Calcified microbes in Neoproterozoic carbonates; implications for our understanding of the Proterozoic/Cambrian transition

• Published in: Palaios 1993-12, Vol.8 (6), p.512-525
• Main Authors: Knoll, Andrew H, Fairchild, Ian J, Swett, Keene
• Format: Article
• Language: English
• Subjects: algal mats; algal structures; Arctic region; assemblages; biogenic structures; calcification; Calcium Carbonate - analysis; Cambrian; carbonate rocks; Carbonates; Crystals; Cyanobacteria; Cyanobacteria - chemistry; dolomicrite; dolostone; Draken Formation; Earth sciences; Earth, ocean, space; Environmental Microbiology; Exact sciences and technology; Fossils; Geologic Sediments - analysis; Geologic Sediments - microbiology; Geology; Geosciences (General); Invertebrate paleontology; Magnesium - analysis; Microfossils; Microorganisms; Oceans; paleoecology; Paleontology; Paleozoic; Polybessurus; Precambrian; Proterozoic; Proterozoic eon; Research Reports; Sea water; Seawater - chemistry; sedimentary rocks; sedimentary structures; Space life sciences; Spitsbergen; stratigraphic boundary; Stratigraphy; Stromatolites; Svalbard; upper Precambrian
• ISSN: 0883-1351; 1938-5323
• DOI: 10.2307/3515029

Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where calcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.