This nitrogen also bears the isotopic fingerprint of nitrogen derived through the inorganic processes of fertiliser manufacture and fossil fuel burning, and not of the way microbes do it.
By analysing the isotopic composition of hydrocarbon molecules from plant waxes of the period, he found what looks like a spike in the amount of recently non-biological carbon (which has a distinctive ratio of light isotopes to heavy ones), lasting between 10, 000 and 20, 000 years.
The records from a particularly warm period called Marine Isotopic Stage 11, which occurred around 400, 000 years ago, suggest that warming of 1.5C compared to the present is enough to cause substantial thawing of permafrost - even in areas far north from its present-day southern limit.