Δ47 Temperature Research Articles

Siderite ‘clumped’ isotope thermometry: A new paleoclimate proxy for humid continental environments

Clumped isotope measurements can be used to exploit the paleoclimatic potential of pedogenic siderite (FeCO3); however, the applicability of this method is held back by the lack of clumped isotope calibrations of mineralogies other than calcite and aragonite. Here we present an inorganic calibration of siderites grown in the laboratory between 21 and 51°C. Linear regression of Δ47 values and temperature (106/T2, K) yields the following relationship (r2=0.997):Δ47-RF=(0.0356±0.0018)×106T2+(0.172±0.019)We demonstrate that this calibration is indistinguishable from calcite at current levels of analytical precision. Our observations suggest that there is likely no large systematic bias in the clumped isotope acid fractionation factors between the two different carbonate minerals. We also present clumped isotope measurements of a natural siderite collected from Holocene sediments of the Mississippi River Delta. We find that siderites record warm season marsh water temperatures instead of mean annual temperatures as it has long been presumed. This finding has important implications for the accuracy of siderite stable isotope and clumped isotope based climate reconstructions.

A revised calibration of the clumped isotope thermometer

A growing number of materials and environmental settings are studied using the carbonate clumped isotope (Δ47) thermometer. The method has been applied in both biogenic and non-biogenic carbonate systems, in marine and terrestrial settings, over a wide range of geological timescales. The current Δ47 temperature calibration gives good temperature estimates for most biogenic materials, however, systematic biases are commonly observed at low temperatures.In this study we report additional calibration data, that covers a wider temperature range, at more evenly distributed temperatures, and are measured at higher analytical precision than the original calibration. Combining these data with the original calibration yields a Δ47–T relationship that is similar to the original calibration, though slightly less steep: Δ47=(0.0526±0.0025)×106/T2+(0.0520±0.0284). This revised calibration is in better agreement with biogenic carbonates, including those grown at low temperatures. The difference between the original and revised calibrations is significant for carbonates forming below 16 °C or above 49 °C (Δ47 values of 0.68‰ and 0.56‰). Additionally, we include a comprehensive analysis of the sources of error associated with Δ47 measurements and estimated temperatures and recommend measurement strategies for obtaining the desired precision.As an illustration, we apply the revised calibration and uncertainty analysis to 3 previously published studies. At low temperatures, the revised calibration results in significant differences from the original calibration and hence affects the interpretation of the environmental signal recorded. In light of our Δ47 errors analysis, in cases where the temperature signals are small, we find that replicate analyses are critical to identify a temperature signal.

The influence of temperature and seawater carbonate saturation state on <sup>13</sup>C–<sup>18</sup>O bond ordering in bivalve mollusks

Abstract. The shells of marine mollusks are widely used archives of past climate and ocean chemistry. Whilst the measurement of mollusk δ18O to develop records of past climate change is a commonly used approach, it has proven challenging to develop reliable independent paleothermometers that can be used to deconvolve the contributions of temperature and fluid composition on molluscan oxygen isotope compositions. Here we investigate the temperature dependence of 13C–18O bond abundance, denoted by the measured parameter Δ47, in shell carbonates of bivalve mollusks and assess its potential to be a useful paleothermometer. We report measurements on cultured specimens spanning a range in water temperatures of 5 to 25 °C, and field collected specimens spanning a range of −1 to 29 °C. In addition we investigate the potential influence of carbonate saturation state on bivalve stable isotope compositions by making measurements on both calcitic and aragonitic specimens that have been cultured in seawater that is either supersaturated or undersaturated with respect to aragonite. We find a robust relationship between Δ47 and growth temperature. We also find that the slope of a linear regression through all the Δ47 data for bivalves plotted against seawater temperature is significantly shallower than previously published inorganic and biogenic carbonate calibration studies produced in our laboratory and go on to discuss the possible sources of this difference. We find that changing seawater saturation state does not have significant effect on the Δ47 of bivalve shell carbonate in two taxa that we examined, and we do not observe significant differences between Δ47-temperature relationships between calcitic and aragonitic taxa.

Natural gas

Clumped isotope compositions of slowly-cooled calcite and dolomite marbles record apparent equilibrium temperatures of roughly 150–200 °C and 300–350 °C, respectively. Because clumped isotope compositions are sensitive to the details of T–t path within these intervals, measurements of the Δ47 values of coexisting calcite and dolomite can place new constraints on thermal history of low-grade metamorphic rocks over a large portion of the upper crust (from ∼5 to ∼15 km depth). We studied the clumped isotope geochemistry of coexisting calcite and dolomite in marbles from the Notch Peak contact metamorphic aureole, Utah. Here, flat-lying limestones were intruded by a pluton, producing a regular, zoned metamorphic aureole. Calcite Δ47 temperatures are uniform, 156 ± 12 °C (2σ s.e.), across rocks varying from high-grade marbles that exceeded 500 °C to nominally unmetamorphosed limestones >5 km from the intrusion. This result appears to require that the temperature far from the pluton was close to this value; an ambient temperature just 20 °C lower would not have permitted substantial re-equilibration, and should have preserved depositional or early diagenetic Δ47 values several km from the pluton. Combining this result with depth constraints from overlying strata suggests the country rock here had an average regional geotherm of 22.3–27.4 °C/km from the late Jurassic Period until at least the middle Paleogene Period. Dolomite Δ47 in all samples above the talc + tremolite-in isograd record apparent equilibrium temperatures of 328-12+13°C (1σ s.e.), consistent with the apparent equilibrium blocking temperature we expect for cooling from peak metamorphic conditions. At greater distances, dolomite Δ47 records temperatures of peak (anchi)metamorphism or pre-metamorphic diagenetic conditions. The interface between these domains is the location of the 330 °C isotherm associated with intrusion. Multiple-phase clumped isotope measurements are complemented by bulk δ13C and δ18O dolomite–calcite thermometry. These isotopic exchange thermometers are largely consistent with peak temperatures in all samples within 4 km of the contact, indicating that metamorphic recrystallization can occur even in samples too low-grade to produce growth of conventional metamorphic index minerals (i.e., talc and tremolite). Altogether, this work demonstrates the potential of these methods to quantify the conditions of metamorphism at sub-greenschist facies.