was like when the creatures died. The oxygen isotope ratios in their shells can also be used as proxies for temperature. Water isotopes and temperature reconstruction Ocean water is mostly H216O, with small amounts of HD16O and H218O, where D denotes deuterium, i.e. hydrogen with an extra neutron. In Vienna Standard Mean Ocean Water (VSMOW) the ratio of D to H is 155.76x10−6 and O-18 to O-16 is 2005.2x10−6. Isotope fractionation occurs during changes between condensed and vapour phases: the vapour pressure of heavier isotopes is lower, so vapour contains relatively more of the lighter isotopes and when the vapour condenses the precipitation preferentially contains heavier isotopes. The difference from VSMOW is expressed as δ18O = 1000‰ ; and a similar formula for δD. δ values for precipitation are always negative. The major influence on δ is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the δ value mostly reflects the temperature where precipitation occurs. Taking into account that the precipitation forms above the inversion layer, we are left with a linear relation: δ 18O = aT + b This is empirically calibrated from measurements of temperature and δ as a = 0.67 ‰/°C for Greenland and 0.76 ‰/°C for East Antarctica. The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations. More recently, borehole thermometry has shown that for glacial-interglacial variations, a = 0.33 ‰/°C, implying that glacial-interglacial temperature changes were twice as large as previously believed. A study published in 2017 called the previous methodology to reconstruct paleo ocean temperatures 100 million years ago into question, suggesting it has been relatively stable during that time, much colder. Membrane lipids A novel climate proxy obtained from peat (lignites, ancient peat) and soils, membrane lipids known as glycerol dialkyl glycerol tetraether (GDGT) is helping to study paleo environmental factors, which control relative distribution of differently branched GDGT isomers. The study authors note, "These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria." , there is a decade of research demonstrating that in mineral soils the degree of methylation of bacteria (brGDGTs), helps to calculate mean annual air temperatures. This proxy method was used to study the climate of the early Palaeogene, at the Cretaceous–Paleogene boundary, and