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The large range of stable oxygen isotope range values of for phosphate-bearing minerals and dissolved phosphate of both inorganic or organic origins requires the use of calibrated material of in-house made internal standards of silver phosphate of which isotopic ratios must closely bracket those of studied samples. Such calibrated materials do not exist yet in the various catalogs of stable isotope calibrated standards. Therefore, we propose a simple and cheap protocol to synthesize in house silver phosphate in with a predictable wide range (≈ 40‰) of oxygen isotope isotopic compositions. It is based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate-water system. Silver phosphate crystals are obtained from a highly soluble salt such as KH2PO4 that is dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water takes place at a desired and constant temperature into Ace Glass™pyrex tubes that are placed in a high precision temperature regulation oven for run-times defined by the user. Samples of dissolved phosphate are withdrawn at desired times, quenched in cold water, and quantitatively precipitated as silver phosphate. We propose an "Excel calculation sheet" that computes the oxygen isotope composition of the precipitated silver phosphate depending as a function of on time t (≤ 1 month), temperature T (recommended range from 110°C to 130°C with a precision of ±0.5°C) and the oxygen isotope compositions of both reactants KH2PO4 and H2O at t = 0. Predicted oxygen isotope compositions of synthesized silver phosphate range from -7 to +31‰ VSMOW for a temperature range comprised between 110°C and 130°C and a range of water delta18O from -20 to +15‰ VSMOW. We present practical applications of this protocol comparing theoretical values with real measurements obtained from phosphate samples generated using this method. [ABSTRACT FROM AUTHOR]

CO 2 is considered the main greenhouse gas involved in the current global warming and the primary driver of temperature throughout Earth's history. However, the soundness of this relationship across time scales and during different climate states of the Earth remains uncertain. Here we explore how CO 2 and temperature are related in the framework of a Greenhouse climate state of the Earth. We reconstruct the long-term evolution of atmospheric CO 2 concentration (pCO 2 ) throughout the Cretaceous from the carbon isotope compositions of the fossil conifer Frenelopsis. We show that pCO 2 was in the range of ca. 150-650 ppm during the Barremian-Santonian interval, far less than what is usually considered for the mid Cretaceous. Comparison with available temperature records suggest that although CO 2 may have been a main driver of temperature and primary production at kyr or smaller scales, it was a long-term consequence of the climate-biological system, being decoupled or even showing inverse trends with temperature, at Myr scales. Our analysis indicates that the relationship between CO 2 and temperature is time scale-dependent at least during Greenhouse climate states of the Earth and that primary productivity is a key factor to consider in both past and future analyses of the climate system.

We have used a high-precision, easy, low-cost and rapid method of oxygen isotope analysis applied to various O-bearing matrices, organic and inorganic (sulfates, nitrates and phosphates), whose (18)O/(16)O ratios had already been measured. It was first successfully applied to (18)O analyses of natural and synthetic phosphate samples. The technique uses high-temperature elemental analysis-pyrolysis (EA-pyrolysis) interfaced in continuous-flow mode to an isotope ratio mass spectrometry (IRMS) system. Using the same pyrolysis method we have been able to generate a single calibration curve for all those samples showing pyrolysis efficiencies independent of the type of matrix pyrolysed. We have also investigated this matrix-dependent pyrolysis issue using a newly developed pyrolysis technique involving 'purge-and-trap' chromatography. As previously stated, silver phosphate being a very stable material, weakly hygroscopic and easily synthesized with predictable (18)O/(16)O values, could be considered as a good candidate to become a reference material for the determination of (18)O/(16)O ratios by EA-pyrolysis-IRMS., (Copyright © 2011 John Wiley & Sons, Ltd.)