An interlaboratory TEX₈₆ and BIT analysis using high-performance liquid chromatography-mass spectrometry

Recently, two new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were proposed, i.e., the TEX₈₆ proxy for sea surface temperature reconstructions and the BIT index for reconstructing soil organic matter input to the ocean. In this study, fifteen laboratories participated in a round robin study of two sediment extracts with a range of TEX₈₆ and BIT values to test the analytical reproducibility and repeatability in analyzing these proxies. For TEX₈₆ the repeatability, indicating intralaboratory variation, was 0.028 and 0.017 for the two sediment extracts or ±1-2°C when translated to temperature. The reproducibility, indicating among-laboratory variation, of TEX₈₆ measurements was substantially higher, i.e., 0.050 and 0.067 or ±3-4°C when translated to temperature. The latter values are higher than those obtained in round robin studies of Mg/Ca and U₃₇ᵏ' paleothermometers, suggesting the need to primarily improve compatibility between labs. The repeatability of BIT measurements for the sediment with substantial amounts of soil organic matter input was relatively small, 0.029, but reproducibility was large, 0.410. This large variance could not be attributed to specific equipment used or a particular data treatment. We suggest that this may be caused by the large difference in the molecular weight in the GDGTs used in the BIT index, i.e., crenarchaeol versus the branched GDGTs. Potentially, this difference gives rise to variable responses in the different mass spectrometers used. Calibration using authentic standards is needed to establish compatibility between labs performing BIT measurements.