Physical processes controlling greenhouse gas emission in upwelling regions of the ocean: An N2O case study
SOPRAN phases I and II suggested turbulent mixing to be the main process bringing nitrous oxide (N2O, laughing gas) from below to the near surface of upwelling systems. The workhorse instrument which served to quantify turbulent mixing on countless profiles during several cruises is the Sea&Sun Microstructure Profiler (Fig.1). A remaining open question since has been why estimated N2O outgassing from the ocean surface to the atmosphere was found substantially higher than the sum of N2O supplies from below.
One part of SOPRAN III activity has been exploring the hypothesis that outgassing is overestimated because common measurements near the surface incorrectly assume that gas concentrations are homogeneous for at least the top 5 to 10 meters (Fig. 2). In order to check this hypothesis, high resolution N2O profiles of the top 10 meters (Fig. 3) were analyzed during SOPRAN cruise Meteor M91 in the upwelling system off Peru.
Preliminary results of this campaign show that N2O gradients similar to Fig. 2 exist, and can explain part of the outgassing overestimation. Surface microlayers (SP 2.2) and/or near surface N2O production (SP 1.10) may also contribute to explain the discrepancy between estimated outgassing and supply from below. Such processes may have profound implications for greenhouse gas emission estimates from upwelling systems.
Fig. 1: The microstructure profiler, carrying airfoil
shear sensors which detect the small-scale water velocity fluctuations that are
caused by turbulence. During descent, the power and data cable is slack in
order to avoid transfer of ship vibrations to the probe.
Fig. 2: Common outgassing estimates are based on
correct estimates of the driving difference between gas concentrations in the
ocean surface and the atmosphere. The sketch illustrates how a near surface gas
gradient can cause measurements from research vessels to overestimate this
concentration difference and thus the outgassing.
Fig. 3: Dinghy based N2O high resolution sampling
of the top 10 meters of the ocean, away from ship influence. The top 1 meter
was sampled with a submersible pump, meter 1 to 10 was sampled with a hand
triggered Niskin bottle.
PI's: M. Dengler, H. Bange, P. Brandt
Contact: Tim Fischer (email@example.com)