Fast and pervasive diagenetic isotope exchange in foraminifera tests is species-dependent

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis without discernible textural changes. Here, we investigate fluid-mediated isotopic exchange in pristine tests of three modern benthic foraminifera species (Ammonia sp., Haynesina germanica, and Amphistegina lessonii) following immersion into an 18O-enriched artificial seawater at 90 °C for hours to days. Reacted tests remain texturally pristine but their bulk oxygen isotope compositions reveal rapid and species-dependent isotopic exchange with the water. NanoSIMS imaging reveals the 3-dimensional intra-test distributions of 18O-enrichment that correlates with test ultra-structure and associated organic matter. Image analysis is used to quantify species level differences in test ultrastructure, which explains the observed species-dependent rates of isotopic exchange. Consequently, even tests considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted.

Microphysical Characteristics and Environmental Isotope Effects of the Micro-Droplet Groups under the Action of Acoustic Waves

Acoustics can cause particles/droplets to agglomerate in the air medium, thereby accelerating gravity sedimentation. To assess the microphysical characteristics and environmental isotope effects of micro-droplet groups under the action of acoustic waves, an air chamber experimental platform was established, and 100 groups of controlled experiments were conducted. The characteristic particle size, size spectrum, isotope values, corresponding linear relationships with hydrogen and oxygen, and d values were analyzed. The isotope exchange equation between the micro-droplet groups and environmental water vapor inside the air chamber was investigated. The results showed that the peak size values of the micro-droplet groups increased under the action of acoustic waves. The characteristic particle size (D90) showed a “trigger effect” with the acoustic operation with a positive deviation in the size spectrum and isotope exchange between the micro-droplet groups and environmental water vapor. The relative variations in theoretical values for different sedimentation conditions were consistent with those of the experimental results. Environment isotopes could be used to trace the acoustic agglomeration process of micro-droplets in the future.

Chloride Ion Catalysed Rearrangement, and Isotope Exchange of the Isomeric Methylallyl Chlorides in Acetonitrile

<p><b>The bromide ion catalysed rearrangement reaction of ⍺-methylallyl bromide in acetone was studied by England and Hughes and assigned an S[n]2' mechanism, and this was the only isomeric rearrangement reaction proceeding by this mechanism which had been thoroughly investigated. 79,120 The kinetics and mechanism of the chloride ion catalysed isomeric rearrangement reactions of ⍺-and ɣ-methylallyl chloride in acetonitrile have now been investigated and an S[n]2' mechanism has been assigned to these reactions, various other possible mechanisms having been excluded. The kinetics and mechanics of the isotope exchange reactions of the two isomers in acetonitrile have also bean studied and these reactions have been assigned en S[n]2 mechanism. The isotope exchange reactions of both the methylallyl bromides and the methylallyl chlorides in acetone were studied by England and Hughes, the reaction medium being unfavourable for a study of the S[n]2' reactions of the latter substrates. A comparison of the kinetic data for the rearrangement end Isotope exchange reactions of the methylallyl chlorides in acetonitrile with that for the corresponding reactions of the methylallyl bromides in acetone la made. The two systems are similar in behaviour as may be seen from the data tabulated below.</b></p> <p>Substrate Mechanism K2 (25° C) Δ H°Δ S° (M⁻¹ sec.⁻¹) (kcal.mole⁻¹) (cal.deg.⁻¹ mole⁻¹)⍺-methylallyl chloride S[n]2 2.87X10⁻⁶ 20.8-13.9⍺-methylallyl chloride S[n]2'1.33X10⁻⁸ 24.2-13.4ɣ-methylallyl chlorideS[n]2 3.15X10⁻⁴ 18.8-11.6ɣ-methylallyl chloride S[n]2' 5.31X10⁻⁹ 24.3-14.9⍺-methylallyl bromide S[n]2 8.79X10⁻⁴ 15.9-19.1⍺-methylallyl bromide S[n]2' 1.49X10⁻⁵ 18.8-17.7ɣ-methylallyl bromide S[n]2 1.41X10⁻¹ 14.1-15.0⍺-methylallyl chloride S[n]2 2.30X10⁻⁷ 19.9-22.1ɣ-methylallyl chloride S[n]2 2.88X10⁻⁵ 15.8-26.2A comparison of the kinetic data for the isotope exchange reactions of the methylallyl chlorides in acetonitrile and acetone is also made. England and Hughes' data for the latter solvent is shown above. Relative rates of nucleophilic attack at saturated carbon, π-bonded carbon and hydrogen are discussed for these substrates, a minor concurrent elimination reaction accompanying the S[n]2' reaction of ⍺-methylallyl chloride in acetonitrile providing information on the latter. The nucleophilic reactivity of the halide ions is much greater for attack at a saturated carbon atom than for attack at a π-bonded carton atom, while toward hydrogen these ions are very weak nucleophiles as shown by the rate ratios given below for the chloride ion catalysed reactions of ⍺-methylallyl chloride in acetonitrile.</p> <p>[see thesis pdf for formula]In the absence of chloride ions a slow surface-catalysed elimination reaction takes place which promotes hydrogen chloride catalysed rearrangement. The addition of 0.05M Et4 NCl prevents the latter reaction from occurring and gives a rata ratio of 5.5 for the chloride ion catalysed and surface-catalysed elimination reactions. By comparison with other dipolar aprotic solvents, acetonitrile appears to be particularly suitable for slow reactions requiring prolonged reaction times at elevated temperatures. Acetone, as mentioned above, and both dimethyl sulphoxide and dimethylformamide were found to be unsuitable for S[n]2' studies, the latter two being: Investigated during the search for a suitable solvent in which to study the reactions of the methylallyl chlorides.</p>

