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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"All these evidences and theoretical analysis imply that rapid carbonate deposition changes were affected by enhanced terrestrial weathering input, abnormal ocean circulation and various geobiological processes, associated with metazoan extinction and the occurrences of unusual sediments. The ocean geochemistry could be reversed as " Strangelove Ocean " , in which the primary productivity was replaced by the terrestrial input as the major contributor of the carbonate factory, during the Permian-Triassic transition (Rampino and Caldeira, 2005; Kump, 1991). Zeebe and Westbroek (2003) demonstrated the carbonate is critical supersaturated in the " Strangelove Ocean " dominated by inorganic precipitation. "
[Show abstract][Hide abstract] ABSTRACT: Various environmental changes were associated with the Permian-Triassic mass extinction at 252.2 Ma. Diverse unusual sediments and depositional phenomena have been uncovered as responses to environmental and biotic changes. Lithological and detailed conodont biostratigraphic correlations within six Permian-Triassic boundary sections in South China indicate rapid fluctuations in carbonate deposition. Four distinct depositional phases can be recognized: (1) normal carbonate deposition on the platform and slope during the latest Permian; (2) reduced carbonate deposition at the onset of the main extinction horizon; (3) expanded areas of carbonate deposition during the Hindeodus changxingsensis Zone to the H. parvus Zone; and (4) persistent mud-enriched carbonate deposition in the aftermath of the Permian-Triassic transition. Although availability of skeletal carbonate was significantly reduced during the mass extinction, the increase in carbonate deposition did not behave the same way. The rapid carbonate depositional changes, presented in this study, suggest that diverse environmental changes played key roles in the carbonate deposition of the Permian-Triassic mass extinction and onset of its aftermath. An overview of hypotheses to explain these changes implies enhanced terrestrial input, abnormal ocean circulation and various geobiological processes contributed to carbonate saturation fluctuations, as the sedimentary response to large volcanic eruptions.
"Seawater acts as the largest reservoir of strontium, by accumulating it from surface runoff throughout 127 the history of the Earth. Strontium in the seawater has a long residence time (∼2.4 × 10 6 yr; Jones and Jenkyns, 2001) relative to a short mixing time (∼10 5 yr; Kump, 1991; Jacobsen and Kaufman, 1999), and therefore, global ocean is considered to be homogenous with respect to its 87 Sr/ 86 Sr ratio (Halverson et al., 2007). The 87 Sr/ 86 Sr ratio of seawater is controlled by the mixing of continental derived strontium, which is radiogenic due to high rubidium content in granitic continental rocks and those from the volcanic sources, especially from the mid-ocean ridge basalts derived from the partial melting (the process though which some minerals in a rock melt initially and accumulate to form a magma) of mantle and are representative of mantle evolution of Rb-Sr isotope system (DePaolo, 1980). "
[Show abstract][Hide abstract] ABSTRACT: Chemostratigraphy or chemical stratigraphy deals with the correlation of sedimentary strata based on systematic variation of a particular chemical composition with time in the history of the Earth. In particular, this method is widely used in decoding the temporal variations seen in marine sediments that are deposited uninterruptedly in deep sea or in shallow marine carbonate depositional environments. In this review, the application of strontium and carbon isotope based chemostratigraphy in marine carbonate sediments is discussed. Irrespective of post depositional geological events such as metamorphism or tectonic displacements to form Mountain chains during continental collision, chemostratigraphy helps in determining the apparent depositional ages of sedimentary rocks in the Precambrian time, where biostratigraphy is not applicable. Although carbonate rocks are vulnerable to post depositional alterations, a systematic geochemical screening can guide in identifying the best chemically preserved carbonate rocks. A novel method of using Mn/Sr ratios for determining the best estimate of strontium initial ratio for carbonate rocks is presented, on the basis of a worked example of metamorphosed carbonate rocks from the Sør Rondane Mountains in the Dronning Maud Land, East Antarctica. The future potential of chemostratigraphy in decoding the early history of the Earth is also discussed.
Full-text · Article · Apr 2015 · Journal of the Indian Institute of Science
"N 13 C is therefore driven towards heavier values through high bio-production (Berger and Vincent, 1986), increased storage of organic carbon in deep anoxic oceans and sediments, or diminished carbonate production and/or enhanced carbonate erosion . A lighter isotopic composition of N 13 C carb in whole rock samples is caused by lighter N 13 C values of the reservoir due to low bio-production (Kump, 1991), diminished storage of organic carbon as a consequence of better ventilation of deep oceans and enhanced currents, or increased carbonate production and/or erosion and oxidation of organic carbon. "
[Show abstract][Hide abstract] ABSTRACT: A more than 2000-m-thick Cambrian^Ordovician carbonate platform succession developed on the exotic Argentine Precordillera terrane during rifting from Laurentia and drifting towards Gondwana. On base of these carbonates, a carbon isotope curve could be developed for the Cambrian^Ordovician. We measured Delta13Ccarb and Delta13Corg values on bulk rocks, selected components and diagenetic cements. Whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis, most subtidal carbonates exhibit primary marine values. We can report reliable curves for the Middle^Late Cambrian transition and for the latest Cambrian to earliest Middle Ordovician. The Delta13C curve matches well with the published data from global Cambrian^Ordovician boundary sections. Excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy. This indicates the interdependence on sea level which rules the productivity and/or preservation of organic carbon and therefore the partition between Corg and Ccarb burial.