‰PDB±0.4) reflect mean average oceanic surface water composition during their formation; the light oxygen isotopic compositions (mean δ18O=−6.4‰PDB±2.2) are generally similar to those of other little-altered Meso- to Neoproterozoic limestones and dolostones. These molar tooth structures have no features that would support a tectonic origin; they more likely formed through bacterial processes. Carbonate cement filling of these voids occurred soon after their formation, but the mechanism responsible for this carbonate precipitation is currently uncertain. Local restriction of molar tooth structures to early diagenetic nodules suggests that penecontemporaneous lithification was required for the formation, or at least preservation, of these widespread Mesoproterozoic to Neoproterozoic features.

Liaisons of Life, from Hornworts to Hippos, How the Unassuming Microbe has Driven Evolution

Tom Wakeford, 2001, John Wiley & Sons, Inc., New York, 212 p. (Cloth,

Oxygenation of the mid-Proterozoic atmosphere: clues from chromium isotopes in carbonates

24.95) ISBN: 0-471-39972-8. The title pretty much sums it up; Tom Wakefords well-written book tells us about the wonders of microbial interactions with metaphytes, metazoans, and protists. The book weaves science and the history of science together to tell an engaging story about these microscopic, often overlooked denizens of, well, everywhere. The protagonists are diverse and include Beatrix Potter, Lynn Margulis, Paul Buchner, Jules Verne, and Carl Woese. Wakefords message is that intimate, inter-kingdom liaisons between organisms are not only important in lifes history and ecology, but are central to the existence of life on this planet. In recent years, symbiotic relationships have enjoyed a more prominent role in biology and even in paleontology than ever before. The intimate symbiotic relationship between tubeworms and chemosynthetic bacteria, described relatively recently, stands as one such example. Symbioses between legumes and N-fixing bacteria are well established and reasonably well understood at the ecological level, if not at the molecular level. In the fossil record, paleontologists hunt for evidence of symbiotic relationships between ancient corals and primary producers, and for associations between mollusks and microbes. Few biologists today would argue that symbioses are unimportant in ecology, and fewer still would claim that they are rare events. However, these liaisons of life do pose some difficulty for taxonomists. What is a lichen? Is it a fungus? A cyanobacterium? An alga? The language of biology assumes that only one organism is being named (usually the macroscopic one). The symbiont, when classified, is classified according to its position in the taxonomy of free-living microbes. Microbiologists presently are coming to grips with the ever more apparent fact that not all microbes are free-living, and many, …