Carbonate results guide

A Kiel IV-MAT 253 results file includes several tabs. A results summary is in the Final Report tab.

All data are observed directly and recorded by Thermo Scientific Isodat software during automated analysis. This output is recorded in the Kiel and Kielresult tabs. These C and O isotope ratio data are normalized (also called corrected) to international reference scales according to the calculations explained below. These calculations are documented in the Data Corrections tabs. A comprehensive understanding of the contents of the first three tabs is generally unnecessary for appropriate application of carbonate isotope ratio results in the results summary in the Final Report tab.

Row

Run

Row is the number position of the measurement in the run sequence.

Some results files will be a compilation of multiple runs and will also be annotated with the run number.

Sample weight (µg)

ID (Identifier) is the unique identifier for the sample.

If the sample weight is known, this is also specified as Sample weight.

Est. CaCO3 (µg)

CaCO3 (wt%)

Est. CaCO3 is the equivalent amount of inorganic carbon as calcium carbonate in the acidified sample calculated from exponential regression of CO2 pressure in Kiel IV trap 1 vs. measured weight of sample for pure homogeneous CaCO3.

If the weight of the sample is specified, then wt% CaCO3 is calculated from Est. CaCO3. The error in this measurement is relatively large, because the precision pressure transducer measurement used to estimate CaCO3 is relatively poor and the precision of the weight for relatively small samples prepared for Kiel IV analysis also tends to be poor. Analysis of larger samples via acidometric coulometery produces superior wt%CaCO3 estimates.

Sample Intensity (mV)

Pressure Balance (S/R)

Sample intensity is the voltage intensity of the first of eight cycles of C and O isotope ratio measurements in the MAT 253. Typical sample yields are 1000 to 10,000 mV. Samples intensitiy results near 0 indicate a measurement problem in the Kiel IV or that the sample was too small (<10 µg CaCO3).

Pressure balance is the ratio of sample voltage to reference voltage. For an accurate Kiel IV measurement, this ratio should be 0.9 to 1.1. Depending on the amount of reference gas supplied, samples smaller than ~20 µg CaCO3 may produce isotope ratio data with good accuracy with initial pressure balance of 0.8 to 0.9. This accuracy depends on the pressure balance of the later of the eight measurement cycles, and will be appropriately annotated in the results file.

δ¹³C raw (‰ VPDB)

δ¹³C raw (± 1s)

δ¹⁸O raw (‰ VPDB)

δ¹⁸O raw (± 1s)

The first column block of C and O isotope ratio data are 'raw' data because this is how the measurement in Isodat is reported. This measurement is referenced to the CO2 reference gas, which is calibrated to VPDB. These data is the most accurate set of isotope ratio data for a data set where the number of CM12 and NBS18 standard reference material measurements in the correction are small. This data standardization situation is most common when a run is aborted and not restarted due to technical issues.

The ± 1s data are the standard errors for set of eight C and O isotope ratio measurements for the eight sample-reference cycles for the measurement of a sample. These eight measurements are not identically and independently distributed, so this is not a measurement of external precision of the isotope ratio data.While this standard error is not necessarily an indication of baseline measurement reproducibility, as isotope ratio variance may be correlated between samples, this generally is an indicator of instrument noise and therfor minimum measurement reproducibility.

δ¹³C vs. CM12 (‰ VPDB)

δ¹⁸O vs. CM12 (‰ VPDB)

The second column block of isotope ratio data are standardized to the measurements of CM12 standard reference material in the run or set of runs being standardized. These data is the most accurate set of measurements for a data set where the sample isotope ratios are very near marine and there are many observations of CM12.

δ¹³C vs. CM12 & NBS18 (‰ VPDB)

δ¹⁸O vs. CM12 & NBS18 (‰ VPDB)

The third column block of isotope ratio data are standardized to the measurements of CM12 and NBS18 standard reference material in the run or set of runs being standardized. This two-point correction includes a slope factor that accounts for non-linearity in the delta isotope scale. The process often invoked for this non-linearity is fractionation ion source via differential ionization. This non-linearity is typically insignificant for C, but significant for O, with an ~1 % deviation from linear. Because this two point correction accounts for non-linearity, these data are the most accurate set of measurements for samples that deviate by more than a few permil from typical marine values. These are also the most accurate data for any data set with a large number of independent observation of CM12 and NBS18. Because NBS18 is relatively heterogeneous for a standard reference material, and because its isotopic composition is significantly different than marine, this correction is not appropriate for samples standardized with a small number of NBS18 observations.