Chloride Ion Catalysed Rearrangement, and Isotope Exchange of the Isomeric Methylallyl Chlorides in Acetonitrile

<p><b>The bromide ion catalysed rearrangement reaction of ⍺-methylallyl bromide in acetone was studied by England and Hughes and assigned an S[n]2' mechanism, and this was the only isomeric rearrangement reaction proceeding by this mechanism which had been thoroughly investigated. 79,120 The kinetics and mechanism of the chloride ion catalysed isomeric rearrangement reactions of ⍺-and ɣ-methylallyl chloride in acetonitrile have now been investigated and an S[n]2' mechanism has been assigned to these reactions, various other possible mechanisms having been excluded. The kinetics and mechanics of the isotope exchange reactions of the two isomers in acetonitrile have also bean studied and these reactions have been assigned en S[n]2 mechanism. The isotope exchange reactions of both the methylallyl bromides and the methylallyl chlorides in acetone were studied by England and Hughes, the reaction medium being unfavourable for a study of the S[n]2' reactions of the latter substrates. A comparison of the kinetic data for the rearrangement end Isotope exchange reactions of the methylallyl chlorides in acetonitrile with that for the corresponding reactions of the methylallyl bromides in acetone la made. The two systems are similar in behaviour as may be seen from the data tabulated below.</b></p> <p>Substrate Mechanism K2 (25° C) Δ H°Δ S° (M⁻¹ sec.⁻¹) (kcal.mole⁻¹) (cal.deg.⁻¹ mole⁻¹)⍺-methylallyl chloride S[n]2 2.87X10⁻⁶ 20.8-13.9⍺-methylallyl chloride S[n]2'1.33X10⁻⁸ 24.2-13.4ɣ-methylallyl chlorideS[n]2 3.15X10⁻⁴ 18.8-11.6ɣ-methylallyl chloride S[n]2' 5.31X10⁻⁹ 24.3-14.9⍺-methylallyl bromide S[n]2 8.79X10⁻⁴ 15.9-19.1⍺-methylallyl bromide S[n]2' 1.49X10⁻⁵ 18.8-17.7ɣ-methylallyl bromide S[n]2 1.41X10⁻¹ 14.1-15.0⍺-methylallyl chloride S[n]2 2.30X10⁻⁷ 19.9-22.1ɣ-methylallyl chloride S[n]2 2.88X10⁻⁵ 15.8-26.2A comparison of the kinetic data for the isotope exchange reactions of the methylallyl chlorides in acetonitrile and acetone is also made. England and Hughes' data for the latter solvent is shown above. Relative rates of nucleophilic attack at saturated carbon, π-bonded carbon and hydrogen are discussed for these substrates, a minor concurrent elimination reaction accompanying the S[n]2' reaction of ⍺-methylallyl chloride in acetonitrile providing information on the latter. The nucleophilic reactivity of the halide ions is much greater for attack at a saturated carbon atom than for attack at a π-bonded carton atom, while toward hydrogen these ions are very weak nucleophiles as shown by the rate ratios given below for the chloride ion catalysed reactions of ⍺-methylallyl chloride in acetonitrile.</p> <p>[see thesis pdf for formula]In the absence of chloride ions a slow surface-catalysed elimination reaction takes place which promotes hydrogen chloride catalysed rearrangement. The addition of 0.05M Et4 NCl prevents the latter reaction from occurring and gives a rata ratio of 5.5 for the chloride ion catalysed and surface-catalysed elimination reactions. By comparison with other dipolar aprotic solvents, acetonitrile appears to be particularly suitable for slow reactions requiring prolonged reaction times at elevated temperatures. Acetone, as mentioned above, and both dimethyl sulphoxide and dimethylformamide were found to be unsuitable for S[n]2' studies, the latter two being: Investigated during the search for a suitable solvent in which to study the reactions of the methylallyl chlorides.</p>