Fellowships / Research

Source or sink? Greenhouse gas emissions in the Delta

The newly restored wetland on Sherman Island near Mayberry Slough, below, was flooded in late 2010 and contains patches of emergent vegetation interspersed within open water. PHOTO: G. McNicol

The newly restored wetland on Sherman Island near Mayberry Slough was flooded in late 2010 and contains patches of emergent vegetation interspersed within open water. PHOTO: G. McNicol

Drive less. Turn off the lights. Use efficient appliances. Some steps to reduce greenhouse gas emissions are straightforward; others are trickier. Delta Science Fellow Gavin McNicol has been investigating one of the latter: wetland restoration.

Delta Science Fellow Gavin McNicol

Natural wetlands, like those McNicol is studying in the San Francisco Bay Delta (the Delta), store carbon in a thick layer of peat soil at their base. This soil forms from the slow anaerobic decomposition of submerged wetland plant litter, a process that stores ten times more carbon than it releases. This imbalance makes wetlands one of nature’s most efficient “sinks” for atmospheric carbon.

So when historic draining of wetlands exposed peat soils to the air, millennia of stored carbon was suddenly free to flood, rather than trickle, into the atmosphere. McNicol says that, in the Delta alone, breakdown of exposed peat soils account for one percent of California’s carbon dioxide (CO2) emissions.

McNicol, a PhD student and biogeochemist at UC Berkeley, is studying what happens to carbon cycles when Delta wetlands are restored. While restoration slows CO2 emissions and resumes carbon sequestration, other chemical processes also kick back into gear. This includes the production of methane, a greenhouse gas several times more potent than CO2.

“Though wetlands are typically sinks of CO2, they are also the largest natural source of methane,” says McNicol. In order to discover the impact of restored wetlands on greenhouse gas levels, and how this changes as they age, McNicol has set out to solve when, where and how much methane is leaving two restored wetlands in the Delta.

“Methane is also a greenhouse gas, and it is present in the atmosphere at much lower concentrations than CO2,” says McNicol. Yet every molecule of methane is 34 times more effective than CO2 at trapping heat. The result, says McNicol, is that, “Methane is the next most important gas for climate change after CO2.”

Methane can be emitted from a wetland in three possible ways: diffusion through the water column, ebullition (bubbling), and plant-mediated transport. Ebullition is particularly understudied as a pathway. To measure ebullition, McNicol uses a series of floating chambers to capture gas releases.  From there, he can estimate the amount and age of methane released.

Interestingly, McNicol has used naturally occurring radiocarbon to date his methane samples at 600-2200 years old, indicating that a substantial amount of “younger” carbon (closer to the surface) was already lost as CO2 prior to restoration. McNicol is also using carbon dating to trace the spatial and seasonal patterns of methane production.

Ultimately, McNicol hopes to understand what drives the variation and concentration of methane emissions in the Delta. “Understanding of what causes bubble release events, such as strong winds or shifting water levels, helps wetland biogeochemists develop models to predict future rates of bubbling,” says McNicol. “Then they can use those annual estimates to construct greenhouse gas budgets for the wetland.”

And as for whether restoration returns the Delta to being a carbon sink?

“The radiocarbon data can tell us how effective restoration is at stabilizing soil carbon,” McNicol says. “My work is part of the puzzle to understand the past, present and future of the Delta as a source and sink of greenhouse gases.”

Written by Deborah Seiler

Related article:
Wetland carbon budgets

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