Arctic Photochemistry

Turning on the lights – Photochemical and microbial processing of newly exposed carbon in arctic ecosystems

Rose Cory and George Kling, University of Michigan

This National Science Foundation project (NSF OPP-1023270) focuses on determining the controls and rates of organic carbon (C) degradation in arctic systems.  These controls include microbial respiration, photochemical reactions, and chemical (abiotic) reactions in soils and surface waters.  Tremendous stores of organic C reside in the permanently frozen layers of soil found in arctic and boreal regions, but this C must be unfrozen, dissolved, and oxidized before it can feedback to the atmosphere as CO2.  Recent evidence indicates that these landscapes face increasing disturbance from melting permafrost, thermokarst failures, and fire.  These disturbances will transform the Arctic in terms of (1) exposing previously buried or frozen C to the land surface ("newly released" C), (2) modifying C by being burned, unfrozen, and dissolved, (3) altering C when exposed to sunlight during transport through surface waters, and (4) determining the fate of newly exposed carbon.  The fate of the newly exposed, dissolved organic carbon (DOC) is complete oxidation to CO2 and release to the atmosphere, partial oxidation and transport as DOM in rivers to oceans in more recalcitrant forms, or trapped by deposition in lakes and quickly re-buried with little alteration.

This project has provided evidence to support the following main conclusions.  First, we found that permafrost carbon brought to the surface in thermokarst failures (landslides) is 40% more labile to bacterial degradation when exposed to light than is carbon held in the dark (PNAS 110:3429)Second, we showed that photochemical C degradation in surface waters was up to 94% of the total C processed, and dominates over bacterial respiration of C in arctic inland waters (Science 345:925); this conclusion is highlighted by the conceptual overview figure shown below.  Third, we show that a novel, chemical (abiotic) reaction with hydroxyl radical that occurs in the dark is an important pathway of C degradation in soils (Env. Sci. & Tech. 47:12860).

By integrating our process-based research with ongoing projects studying the formation of thermokarst failures and the impacts of burning (creation of 'black carbon') on plant and soil C dynamics, we are working to answer questions such as whether C export from tundra to oceans will rise or fall, how reactive is the C exported to oceans, and what will be the ultimate impact of impending disturbances, including climate change, on the net C balance of the Arctic and its potential feedback to an acceleration of global warming.

Conceptual Overview. Overview of processing of DOC released from soils (left) which can be completely oxidized to CO2 (dark bacterial respiration or photo-mineralization) or partially oxidized (partial photo-oxidation, DOCox) and remain in the DOC pool to be transported in rivers to oceans (right) DOC in headwater streams fed directly by soil waters has low prior light exposure, and is more labile to photochemical mineralization to CO2 (e.g., red piece of pie for Imnavait Creek) compared to Sagavanirktok River water where partial photo-oxidation to DOCox dominates.