Proterozoic Oceans Research Articles

Sulfur isotopes in Early Proterozoic volcanogenic massive sulfide deposits: new data from Arizona and implications for ocean chemistry

The δ 34S range of sulfides from Early Proterozoic volcanogenic massive sulfides of Arizona is narrow, 129 out of 133 measurements lying within a range of −2.8 to + 3.2‰ . Other Early Proterozoic and Archean deposits of comparable type and mineralogy have similar S-isotope characteristics. These deposits, unlike Phanerozoic analogues, do not appear to have incorporated reduced sulfur derived from marine sulfate. Early Proterozoic oceans are known to have contained sulfate; in this paper it is a major tenet that sulfate was confined to an upper water layer. It is proposed that most volcanogenic massive sulfide hydrothermal systems prior to 1.7 Ga were recharged with sulfate-free water from an anoxic lower layer, and in general discharged into reducing environments. Rare deposits with exceptional mineralogy or S-isotope compositions may reflect hydrothermal recharge from the oxidized upper layer, or deposition from fluids discharging into, and mixing with, the upper water layer.

The North American Midcontinent Rift beneath Lake Superior from Glimpce seismic reflection profiling

The Midcontinent rift system is a 1.1‐b.y.‐old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on‐shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low‐angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben‐bounding normal faults were reactivated as high‐angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.

The Proterozoic biosphere: a multidisciplinary study

List of contributors Preface Part I: 1. Geology and paleobiology of the Archean Earth 2. Geological evolution of the Proterozoic Earth 3. Proterozoic biochemistry 4. Proterozoic atmosphere and ocean 5. Proterozoic and selected early Cambrian microfossils: prokaryotes and protists 6. Modern mat-building microbial communities: a key to the interpretation of Proterozoic stromatolitic communities 7. Proterozoic and earliest Cambrian carbonaceous remains, trace and body fossils 8. The Proterozoic-early Cambrian evolution of metaphytes and metazoans 9. Molecular phylogenetics, molecular paleontologym and the Proterozoic fossil record 10. Biostratigraphy and paleobiology of the Proterozoic 11. Biotic diversity and rates of evolution during Proterozoic and earliest Phanerozoic time 12. A paleogeographic model for Vendian and Cambrian time 13. Evolution of the Proterozoic biosphere: benchmarks, tempo and mode Part II: 14. Geographic and geologic data for PPRG rock samples 15. Flow chart and processing procedures for rock samples 16. Procedures of whole rock and kerogen analysis 17. Abundances and isotopic compositions of carbon and sulphur species in whole rock and kerogen samples 18. Procedures for analysis of extractable organic matter 19. Composition of extractable organic matter 20. Modern mat-building microbial communities: methods of investigation and supporting data 21. Construction and use of geological, geochemical and paleobiological databases 22. Proterozoic and selected early Cambrian microfossils and microfossil-like objects 23. Described taxa of Proterozoic and selected earliest Cambrian Carbonaceous remains, trace and body fossils 24. Atlas of representative Proterozoic microfossils 25. Informal revised classifications of Proterozoic microfossils 26. Models for Vendian-Cambrian biotic diversity and for Proterozoic atmospheric and ocean chemistry 27. Glossary of technical terms References cited Subject index Index to geologic units Taxonomic index.

PALEOBIOLOGY OF DISTINCTIVE BENTHIC MICROFOSSILS FROM THE UPPER PROTEROZOIC LIMESTONE‐DOLOMITE “SERIES,” CENTRAL EAST GREENLAND

Populations of Polybessurus bipartitus Fairchild ex Green et al., a large and morphologically distinctive microfossil, occur in silicified carbonates of the Upper Proterozoic (700–800 Ma) Limestone‐Dolomite “Series,” central East Greenland. Large populations of well‐preserved individuals permit reconstruction of P. bipartitus as a coccoidal unicell that “jetted” upward from the sediment surface by the highly unidirectional secretion of extracellular mucopolysaccharide envelopes. Reproduction by baeocyte formation is inferred on the basis of clustered envelope stalks produced by small cells. Sedimentological evidence indicates that P. bipartitus formed surficial crusts locally within a shallow peritidal carbonate platform. Among living microorganisms a close morphological, reproductive, and behavioral counterpart to Polybessurus is provided by populations of an as yet undescribed cyanobacterium found in coastal Bahamian environments similar to those in which the Proterozoic fossils occur. In general morphology and “jetting” behavior, this population resembles species of the genus Cyanostyton Geitler (1925), but reproduces via baeocyte formation. Polybessurus is but one of the more than two dozen taxa in the richly fossiliferous biota of the Limestone‐Dolomite “Series.” This distinctive population, along with co‐occurring filamentous cyanobacteria and other microfossils, contributes to an increasingly refined picture of ecological heterogeneity in late Proterozoic oceans.

