Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved

Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

International audienceThe Wonoka–Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (δ18Ow) with which carbonate minerals equilibrated. With δ18Ow known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed δ18Ow. Previous hypotheses, based on the covariance of δ18Ocarb vs. δ13Ccarb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed δ18Ow values do not covary with carbon isotope compositions (δ13Ccarb) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka–Shuram-aged carbonates experienced equilibration with fluids under increasingly closed-system conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (δ7Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the δ7Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals)

Aspectos clínicos e radiográficos do pericárdio bovino como substituto do ligamento cruzado cranial de cães Clinical and radiographic aspects of the bovine pericardium as a substitute of the canine cranial cruciate ligament

Avaliaram-se os aspectos clínicos e radiográficos do enxerto de pericárdio bovino, preservado em glicerina, como substituto do ligamento cruzado cranial. Quinze cães machos, sem raça definida, pesando entre 17,4 e 31,6 kg, foram submetidos à ruptura experimental do ligamento cruzado cranial e à substituição por pericárdio, via videoartroscopia. Os cães foram divididos em três grupos de cinco e avaliados aos 30, 90 e 120 dias. O membro operado foi imobilizado por duas semanas e procederam-se avaliações clínicas semanais. Radiografias foram feitas mensalmente e foram realizadas colheita de líquido sinovial nos tempos descritos. Clinicamente, os cães mostraram claudicação acentuada a moderada, hipotrofia muscular no membro operado e acentuado deslocamento cranial da tíbia em relação ao fêmur. As alterações degenerativas foram observadas nas radiografias. Observou-se instabilidade acentuada em todos os animais. O líquido sinovial tinha características de inflamação. Concluiu-se que o enxerto rompeu precocemente, provocou reação inflamatória persistente e fenômenos de rejeição, não sendo, portanto, recomendado para substituição do ligamento cruzado cranial de cães.<br>The clinical and radiographic aspects of the bovine pericardium preserved in glicerin, were evaluated as a substitute for canine cranial cruciate ligament. Fifteen male mongrel dogs weighing between 17.4 and 31.6kg had the ligament experimentally ruptured and the stifle joint stabilized by an arthroscopical technique with bovine pericardium as a graft. The dogs were divided into three groups of five animals each. They were evaluated at 30, 90 and 120 days. The operated limb was imobillized for two weeks and clinical examination was performed weekly. Radiographs were taken monthly and the sinovial fluid was collect at 30, 90 and 120 days. Clinically, dogs presented high to moderate lameness, muscle hipotrophy in the operated limb and accentuated cranial drawer movement. Degenerative disease was detected in radiography. All dogs showed total rupture of the graft. Sinovial fluid analysis showed characteristics of inflammation. It can be concluded that pericardium graft failed prematurely, incited persistent inflammatory reaction and rejection phenomena. Thus, it cannot be recommended as a xenograft for cranial cruciate ligament replacement