RESUMEN
RATIONALE: Clumped isotope (Δ47 ) analysis of bioapatite-derived CO2 is a powerful tool to determine body temperatures of extinct vertebrates. The common acid bath technique in combination with dual-inlet-based mass spectrometric measurements has been the preferred method of choice for this purpose, but the large amount of material necessary and the presence of secondary calcite represent obstacles. METHODS: We analyzed the Δ47 composition of carbonate-bearing (bio)apatites using a Kiel IV device, which - in general - allows a reduction of sample replicate size by a factor of ~40 over dual-inlet-based techniques. The Kiel IV device was tested in two different modes: without and with additional water sinks for improved water removal. Furthermore, we tested a pretreatment technique based on 1 M acetic acid (pH = 5) to selectively remove secondary calcite from the carbonate-bearing (bio)apatite phase. RESULTS: Significantly lower Δ47 values were obtained for a given bioapatite after the installation of the two water sinks. With this setup, Δ47 of (bio)apatites followed a temperature relationship that is indistinguishable from the unified one for pure carbonates, provided a dentine sample, rich in organic matter, was excluded. The original bioapatite Δ47 value was restored from a bioapatite/calcite mixture if the mixed material was treated for 1 h with 1 M acetic acid (pH = 5). CONCLUSIONS: (Bio)apatites having low organic matter content such as enamel(oid) can be analyzed accurately for Δ47 using a Kiel IV equipped with water sinks that ensure effective removal of water. Secondary calcite can be effectively removed from carbonate-bearing apatite by pretreatment with 1 M acetic acid (pH = 5) for 1 h.
RESUMEN
Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first in situ δ34S data from Mars at Gale crater display a â¼75 range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δ34S fractionations >21 as indicative of a biological origin, and studies of δ34S at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δ34S, total organic carbon wt%, and scanning electron microscope imaging) on multiple lithologies from the Nördlinger Ries impact crater, focusing on hydrothermally altered impact breccias and associated sedimentary lake-fill sequences to determine whether the δ34S properties define a biosignature. The differences in δ34S between the host lithologies may have resulted from thermochemical sulfate reduction, microbial sulfate reduction, hydrothermal equilibrium fractionation, or any combination thereof. Despite abundant samples and instrumental precision currently exclusive to Earth-bound analyses, assertions of biogenicity from δ34S variations >21 at the Miocene Ries impact crater are tenuous. This discourages the use of δ34S as a biosignature in similar environments without independent checks that include the full geologic, biogeochemical, and textural context, as well as a comprehensive acknowledgment of alternative hypotheses.
RESUMEN
RATIONALE: The analytical method to determine the stable oxygen isotope (18 O/16 O) composition of carbonates via phosphoric acid digestion leads to temperature- and solid-dependent kinetic isotope fractionation. Values for the double carbonate norsethite (BaMg(CO3 )2 ) have been unknown so far. METHODS: The temperature dependence of kinetic oxygen isotope fractionation during the reaction of synthetic and natural BaMg(CO3 )2 with orthophosphoric acid (H3 PO4 ) according to the overall reaction BaMg(CO3 )2 + 2H3 PO4 = Ba2+ + Mg2+ + 2HPO4 2- + 2CO2 + 2H2 O has been examined for the first time using separate carbonate decomposition via fluorination or phosphoric acid digestion, with the resulting gases analyzed by isotope ratio monitoring mass spectrometry. RESULTS: In the temperature range between 25 and 70°C the kinetic fractionation factor between acid-generated CO2 and artificial and natural norsethite is described by (T in K): [Formula: see text] with A = 4.15 and B = 6.47 for natural norsethite, and A = 4.77 and B = 5.94 for synthetic norsethite. The fractionation factor measured for a poorly crystallized synthetic carbonate agrees with those for the other samples at 25°C, but is slightly lower at 50 and 70°C. No carbon isotope fractionation was found during the unidirectional acid dissolution. CONCLUSIONS: The kinetic oxygen isotope fractionation during phosphoric acid liberation of CO2 from BaMg(CO3 )2 is quantified. Based on published results for endmember carbonates, the results at 25°C for other double carbonates are estimated.
RESUMEN
In the present study we investigated the isotope effects associated with water loss from closed low-density polyethylene (LDPE) bottles via diffusion at temperatures between 4 and 60 °C. While at low temperatures (4 and 10 °C) no substantial diffusional loss of water was observed within storage time, a pronounced loss was found for the experiments at room temperature and 60 °C. The latter was associated with a substantial increase in δ 18O, δ 17O, and δ 2Η values, and a decrease in the deuterium excess. The magnitude of the isotope effects essentially depended on the extent of water evaporation from the closed bottles through the LDPE membrane.