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

Abstract

[2] Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX 86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX 86 values (reproducibility 63-4 C when translated to temperature) but a large spread in BIT measurements (reproducibility 60.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX 86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX 86 values ranged from 1.3 to 3.0 C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX 86 and BIT indices are not significantly affected by interlaboratory differences in sediment 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 poor reproducibility and a large overestimation of the ''true'' (i.e., molarbased) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

A four-year record of U K′ 37 - and TEX 86 -derived sea surface temperature estimates from sinking particles in the filamentous upwelling region off Cape Blanc, Mauritania

“…The results of the selected repeat analyses are identical within error with the measurements in Bremen, as was to be expected according to the inter-laboratory comparison experiment (Schouten et al, 2013a).…”

Reconstructing Holocene temperature and salinity variations in the western Baltic Sea region: a multi-proxy comparison from the Little Belt (IODP Expedition 347, Site M0059)

“…The TEX L 86 does, therefore, not seem to be biased by large allochthonous contributions of isoprenoid GDGTs. This is also supported by an absence of correlation between the branched isoprenoid tetraether index and TEX L 86 values (r 2 = 0.04; p > 0.001), which is only to be expected if the terrestrial influence on the TEX L 86 is negligible (Schouten et al, 2013a). In fact, a TEX L 86 temperature record with similar trends, covering the last 1000 cal yr BP with temperature maxima during the MWP and the modern hypoxic period, has been reported from the Gotland Basin in the central Baltic Proper (Kabel et al, 2012), suggesting that the TEX L 86 indeed traces climatedriven variations in SWT in the brackish-marine Littorina Sea stage.…”

“…Analysis of GDGTs was performed using an Alliance 2695 (Waters, UK) high-performance liquid chromatograph coupled to a ZQ (Micromass, UK) single quadrupole mass spectrometer (MS) as detailed in Zink et al (2016) following the analytical protocols of Hopmans et al (2000) and Liu et al (2012). Isoprenoid and branched GDGTs were detected using single ion recording of their protonated molecules [M+H] + as outlined in Schouten et al (2013a). The TEX 86 , TEX L 86 , and TEX H 86 values were calculated using the equations given by Schouten et al (2002) and Kim et al (2010):…”

Black Sea temperature response to glacial millennial‐scale climate variability

“…The mean standard deviation for duplicate runs was 0.006 for TEX 86 (TetraEther IndeX of lipids with 86 carbon atoms), corresponding to 0.3°C. The laboratory setting was tested through two interlaboratory comparison experiments carried out in 2009 and 2013 [Schouten et al, 2013a]. The TEX 86 values were converted to mean annual surface temperature estimates using the global lake calibration [Powers et al, 2010].…”