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N J Butterfield

Publications and source records attributed to N J Butterfield.

6 recordsLinked to original sources
PubMed1991

Strontium isotopic variations of Neoproterozoic seawater: implications for crustal evolution.

We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively well-dated Mackenzie Mountains Supergroup constrain Shaler deposition to approximately 770-880 Ma, a range corroborated by 723 +/- 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb/86Sr ratios (<0.01) were selected for Sr isotopic analysis. Delta 18O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn/Sr (> or = 2) and variable delta 18O; most are dolomites. The data indicate that between ca. 790-850 Ma the 87Sr/86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr/86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr/86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (approximately 900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr/86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790-850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows us to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500-900 Ma. The results indicate that the hydrothermal flux reached a maximum value at ca. 830 Ma. In contrast, a large peak in erosion rate is indicated at ca. 570 Ma. The peaks in hydrothermal flux and erosion rate are most likely related to developments in the Pan-African and related orogenic events, whose initial development is characterized by production of juvenile crust during supercontinental break up and rifting. The time ca. 570 Ma is characterized by continent-continent collision and production of recycled crust. Sr isotope data from Proterozoic carbonates offer a valuable resource for understanding large-scale crust dynamics.

Africa, Western↗
PubMed1990

A bangiophyte red alga from the Proterozoic of arctic Canada.

Silicified peritidal carbonate rocks of the 1250- to 750-million-year-old Hunting Formation, Somerset Island, arctic Canada, contain fossils of well-preserved bangiophyte red algae. Morphological details, especially the presence of multiseriate filaments composed of radially arranged wedge-shaped cells derived by longitudinal divisions from disc-shaped cells in uniseriate filaments, indicate that the fossils are related to extant species in the genus Bangia. Such taxonomic resolution distinguishes these fossils from other pre-Ediacaran eukaryotes and contributes to growing evidence that multicellular algae diversified well before the Ediacaran radiation of large animals.

Biological Evolution↗
PubMed1988

Exceptional preservation of fossils in an Upper Proterozoic shale.

Late Proterozoic organisms must have been diverse and widely distributed, but in general their fossil record is both taxonomically and environmentally limited. Exceptional preservation of Proterozoic fossils is not unknown, but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments. We report here an exceptionally well preserved and distinctive assemblage of Late Proterozoic fossils from subtidal marine shales. In addition to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in Proterozoic rocks, this assemblage includes multicellular algae ('seaweeds'), a diverse assortment of morphologically complex protistan vesicles, and probable heterotrophic bacteria. Thus, it provides one of the clearest and most taxonomically varied views of Proterozoic life yet reported.

Bacteria↗