Interpreting carbon-isotope excursions: Stranglove oceans
ABSTRACT Large negative excursions in marine carbonate Î´Â¹Â³C are commonly associated with period boundaries and mass extinctions. Explanations for these events must be consistent with limitations imposed by carbon-isotope mass balance. At steady state (i.e., for excursions lasting more than 10âµ yr), the surface ocean Î´Â¹Â³C is set by the organic fraction of the total carbon burial rate and the magnitude of the photosynthetic isotope effect. The Î´Â¹Â³C of the deep ocean and the surface-to-deep isotope gradient are set by both the organic fraction of the ocean's remineralized particulate flux and the magnitude of the isotope effect. Thus it is the carbon-isotope composition of the deep ocean that is most reflective of internal oceanic processes; the surface ocean records changes in the longer term throughput of carbon in the system. The cessation of organic export from the surface ocean, such as is presumed to have caused the Strangelove ocean condition of the Cretaceous/Tertiary (K/T) boundary, leads to an isotopically homogeneous ocean in decades to centuries. If this condition persists, the ocean's isotopic composition approaches that of the riverine weathering input (in 10âµ yr). Failure to approach this value during the K/T event suggests continued production and burial of organic carbon, dominantly in either terrestrial or shallow-marine environments.
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ABSTRACT: The Kyrpy Group of the East European platform is regarded by tradition as correlative with the Lower Riphean Burzyan Group of the Bashkirian meganticlinorium in the southern Urals. Age and correlation of the Kyrpy Group remain problematic, however, because of a limited geochronological information and controversial interpretation of paleontological materials. Data of C-and Sr-isotope chemostratigraphy contribute much to the problem solution. In the Kyrpy Group of the Kama-Belaya aulacogen, the Kaltasy Formation carbonates 1300 to 2400 m thick (boreholes 133 and 203 of the Azino-Pal’nikovo and Bedryazh areas) show 87Sr/86Sr ratios ranging around 0.7040 and narrow diapasons of δ13C values: about 0.5‰ (V-PDB) in shallow-water facies and-2.0‰ (V-PDB) in sediments of deeper origin. Despite the facies dependence of carbon isotope composition, δ13C variations not greater than ±1.0‰ are depicted in chemostratigraphic profiles of carbonate rocks characterizing separate stratigraphic intervals up to 800 m thick in the above borehole sections. Low 87Sr/86Sr ratios and almost invariant δ13C values in carbonates of the Kaltasy Formation are obviously contrasting with these parameters in the Middle and Upper Riphean deposits, being comparable with isotopic characteristics of the Lower Riphean sediments (Mesoproterozoic deposits older than 1300 Ma). Consequently, the results obtained evidence in favor of the Early Riphean age of the Kaltasy Formation and the Kyrpy Group as a whole.Stratigraphy and Geological Correlation 04/2012; 15(1):12-29. · 0.55 Impact Factor
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ABSTRACT: IntroductionSince the first case descriptions of dementia praecox (Diem, 1903), the diagnosis of simple schizophrenia has continued to be controversial. The questioning of its descriptive validity and its reliability, as well as its infrequent use, has led to it being eliminated as a sub-type of schizophrenia in the DSM-III. Criteria for the diagnosis of «simple deteriorative disorder» are currently included in the DSM-IV-TR as a disorder requiring more studies for its possible inclusion.Precambrian Research - PRECAMBRIAN RES. 01/2012;
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ABSTRACT: Carbonate rocks from the Middle Ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in Earth history. This finding suggests a dramatic reorganization of Earth's carbon cycle. Named the Shuram excursion for its original discovery in the Shuram Formation, Oman, the anomaly closely precedes impressive events in evolution, including the rise of large metazoans and the origin of biomineralization in animals. Instead of a true record of the carbon cycle at the time of sedimentation, the carbon isotope signature recorded in the Shuram excursion could be caused by alteration following deposition of the carbonate sediments, a scenario supported by several geochemical indicators. However, such secondary processes are intrinsically local, which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe. Both possibilities are intriguing: if the Shuram excursion preserves a genuine record of ancient seawater chemistry, it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern Earth. If, however, it represents secondary alteration during burial of sediments, then marine sediments must have been globally preconditioned in a unique way, to allow ordinary and local processes to produce an extraordinary and widespread response.Nature Geoscience 01/2011; · 11.67 Impact Factor