The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurements • Measurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishable • We propose new consensus values for a set of 7 carbonate reference materials and updated guidelines to report clumped isotope measurements

Shallow-water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of Earth's chemical and climatic history. One of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post-depositional diagenetic alteration. In this study, we use paired measurements of calcium (44 Ca/ 40 Ca) and magnesium (26 Mg/ 24 Mg) isotope ratios in sedimentary carbonates and associated pore-fluids as a tool to understand the mineralogical and diagenetic history of Neogene shallow-water carbonate sediments from the Bahamas and southwest Australia. We find that the Ca and Mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis. The observed variability in bulk sediment Ca isotopes is best explained by changes in the extent of fluid-dominated early marine diagenesis in both platform and slope sediments. Our results indicate that this process, together with variations in carbonate mineralogy (aragonite, calcite, and dolomite), likely plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers (d 13 C, d 18 O, major, minor, and trace elements) in shallow water carbonate sediments in the geologic record.

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Experiments have been conducted in which CO2 gases with varying C and O isotopic compositions and with stochastic and nonstochastic Δ47 values have been allowed to equilibrate with phosphoric acid of two concentrations in reaction vessels of varying dimensions at temperatures of 25 and 90 °C. Rates of 13C18O and 18O exchange between the CO2 and the phosphoric acid varied as a function of the length of exposure, volume of reaction vessel, acid strength, and difference of the initial Δ47 and δ18O values of the CO2 from theoretical equilibrium values. The Δ47 values were also altered by heated stainless steel surfaces such as those found within the Kiel device and other preparation systems. These results have been used to explain variations in the differences in the fractionation between 25 and 90 °C reported for calcite by different workers as well as differences in the slopes between temperature and Δ47 values produced by reacting samples at different temperatures (25 and 90 °C).

The reconstruction of pre-depositional cooking treatments used by prehistoric coastal populations for processing aquatic faunal resources is often difficult in archaeological shell midden assemblages. Besides limiting our knowledge of various social, cultural, economic and technological aspects of shell midden formation, unknown pre-depositional cooking techniques can also introduce large errors in palaeoclimate reconstructions as they can considerably alter the geochemical proxy signatures in calcareous skeletal structures such as bivalve shells or fish otoliths. Based on experimental and archaeological data, we show that carbonate clumped-isotope thermometry can be used to detect and reconstruct prehistoric processing methods in skeletal aragonite from archaeological shell midden assemblages. Given the temperature-dependent re-equilibration of clumped isotopes in aragonitic carbonates, this allows specific processing, cooking or trash dispersal strategies such as boiling, roasting, or burning to be differentiated. Besides permitting the detailed reconstruction of cultural or technological aspects of shell midden formation, this also allows erroneous palaeoclimate reconstructions to be avoided as all aragonitic shells subjected to pre-historic cooking methods show a clear alteration of their initial oxygen isotopic composition.

Rationale Information on the temperature of formation or alteration of carbonate minerals can be obtained by measuring the abundance of the isotopologues 47 and 48 (Δ47 and Δ48 values) of CO2 released during acid dissolution. The combination of these two proxies can potentially provide a greater insight into the temperature of formation, particularly if the carbonate minerals form by non‐equilibrium processes. Methods We have precipitated calcium carbonates at seven temperatures between 5 and 65°C and measured their δ48 values using a Thermo‐253 plus isotope ratio mass spectrometer. The values were transformed to Δ48 values in the conventional manner and then converted to the carbon dioxide equilibrium scale. Results Using the Δ48 values, we have established an empirical calibration between temperature and Δ48 values: Δ48=0.0142)(±0.0012×106/T2+0.0880.25em)(±0.0140.25em)(R2=0.96. Conclusions The calibration line produced allows the determination of the temperature of natural carbonates using the Δ48 values and agrees with the measurements of the Δ47 and Δ48 values of some carbonates assumed to have formed under equilibrium conditions.

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In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow-water carbonate sediments the δ 34 S, Δ 47 , δ 13 C, δ 18 O, δ 44/40 Ca and δ 26 Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ 34 S values similar to those found in contemporaneous seawater and δ 44/40 Ca, δ 26 Mg values, Sr contents and Δ 47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluidbuffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ 34 S values indicating dolomitization in a more sediment-buffered diagenetic system where bacterial sulphate reduction enriches the residual SO 2À 4 in 34 S, consistent with high sediment Sr concentrations and low δ 44/40 Ca and high δ 26 Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ 34 S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ 44/40 Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid-buffered or closed sediment-buffered diagenetic system have similar δ 26 Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ 47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow-water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.