Explaining High Frequency CO2 Fluctuations Observed in Snowpacks

Abstract:

Northern soil organic carbon (C) reserves continue to lose stability due to anthropogenic climate change. Often overlooked and deemed negligible, winter soil carbon dioxide (CO₂) respiration is a significant and understudied component of the global C cycle. Datasets have shown that winter soil CO₂ fluxes can be surprisingly variable owing to physical factors such as snowpack properties and wind. Using in-situ measurements and soil transport modeling, the purpose of our study was to explain high frequency CO₂ fluctuations observed in snowpacks. Continuous measurements consisted of CO₂ concentration gradients and meteorological data at two upland sites of differing snow cover in Nova Scotia, Canada. Periods of equilibrated snow depth were extracted to determine the relationship between CO₂ in snow and wind speeds during these intervals. We adapted a soil CO₂ diffusion model for the soil-snow system, and simulated step changes in transport rate over a broad range of plausible synthetic cases. The goal was to mimic changes we observed in CO₂ snowpack concentration to help elucidate the mechanisms (diffusion, advection) responsible for observed variations. Field data results showed the effect of advective transport from wind on the concentration of CO₂ in the snowpack. On short time spans (6-48 hrs) with varying winds and constant snow levels, a strong negative relationship between wind speed and CO₂ concentration within the snowpack was often identified. This relationship was observed at both sites, though deeper snow resulted in more frequent and stronger occurrences. Modeling clearly demonstrated that diffusion alone was unable to replicate the high frequency CO₂ fluctuations, but simulations using above-atmospheric snowpack diffusivities (simulating advective transport within the snowpack) reproduced snow CO₂ changes of the observed magnitude and speed. This confirmed that wind-induced ventilation was contributing to episodic pulsed emissions from the snow surface and suppressed snowpack concentrations. This study improves our understanding of winter CO₂ dynamics to aid in continued quantification of the annual global C cycle and will inform our use of measurement methodologies suited to winter environments.