Non‐linear mixing effects on mass‐47 CO2 clumped isotope thermometry: Patterns and implications

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110849/1/rcm7175.pd

Development of the Mass-47 Clumped Isotope Paleothermometer: Methods, Theory, and Application to Climate and Diagenetic Reconstructions.

The mass-47 CO2 clumped isotope paleothermometer is one of the most exciting developments in the field of paleothermometry of the last 50 years. This technique improves upon previous approaches in that it constrains the formation temperature of carbonate minerals in a single measurement without assumptions about the composition of the precipitating fluid. This allows the clumped isotope thermometer to be applied to situations where traditional techniques cannot discern the relative influences of temperature versus fluid composition, and allows more accurate reconstructions of Earth climate. Several challenges remain before the thermometer can be accurately applied to Earth materials. Two separate calibrations must be employed to convert measured clumped isotope abundance into temperature estimates: a calibration of the effects of phosphoric acid digestion on CO2 isotopologues; and a calibration of the corrected mass-47 CO2 abundance to formation temperatures. Investigation of these calibrations reveals that they are likely mineralogy independent, suggesting that the cation and crystallographic structure plays a negligible role in clumped isotope equilibria. A second challenge relates to the sample size required for clumped isotope studies: at ~ 20 mg per sample, sample homogeneity cannot be assumed. Full investigation of mixing effects using a computer model and laboratory tests shows that sample heterogeneity can have a measureable effect on clumped isotope temperature estimates, but that these effects can be mitigated by the use of subsampling and mathematical mixing calculations. Further application of the clumped isotope thermometer requires detailed understanding of different proxy materials and the effects of geologic processes on these materials. Calcite formed by meteoric diagenesis was investigated in Pleistocene rocks and sediments. It was determined that meteoric phreatic cements accurately record mean annual temperature, while carbonate from vadose environments record kinetic departures from isotopic equilibrium. To demonstrate the utility of clumped isotopes a suite of Cenozoic New Zealand bivalves were examined. The clumped isotope thermometer reveals that New Zealand climate warmed from the Eocene to Miocene, which is the opposite trend inferred using traditional approaches. The ability to constrain temperatures independent of water isotopic composition is a major breakthrough, and will continue to refine our understanding of Earth history.PHDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108868/1/defliese_1.pd

Analytical effects on clumped isotope thermometry: Comparison of a common sample set analyzed using multiple instruments, types of standards, and standardization windows

International audienceRationale: Carbonate clumped isotope geothermometry is being increasingly used in multiple disciplines in the geosciences. However, potential interlaboratory issues are arising from different standardization procedures that may contribute to the multiple Δ47-temperature calibrations reported in the literature. We investigate this issue by comparing a common temperature calibration sample set across three different mass spectrometers, using multiple standardization methods.Methods: The same temperature calibration sample set was analyzed on three different mass spectrometers. Several standardization methods were utilized, including the use of carbonate versus gas standards, and different types of background correction were applied to the raw data.Results: All standardization types applied resulted in statistically indistinguishable Δ47-temperature slopes, with the exception of standardization calculations that did not correct for background effects. Some instruments and standardizations showed different intercepts relative to each other. The use of carbonate standards improved comparability between different instruments relative to gas standards.Conclusions: Our results show that background effects are the largest factor potentially affecting Δ47 results, and there may be an improvement in interlaboratory precision using carbonate standards. Critically, all techniques used for standardizing Δ47 results converge on a common slope as long as background effects are properly corrected. The use of carbonate standards is recommended as a component of standardization procedur

Evaluation of meteoric calcite cements as a proxy material for mass-47 clumped isotope thermometry

Meteoric diagenetic cements are ubiquitous throughout geologic history, affecting most carbonate exposures worldwide. They can often be difficult to interpret, as it is usually difficult to separate the influences of water δO and temperature on isotopic signals contained within the carbonate rock body. Despite this difficulty in interpretation, meteoric phreatic cements can potentially be a useful proxy material, as they form slowly in equilibrium at mean annual temperature and are not affected by any biogenic effects that can bias other proxy materials.We applied the mass-47 clumped isotope paleothermometer to Pleistocene and Holocene carbonates from Bermuda and Barbados in order to investigate the effects of meteoric diagenesis on δ signals, and to determine their suitability as a paleotemperature proxy. Phreatic calcite cements are found to record the same temperatures as unaltered carbonate sediments, while any sample exhibiting vadose characteristics is biased towards unreasonably hot apparent formation temperatures. Burial heating and re-equilibration are not geologically viable explanations for the anomalously hot temperatures recorded in vadose cements, as neither Bermuda or Barbados has any burial history. Instead, it is likely that precipitation in the vadose zone occurs on timescales quicker than isotopic equilibrium can be achieved, driven by a combination of CO degassing and evaporation, which have been previously shown to cause problems in speleothems and pedogenic carbonates.We conclude by suggesting that meteoric phreatic calcites may be an ideal phase for paleotemperature reconstruction, as they accurately record mean annual temperatures and form under equilibrium conditions, while also being resistant to further mineral driven diagenesis. Vadose cements, and any sample likely affected by processes similar to vadose diagenesis, should be avoided for climate reconstructions using the mass-47 clumped isotope thermometer

