N<sub>2</sub>O changes from the Last Glacial Maximum to the preindustrial – Part 1: Quantitative reconstruction of terrestrial and marine emissions using N<sub>2</sub>O stable isotopes in ice cores

Abstract

<jats:p>Abstract. 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 δ13C(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. </jats:p>

N&lt;sub&gt;2&lt;/sub&gt;O changes from the Last Glacial Maximum to the preindustrial – Part 2: terrestrial N&lt;sub&gt;2&lt;/sub&gt;O emissions and carbon–nitrogen cycle interactions

“…We start the presentation of results by summarizing the main feature of the terrestrial N 2 O emission record (Fig. 3, green line) presented in part 1 of this study (Fischer et al, 2019). In part 1, the global N 2 O emissions from land and from the ocean are jointly reconstructed by deconvolving novel icecore data of N 2 O and of its isotopic signature, δ 15 N(N 2 O), using an established method and relying on differences in the isotopic signature of land versus marine N 2 O emissions.…”

“…Terrestrial N 2 O emissions show a 40 % increase from the Last Glacial Maximum to the late preindustrial period (Fischer et al, 2019). Most of the deglacial increase was realized in two large steps, linked to rapid, decadal-scale, and widespread Northern Hemisphere warming and to shifts in the Intertropical Convergence Zone (ITCZ) and precipitation patterns.…”

“…Most frequently, empirical models are used, describing BNF as a linear function of evapotranspiration (Cleveland et al, 1999) or as an exponential function of NPP (Thornton et al, 2007). More process-oriented models heuristically account for the dependency of symbiotic BNF on N demand by vegetation, soil N status, and light limitations in extra-tropical regions (Gerber et al, 2010) or on the optimization of plant C investment in resource acquisition (Fisher et al, 2010). Meyerholt et al (2016) describe asymbiotic BNF as a function of temperature, shading, and soil moisture.…”

“…In part 1 of this study (Fischer et al, 2019), this earlier work was extended to reconstruct the evolution in terrestrial versus oceanic emissions of N 2 O from the Last Glacial Maximum (LGM; 21 ka) to the late preindustrial Holocene using novel, high-precision N 2 O stable isotope data. The age scales of the records were aligned with the absolutely counted GICC05 age scale (Rasmussen et al, 2006) using high-resolution methane measurements leading to particularly small age-scale uncertainties around past rapid warming events, namely, at the onset of the Bølling-Allerød Northern Hemisphere warm period and of the Holocene interglacial.…”

“…Controls on variations in terrestrial emissions are elucidated in the framework of a dynamic global vegetation model. Part 1 of this study (Fischer et al, 2019) presents the ice-core N 2 O concentration and isotope data and the reconstruction of global terrestrial and marine N 2 O emissions for the past 21 000 years and provides a discussion of the reconstructed marine emissions in the context of past climate and oceanographic changes. A model-based interpretation of the reconstructed marine N 2 O emission changes during past abrupt climate events is given by Joos et al (2019).…”