We currently witness in the Arctic:

1. a decrease in summer ice cover that exposes sea surface to solar radiation and physical forcings;
2. permafrost thawing and increased river runoff, both leading to an increase in the export to the ocean of organic carbon previously sequestered in the Tundra;
3. an increase in ultraviolet radiation.

These three phenomena favour a growing mineralization of organic carbon through photo-oxidation and bacterial activity, amplifying the increase in atmospheric CO2. At the same time, the exposure of a larger fraction of ocean surface to sun light and the increase in nutrients brought by rivers lead to larger autotrophic production and sequestration of organic carbon. To predict the balance of these processes, we conducted an extensive study in the Mackenzie River / Beaufort Sea system in July, August and September 2009 onboard the Canadian research ice-breaker CCGS Amundsen. The spatial distribution of organic carbon stocks (living and detrital) in the water column and sediments was determined on the shelf and beyond. The magnitude and variability of organic carbon mineralization through photo-oxidation and bacterial activity, and production through photosynthesis was measured. These targeted studies allows the monitoring of these processes using remote sensing in the coming years and decades. A detailed study of microbial biodiversity was conducted to describe the different biocenoses and biotopes and to anticipate their response to climate change. Diagnostic models of the studied processes (primary production, bacterial activity and light-driven mineralization of organic matter) are combined with a coupled physical-biological ecosystem model, and applied using outputs from global climate models to assess the fate of the associated carbon fluxes in the Arctic Ocean during the next decades under different climate change scenarios. Additionally, a retrospective approach was followed to partly answer the Malina questions, based on the analysis of geochemical proxies in the past 1000-y sediments.