Improved procedures for the cleavage and reduction of GDGTs for hydrogen isotope analysis

• Comparison of GDGT ether cleavage and reduction methods. • Improved protocols for ether cleavage and reduction of archaeal and bacterial GDGTs. • Development of a hydrogenation shift correction protocol for archaeal GDGTs. Glycerol dialkyl glycerol tetraether (GDGT) membrane lipids of archaea and bacteria are commonly used to reconstruct past environmental conditions, mainly temperature and pH, based on their relative abundances. In contrast, the stable hydrogen isotope composition (δ 2 H) of GDGTs has received comparatively little attention, although it may hold additional paleoenvironmental information. Established methods for the analysis of δ 2 H require chemical degradation into the constituent alkyl chains via cleavage of the ether bonds of GDGTs and reduction of the halogenated alkyl chains. Two of these GDGT cleavage and reduction methods have been widely used but never been compared: 1) boron tribromide (BBr 3 ) and lithium triethylborohydride (SH), and 2) hydroiodic acid (HI) and platinum oxide (PtO 2 ) combined with H 2 . Here, we compare combinations of these cleavage and reduction methods in terms of yield, recovery, reproducibility, and hydrogen isotopic fractionation to provide improved protocols. Based on our experiments, for archaeal GDGTs we recommend ether cleavage with non-stabilized HI for 4 h at 120 °C followed by hydrogenation with SH for 2 h at 60 °C for optimal efficiency and minimized hydrogen isotopic fractionation. For bacterial GDGTs, we recommend the same cleavage protocol but hydrogenation with PtO 2 and H 2 , as it was not possible to implement a reliable SH isotope fractionation correction for these biomarkers. The improvement of these ether cleavage and hydrogenation protocols represents an important step in the development of δ 2 H of GDGTs as a possible paleoenvironmental proxy and will improve the comparability of future paleorecords using δ 2 H derived from GDGTs.