Pattern Of Isotopic Variation Research Articles

Fluctuation in redox conditions and the evolution of early Cambrian life constrained by nitrogen isotopes in the middle Yangtze Block, South China

AbstractThe Ediacaran–Cambrian (E-C) transition (∼542–517 Ma) witnessed the rapid evolution of Cambrian animals, which was accompanied by carbon cycling anomalies and a significant increase in the concentration of oxygen in Earth’s atmosphere. The mechanisms stimulating the evolution of complex eukaryotes, however, remain problematic, especially concerning the link between biological evolution and contemporaneous changes in the oceanic environment. In this study, integrated δ13Ccarb–δ13Corg–δ15N compositions were analysed from the YD-4 core samples to understand redox fluctuations and nitrogen cycling of the middle Yangtze Block across the E-C transition. Two negative δ13Ccarb excursions (N1 and N2) and a positive δ13Ccarb excursion (P1) are identified from the studied samples and are supposedly of primary origin. Constrained by of the U-Pb age, biolithology and pattern of isotopic variation, N1, P1 and N2 are comparable to the Basal Cambrian Carbon Isotope Excursion (BACE), Zhujiaqing Carbon Isotope Excursion (ZHUCE) and Shiyantou Carbon Isotope Excursion (SHICE). We interpreted the decreased δ15N values in this study as resulting from intensified atmospheric nitrogen fixation driven by enhanced denitrification associated with expanded marine anoxia, as well as partial ammonium assimilation, while increased δ15N values suggest weakened denitrification associated with an amplified oxic water mass. The temporal coincidence of N1 and N2, with two episodes of negative δ15N excursions, and of P1, with a positive δ15N excursion, suggests that variable oceanic redox conditions and nitrogen bioavailability may have influenced the evolution of the Cambrian eukaryote-dominated community.

New map of surface water isotope variation

The isotopic composition of hydrogen and oxygen in water varies in waters of different sources, and isotopic composition changes through mixing of water masses and through processes such as evaporation. For instance, the isotope composition in surface water is often different from the isotope composition in precipitation and groundwater. This isotopic variation can be useful in a variety of hydrological, ecological, and climate science studies. Bowen et al. developed a model based on geographic information system data to predict long‐term annual average surface water isotope ratios across the United States. They validated their model with data from a water monitoring program and showed that their map reproduced the observed pattern of isotopic variation across the United States.

High-resolution carbon isotopic records from the Ordovician of South China: Links to climatic cooling and the Great Ordovician Biodiversification Event (GOBE)

The Great Ordovician Biodiversification Event (GOBE) represented the largest increase in diversity in the marine biosphere during the Phanerozoic Eon, yet its causes and consequences remain poorly understood. Patterns of isotopic variation in high-resolution δ 13C carb and δ 13C org records from a well-exposed section at Honghuayuan in South China may provide important insights regarding the GOBE. The Honghuayuan isotopic profiles, which can be correlated with C-isotopic records from contemporaneous sections globally, reveal large perturbations to the global carbon cycle during the Ordovician. A + 8‰ increase in δ 13C org values in the Floian implies a large, albeit transient increase in the burial rate of organic matter during the mid-Early Ordovician that may have contributed to climatic cooling and played an important role in triggering the GOBE. A + 4‰ increase in δ 13C carb and high-frequency variation in δ 13C org in the Darriwilian to Sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late Middle and early Late Ordovician. This pattern may mark the onset of climate changes culminating in the end-Ordovician Hirnantian glaciation and mass extinction event that terminated the GOBE.

Reflections of surface water, seasonality and climate in stable oxygen isotopes from tyrannosaurid tooth enamel

Patterns of isotopic variation in enamel of eight Albertosaurus teeth from the Campanian/Maastrichtian Horseshoe Canyon Formation, Alberta, were measured by serial sampling along the growth axis. Cycles in the isotopic values appear to correlate to published tooth growth rates for tyrannosaurids. Tyrannosaurid dinosaur teeth of ~50 mm in length provide ideal repositories of terrestrial climatic information as each records a full annual cycle of body water isotopic variation. The relatively thin enamel precludes problems associated with time-averaging associated with complex growth patterns found in thick mammalian enamel. The most consistent and striking pattern associated with all of the teeth are two relatively closely spaced subequal maxima. These appear to be associated with onset of a seasonal episode of high humidity after the activation of the “amount effect,” where the influence of ambient temperature on surface water (and body water) δ 18O ends while temperatures remain above 20 °C. Teeth within a single fossiliferous horizon show the same general isotopic pattern and range. A notable difference in pattern shape and magnitude of isotopic variation between tyrannosaurid teeth from several stratigraphic intervals of the lower Horseshoe Canyon Formation section indicates a climatic changes from strong seasonality (high annual temperature range with distinct precipitation and humidity maxima during each year) at the base of the section to milder seasonality at the top.

Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison meteorite.

The origin of the organic matter in carbonaceous meteorites remains controversial despite extensive study over the past 20 yr. Motivated by the expectation that the patterns of isotopic variation with molecular structure among the organic compounds would contain important clues to their origin, we have measured the carbon isotopic compositions for individual hydrocarbons and monocarboxylic acids from Murchison meteorite, a C2 carbonaceous chondrite which fell in Australia in 1969. With few exceptions, notably benzene, the volatile products are substantially isotopically heavier than their terrestrial counterparts, signifying their extraterrestrial origin. For both classes of compounds, the ratio of 13C to 12C decreases with increasing carbon number in a roughly parallel manner, and each carboxylic acid exhibits a higher isotopic ratio than the hydrocarbon containing the same number of carbon atoms. These trends are consistent with the kinetically controlled synthesis of higher homologues from lower ones. The results suggest the possibility that the production mechanisms for hydrocarbons and carboxylic acids may be similar; they also impose constraints on the identity of the reactant species.