N2O changes from the Last Glacial Maximum to the preindustrial – Part 1: Quantitative reconstruction of terrestrial and marine emissions using N2O stable isotopes in ice cores

Fischer, Hubertus; Schmitt, Jochen; Bock, Michael; Seth, Barbara; Joos, Fortunat; Spahni, Renato; Lienert, Sebastian; Battaglia, Gianna; Stocker, Benjamin D.; Schilt, Adrian; Brook, Edward J. (2019). N2O changes from the Last Glacial Maximum to the preindustrial – Part 1: Quantitative reconstruction of terrestrial and marine emissions using N2O stable isotopes in ice cores. Biogeosciences, 16(20), pp. 3997-4021. European Geosciences Union 10.5194/bg-16-3997-2019

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Using high-precision and centennial-resolution ice
core information on atmospheric nitrous oxide concentrations
and its stable nitrogen and oxygen isotopic composition,
we quantitatively reconstruct changes in the terrestrial
and marine N2O emissions over the last 21 000 years.
Our reconstruction indicates that N2O emissions from land
and ocean increased over the deglaciation largely in parallel
by 1.7+-0.3 and 0.7+-0.3 TgN yr-1, respectively, relative
to the Last Glacial Maximum level. However, during the
abrupt Northern Hemisphere warmings at the onset of the
Bølling–Allerød warming and the end of the Younger Dryas,
terrestrial emissions respond more rapidly to the northward
shift in the Intertropical Convergence Zone connected to the
resumption of the Atlantic Meridional Overturning Circulation.
About 90% of these large step increases were realized
within 2 centuries at maximum. In contrast, marine emissions
start to slowly increase already many centuries before the
rapid warmings, possibly connected to a re-equilibration of
subsurface oxygen in response to previous changes. Marine
emissions decreased, concomitantly with changes in atmospheric
CO2 and d13C(CO2), at the onset of the termination
and remained minimal during the early phase of Heinrich
Stadial 1. During the early Holocene a slow decline in marine
N2O emission of 0.4 TgN yr-1 is reconstructed, which
suggests an improvement of subsurface water ventilation in
line with slowly increasing Atlantic overturning circulation.
In the second half of the Holocene total emissions remain
on a relatively constant level, but with significant millennial
variability. The latter is still difficult to attribute to marine or
terrestrial sources. Our N2O emission records provide important
quantitative benchmarks for ocean and terrestrial nitrogen
cycle models to study the influence of climate on nitrogen
turnover on timescales from several decades to glacial–
interglacial changes.

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