The study of organic matter in a well-preserved meteorite provides insight into processes that affected its parent asteroids.

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[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.

Abstract. Recent findings show that the amount of organic carbon stored in high-latitude permafrost regions has been greatly underestimated. While concerns are rising that thawing permafrost and resultant CO 2 and methane emissions are a positive feedback mechanism at times of anthropogenic global warming, the potential role of permafrost carbon dynamics on glacial-interglacial timescales has received little attention.Here we present new results from a well-studied permafrost loess-paleosol sequence in north-east Siberia that almost spans two glacial cycles (∼220 ka). We analysed the deuterium/hydrogen isotopic ratios (δD) of alkanes, which serve as proxy for paleo-temperature. Thus circumventing difficulties to obtain exact age control for such sequences, the results corroborate our previous notion that more soil organic carbon was sequestered during glacials than during interglacials. This fact highlights the role of permafrost in favouring preservation of soil organic matter. Reduced biomass production during glacials may have been of second-order importance on these timescales.Although future studies are needed to evaluate existing large estimates of carbon dioxide releases from thawing permafrost during the last termination (>1000 Pg C), we suggest that permafrost carbon dynamics contributed to the observed glacial-interglacial variation in atmospheric CO 2 and need to be included in carbon cycle and climate models.

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