Carbonate “clumped” isotope signatures in aragonitic scleractinian and calcitic gorgonian deep-sea corals

Abstract

Abstract. Deep-sea corals are a potentially valuable archive of the temperature and ocean chemistry of intermediate and deep waters. Living in near-constant temperature, salinity, and pH and having amongst the slowest calcification rates observed in carbonate-precipitating biological organisms, deep-sea corals can provide valuable constraints on processes driving mineral equilibrium and disequilibrium isotope signatures. Here we report new data to further develop "clumped" isotopes as a paleothermometer in deep-sea corals as well as to investigate mineral-specific, taxon-specific, and growth-rate-related effects. Carbonate clumped isotope thermometry is based on measurements of the abundance of the doubly substituted isotopologue 13 C 18 O 16 O 2 in carbonate minerals, analyzed in CO 2 gas liberated on phosphoric acid digestion of carbonates and reported as 47 values. We analyzed 47 in live-collected aragonitic scleractinian (Enallopsammia sp.) and high-Mg calcitic gorgonian (Isididae and Coralliidae) deep-sea corals and compared results to published data for other aragonitic scleractinian taxa. Measured 47 values were compared to in situ temperatures, and the relationship between 47 and temperature was determined for each group to investigate taxon-specific effects. We find that aragonitic scleractinian deep-sea corals exhibit higher values than high-Mg calcitic gorgonian corals and the two groups of coral produce statistically different relationships between 47 -temperature calibrations. These data are significant in the interpretation of all carbonate clumped isotope calibration data as they show that distinct 47 -temperature calibrations can be observed in different materials recovered from the same environment and analyzed using the same instrumentation, phosphoric acid composition, digestion temperature and technique, CO 2 gas purification apparatus, and data handling.There are three possible explanations for the origin of these different calibrations. The offset between the corals of different mineralogy is in the same direction as published theoretical predictions for the offset between calcite and aragonite although the magnitude of the offset is different. One possibility is that the deep-sea coral results reflect highMg and aragonite crystals attaining nominal mineral equilibrium clumped isotope signatures due to conditions of extremely slow growth. In that case, a possible explanation for the attainment of disequilibrium bulk isotope signatures and equilibrium clumped isotope signatures by deep-sea corals is that extraordinarily slow growth rates can promote the occurrence of isotopic reordering in the interfacial region of growing crystals. We also cannot rule out a component of a biological "vital effect" influencing clumped isotope signatures in one or both orders of coral. Based on published experimental data and theoretical calculations, these biological vital effects could arise from kinetic isotope effects due to the source of carbon used for calcification, temperature-and pH-dependent rates o...

Comparison of clumped isotope signatures of dolomite cements to fluid inclusion thermometry in the temperature range of 73–176 °C

“…As described in Section 1, many of the previously published Δ 47 to temperature relationships lie in one of two main clusters: one cluster that resembles the equation initially derived by the group at Caltech (Ghosh et al, 2006a;Ghosh et al, 2007;Tripati et al, 2010Tripati et al, , 2015Thiagarajan et al, 2011;Grauel et al, 2013;Zaarur et al, 2013;Came et al, 2014;Kimball et al, 2015); and a second cluster that resembles an equation derived by the group at Harvard (Dennis and Schrag, 2010;Tang et al, 2014;Wacker et al, 2014;Defliese et al, 2015;Kimball et al, 2015;Kluge et al, 2015). Several possible explanations for these apparent calibration differences have been explored, such as the acid digestion fractionation factor (Defliese et al, 2015), sample size (Wacker et al, 2013), a sensitivity of phosphoric acid fractionation to the bulk or clumped isotopic compositions of carbonates (Wacker et al, 2014), differences in the procedures utilized by laboratories (e.g., Henkes et al, 2013;Came et al, 2014;Fernandez et al, 2014;Wacker et al, 2014;Defliese et al, 2015), kinetic isotope effects (Hill et al, 2014a;Tripati et al, 2015), and/or the substitution of different cations in the mineral lattice (Hill et al, 2014b;Yuan, 2015).…”

“…Wacker et al, 2014;Defliese et al, 2015), from actual differences in the Δ 47 to temperature relationships of various types of carbonates (i.e., kinetic isotope effects as discussed in Hill et al, 2014a;Tripati et al, 2015), or cation substitution (per Hill et al, 2014b, Yuan, 2015. One striking observation, however, is that there appear to be two main clusters of calibrations: one cluster that resembles the equation initially derived by the group at Caltech (Ghosh et al, 2006a;Ghosh et al, 2007;Tripati et al, 2010Tripati et al, , 2015Thiagarajan et al, 2011;Grauel et al, 2013;Zaarur et al, 2013;Came et al, 2014;Kimball et al, 2015); and another cluster that resembles an equation derived by the group at Harvard (Dennis and Schrag, 2010;Tang et al, 2014;Wacker et al, 2014;Defliese et al, 2015;Kluge et al, 2015). The establishment of a common interlaboratory "absolute reference frame" for reporting Δ 47 (Dennis et al, 2011) has not brought the discrepant calibrations into agreement.…”

“…Notably, both types of calibrations have been reported based on measurements of samples precipitated at <50 ºC using different methods, made on the same instrument and reacted for analysis using the same digestion apparatus (Tang et al, 2014;Tripati et al, 2015). In addition, both types of calibrations have been observed in deep-sea coral of variable mineralogy, also measured on the same system (Kimball et al, 2015).…”

Exploring the Temperature Dependence of Clumped Isotopes in Modern Porites Corals

“…To discriminate between the potential factors governing isotopic fractionation in corals observed in this study, we explored the isotopic signatures in the Δ(Δ 47 )‐Δ(δ 18 O) and Δ(Δ 47 )‐Δ(δ 13 C) panels (Figure ). The Δ values of δ 13 C, δ 18 O, and Δ 47 have previously been defined as the offsets of measured values relative to their respective equilibrium values (Kimball et al, ; Saenger et al, ; Spooner et al, ; Thiagarajan et al, ). In our study, we defined the Δ values as the offsets of the binned isotopic compositions in track 16SYST1 relative to 16SYST2 (Figures a and b).…”

“…The Δ 47 values of modern corals have been analyzed to assess their capacity for constraining seawater temperatures (Ghosh et al, ; Kimball et al, ; Saenger et al, ; Saenger et al, ; Spooner et al, ; Thiagarajan et al, ). Significant positive deviations in Δ 47 values relative to those of inorganic carbonates at the same temperature have been observed in some Porites corals (Ghosh et al, ; Saenger et al, ).…”