An interlaboratory study of TEX86 and BIT analysis of sediments, extracts and standard mixtures

Abstract

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 paleothermometer for sea-surface temperature reconstructions and theBIT index for reconstructing soil organic matter input to the ocean. An initial round-robin studyof two sediment extracts, in which fifteen laboratories participated, showed relatively consistentTEX86 values (reproducibility ±3-4 °C when translated to temperature) but a large spread in BITmeasurements (reproducibility ±0.41 on a scale of 0 to 1). Here we report results of a secondround-robin study with thirty-five laboratories in which three sediments, one sediment extractand two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3-3.0 °C when translated totemperature. These results are similar to those of other temperature proxies used inpaleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences insediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poorreproducibility and a large overestimation of the =true‘ (i.e., molar-based) BIT index. The latter is likely due to, amongst other factors, the higher mass spectrometric response of branched GDGTscompared to crenarchaeol, which also varies among mass spectrometers. Correction for thisdifferent mass spectrometric response showed a considerable improvement in the reproducibilityof BIT index measurements among laboratories, as well as a substantially improved estimationof molar-based BIT values. 230 This suggests that standard mixtures should be used in order toobtain consistent, and molar-based, BIT values.