Marine N2O emissions during a Younger Dryas-like event: the role of meridional overturning, tropical thermocline ventilation, and biological productivity

Fortunat Joos,Aurich Jeltsch-Thömmes,Hubertus Fischer,Jochen Schmitt,Gianna Battaglia

doi:10.1088/1748-9326/ab2353

Abstract

Past variations in atmospheric nitrous oxide (N2O) allow important insight into abrupt climate events. Here, we investigate marine N2O emissions by forcing the Bern3D Earth System Model of Intermediate Complexity with freshwater into the North Atlantic. The model simulates a decrease in marine N2O emissions of about 0.8 TgN yr−1 followed by a recovery, in reasonable agreement regarding timing and magnitude with isotope-based reconstructions of marine emissions for the Younger Dryas Northern Hemisphere cold event. In the model the freshwater forcing causes a transient near-collapse of the Atlantic Meridional Overturning Circulation (AMOC) leading to a fast adjustment in thermocline ventilation and an increase in O2 in tropical eastern boundary systems and in the tropical Indian Ocean. In turn, net production by nitrification and denitrification and N2O emissions decrease in these regions. The decrease in organic matter export, mainly in the North Atlantic where ventilation and nutrient supply is suppressed, explains the remaining emission reduction. Modeled global marine N2O production and emission changes are delayed, initially by up to 300 years, relative to the AMOC decrease, but by less than 50 years at peak decline. The N2O perturbation is recovering only slowly and the lag between the recovery in AMOC and the recovery in N2O emissions and atmospheric concentrations exceeds 400 years. Thus, our results suggest a century-scale lag between ocean circulation and marine N2O emissions, and a tight coupling between changes in AMOC and tropical thermocline ventilation.

Highlights

Nitrous oxide (N2O) is an atmospheric trace gas that contributes to the greenhouse effect and to the destruction of stratospheric ozone
The reconstructed decrease in marine N2O emissions during the YD is mainly explained by N2O production changes in the low-latitude thermocline in response to altered ventilation and O2 concentrations after a slowing down of the Atlantic Meridional Overturning Circulation (AMOC)
Taking the model results at face value, this would imply that the strong decrease in marine N2O emissions reconstructed from N2O concentrations and their nitrogen isotopic composition in ice cores for the onset of the YD (Schilt et al 2014, Fischer et al 2019) should be delayed relative to the onset of the AMOC reduction by a few hundred years

Summary

Introduction

Nitrous oxide (N2O) is an atmospheric trace gas that contributes to the greenhouse effect and to the destruction of stratospheric ozone. Ice core reconstructions have uncovered natural variations of atmospheric N2O between ∼195 and 290 ppb over the past 800 000 years, synchronous with variations in climate (Flückiger et al 1999, 2004, Sowers et al 2003, Spahni et al 2005, Schilt et al 2010a, 2010b, 2013, 2014). Since the reconstruction by Schilt et al (2014), new measurements of N2O and its isotopic composition have emerged and reconstructions of N2O emission changes from terrestrial and marine sources over the past 21 000 years are available (Fischer et al 2019). The interpretation of the relative phasing of AMOC changes and marine emission reconstructions using N2O isotopes is hampered by independent age scales and the resolution in both marine sediment and ice core records (Fischer et al 2019), the timing and strength of reconstructed emissions provide important benchmarks for model evaluation and process understanding

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