Organic carbon cycling along high-latitude continental margins and implications of climate change

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The carbon cycle involves the atmosphere, land and ocean and influences global climate. Organic carbon (OC) is the portion associated with living or dead matter; produced by plants from CO2, transformed by microbes and animals, and remineralized back to CO2 by respiration, degradation etc. Continental margins (nearshore zones and shelves) are important sites of OC cycling. They receive OC from both the land and the ocean and its production and destruction makes them sites of dynamic CO2 exchange with the atmosphere. Continental margins also receive sediment from rivers and coastal erosion, which promotes burial (sequestration) of OC. About 75% of the world's population lives along margins and their ecosystem services are worth $25 trillion annually. High-latitude areas are experiencing rapid climate change, including loss of sea ice, altered river runoff and thawing permafrost. The OC cycle will thus be affected by changes in sources and altered processing of OC in warmer, fresher, more ice-free, and possibly more productive coastal waters. Food webs will be impacted by these changes, as will CO2 exchange. One of the largest unknowns is how old terrigenous carbon will contribute to change. This carbon has been locked up in subsurface soils and permafrost for thousands of years, but is released when permafrost thaws and can be remineralized to CO2 in margin areas, particularly when highly-degradable marine OC exerts a `priming effect'. Because there is a very large amount of old terrigenous OC, its degradation represents a significant potential feedback to climate change. The overall objective of this proposed Discovery program is to improve our understanding of the processes that determine the fate of terrigenous OC along high-latitude continental margins. Some of these processes occur in the water column and some in surface sediments. Because the margin is variable in space and time, we will focus on "hot spots" and "hot moments", conducting field studies along i) dynamic, river-dominated open shelf areas, where we know old terrigenous OC is being released; and ii) protected coastal embayments, used extensively by wildlife and Inuit, but virtually unstudied in terms of their OC cycling. Working with at least nine southern HQP and a similar number of Northerners, I will study the distributions of OC in the water column; compare winter and summer, and collect annual time series using sediment traps. We will examine the OC in sediments to inform us about recent change. Box models that constrain the rates of OC losses will immediately help put better numbers and timelines on the climate feedback associated with old terrigenous OC. Ultimately, the results will help improve how the land-ocean interface is represented in physical models, develop robust future projections, and better design monitoring plans for tracking impacts of climate change.