Paleobiology of Distinctive Benthic Microfossils from the Upper Proterozoic Limestone-Dolomite "Series," Central East Greenland

Populations of Polybessurus bipartitus Fairchild ex Green et al., a large morphologically distinctive microfossil, occur in silicified carbonates of the Upper Proterozoic (700-800 Ma) Limestone-Dolomite "Series," central East Greenland. Large populations of well-preserved individuals permit reconstruction of P. bipartitus as a coccoidal unicell that "jetted" upward from the sediment by the highly unidirectional secretion of extracellular mucopolysaccharide envelopes. Reproduction by baeocyte formation is inferred on the basis of clustered envelope stalks produced by small cells. Sedimentological evidence indicates that P. bipartitus formed surficial crusts locally within a shallow peritidal carbonate platform. Among living microorganisms a close morphological, reproductive, and behavioral counterpart to Polybessurus is provided by populations of an as yet underscribed cyanobacterium found in coastal Bahamian environments similar to those in which the Proterozoic fossils occur. In general morphology and "jetting" behavior, this population resembles species of the genus Cyanostylon, Geitler (1925), but reproduces via baeocyte formation. Polybessurus is but one of the more than two dozen taxa in the richly fossiliferous biota of the Limestone-Dolomite "Series." This distinctive population, along with co-occurring filamentous cyanobacteria and other microfossils, contributes to an increasingly refined picture of ecological heterogeneity in late Proterozoic oceans.

Manikewan: An early proterozoic ocean in central Canada, its igneous history and orogenic closure

During Early Proterozoic time (Aphebian Period) much of the southwestern Churchill province of the Canadian shield was covered by a sea, called here the Manikewan Ocean. Although the original dimensions of this ocean are unknown, the highly deformed and metamorphosed rocks that represent its evolution and final closure as the Manikewan mobile belt now crop out over an area of at least 6 × 10 5 km 2. Along strike (NE—SW) these rocks are buried at both ends by flat-lying Phanerozoic sediments, whereas, across strike, they are bounded by Archaean cratons, the Western craton to the northwest and Superior province to the southeast. Part of the Western craton and Manikewan mobile belt is overlain by the flat-lying, Middle Proterozoic, Athabasca sandstone, a remnant of a once wide-spread, predominantly fluvial group of rocks. Manikewan rocks can be divided into two major zones: 1. (1) The Cree Lake zone in the northwest consists of a predominantly meta-sedimentary succession which overlies unconformably the southeastern extension of the Western craton. This region appears to have been an ensialic stable platform to miogeosyncline during Aphebian time. 2. (2) The Reindeer Lake zone in the southeast consists of thick sequences of mainly mafic, island-arc type, meta-volcanic rocks (e.g., Amisk volcanics) and their meta-sedimentary equivalents. Locally these are overlain unconformably by thick molasse deposits (e.g., Missi Fm.). This region appears to have been an ensimatic eugeosyncline during at least part of Aphebian time. Numerous stocks and batholiths intrude the Manikewan mobile belt and form an integral part of it. They are mainly granitic and predominantly syn-tectonic. The largest, the Wathaman batholith is over 900 km long, making it the largest known Precambrian batholith. It lies along the junction of the Reindeer Lake and the Cree Lake zones. Radiometric dating is scanty, but what has been done indicates that closure of the Manikewan Ocean was well advanced by 1865 Ma and complete by 1800 Ma (Hudsonian orogeny).

Formula omitted] in Late Proterozoic carbonates: evidence for a “mantle” event at ∼900 Ma ago

Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the 87 Sr 86 Sr ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (∼0.711). Consequently, the non-radiogenic 87 Sr 86 Sr value at ∼900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.

The Grenville orogenic belt in the North Atlantic continents

Abstract When allowance is made for the most recent opening of the Atlantic Ocean, the Grenville orogeny is seen to have affected a huge, roughly ‘Y’-shaped region. The three parts of this system are called the Amer-African, Eur-African and Scandinavian arms. The presence of Upper Proterozoic ophiolites in several areas of North Africa supports the idea that parts of the Grenville orogen formed as a result of ocean closure and continental collision between Africa, Europe and America. The smaller Scandinavian arm may be a ‘failed arm’ of the Grenville orogen or may have formed by closure of a second Proterozoic ocean along the western (?) margin of the Scandinavian arm. In addition to development of these pre-Grenville oceanic areas, geologic evidence suggests that oceanic crust was subsequently generated at least three more times in the same general area.

A marginal Late Proterozoic ocean basin in the Welsh region

SummaryRe-assessment of the idea that the Monian and Longmyndian accumulated along the margin of a palaeo-ocean suggests that Monian sedimentation occurred near the eastern shoreline of a micro-continent. If Monian igneous activity, metamorphism and deformation are attributed to an underlying subduction zone, this must have dipped northwestwards away from a small ocean basin to the southeast. Consumption of this ocean floor brought about collision with the continental shield and simple deformation of the Longmyndian.

Carbon and sulfur isotope records of Ediacaran carbonates of Lesser Himalayas: implications on oxidative state of the contemporary oceans

DOI = 10.3126/hjs.v5i7.1232 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.21