Compositional and temperature effects of phosphoric acid fractionation on Delta(47) analysis and implications for discrepant calibrations

An essential procedure to increase the analytical efficiency of Delta(47) measurements requires raising the temperature of phosphoric acid digestion for carbonate materials. This temperature change introduces a fractionation offset in Delta(47) that must be accounted for prior to calculation of temperatures of carbonate formation and to allow interlaboratory comparison of results. We measured the phosphoric acid fractionation factor relative to reaction at 25 degrees C for calcite, aragonite, and dolomite across a temperature range from 25 to 90 degrees C. Significantly, all three minerals behave similarly during phosphoric acid digestion, allowing for a single temperature dependent acid fractionation relationship:1000 ln infinity CO2(Acid)-Delta(47) = (0.022434 +/- 0.001490) * 10(6)/T-2 - (0. 2524 +/- 0: 0168)where a is the phosphoric acid fractionation factor, and T is in degrees Kelvin. Mineralogical or isotopic compositional effects on the fractionation factorwere not observed, suggesting that this acid fractionation factor may be valid for all carbonate minerals. We also present inorganic temperature calibrations for both calcite and aragonite at low temperatures ( 5-70 degrees C) and find them to agree with prior published data. Using the new acid fractionation factor, published Delta(47)-temperature calibrations are recalculated. This analysis confirms a statistically significant Delta(47)-temperature calibration difference between data analyzed at 25 degrees C versus higher temperatures. The origin of the discrepancy remains unknown, but it appears that the acid fractionation factor is not the cause. (C) 2015 Elsevier B.V. All rights reserved

Investigation of the thermal history of the Delaware Basin (West Texas, USA) using carbonate clumped isotope thermometry

We utilized carbonate clumped isotope thermometry to explore the thermal history of the Delaware Basin, West Texas, USA. Carbonate wellbore cuttings from five oil/gas wells across the basin yielded clumped isotope temperatures (T(Delta(47))) ranging from 27 degrees C to 307 degrees C, interpreted to reflect a combination of initial precipitation/recrystallization temperature and solid-state C-O bond reordering during burial. Dolomite samples generally record lower apparent T(Delta(47))s than calcite, reflecting greater resistance to reordering in dolomite. In all five wells, clumped isotope temperatures exceed modern downhole temperature measurements, indicating higher heat flow in the past. Using modelled burial curves based on sedimentological history, we created unique time-temperature histories by linearly applying a geothermal gradient. Applying two different thermal history reordering models, we modelled the extent of solid-state C-O bond reordering to iteratively find the time-averaged best-fit geothermal gradients for each of the five wells. Results of this modelling suggest that the shallower, southwestern portion of the study area experienced higher geothermal gradients throughout the sediment history (similar to 45 degrees C/km) than did the deeper, southeastern portion (similar to 32 degrees C/km), with the northern portion experiencing intermediate geothermal gradients (similar to 35-38 degrees C/km). This trend is in agreement with the observed gas/oil ratios of the Delaware Basin, increasing from east to west. Furthermore, our clumped isotope temperatures agree well with previously published vitrinite reflectance data, confirming previous observations and demonstrating the utility of carbonate clumped isotope thermometry to reconstruct basin thermal histories

Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ47) measurements: insights from a long-term dataset

Carbonate clumped isotope (Δ ) thermometry examines the temperature-dependent excess abundance of the C- O bond in the carbonate lattice. Inconsistent temperature calibrations and standard values have been reported among laboratories, which has led to use of equilibrated gases and carbonate standards for standardization. Furthermore, different acid fractionation factors and isotopic parameter sets have been proposed to improve inter-laboratory data comparability. However, few long-term datasets have been generated to explore the effects of these factors on the long-term reproducibility of Δ data within a laboratory.Four standards (ISTB-1, NBS-19, GBWO4416, and GB04417) were analyzed as unknowns from 2015 to 2019. Δ values were calibrated using the ETH standards. We investigated the Assonov, Brand, and Gonfiantini isotope parameter sets for carbon and oxygen isotopes, as well as two correction schemes of equilibrated gas and carbonate standardization, using the same sample measurements to determine which procedures enhanced reproducibility. ISTB-1 (calcite) and ZK312-346W (dolomite) were measured to determine the 90 C acid fractionation factor.The corrected 90 C acid fractionation factors are 0.076±0.008‰ for ISTB-1 and 0.077±0.009‰ for ZK312-346W. The choice of isotope parameter set had no significant influence on final Δ values in this study. However, using the Assonov parameters to calculate Δ values improved the reproducibility of results. The use of carbonate standards improved reproducibility through time compared to use of equilibrated gases for standardization.At 90 C, the acid fractionation factors of calcite and dolomite are statistically indistinguishable. We find an insignificant effect from changing the isotope parameter set, suggesting that the choice of isotope parameter set among laboratories is not a major factor affecting interlaboratory reproducibility. We find that using carbonate standards improved the reproducibility of results, suggesting that the use of carbonate standards may help achieve interlaboratory comparability of results in future studies