Seasonal Marine Carbon System Processes in an Arctic Coastal Landfast Sea Ice Environment Using an Innovative Underwater Sensor Platform

The marine carbonate system is a critical component of global biogeochemical cycles. It determines a given marine region’s status as a source or sink for atmospheric CO₂, and vulnerability to long term changes (i.e. ocean acidification) that can affect key ecosystem functions. Carbonate system processes are highly variable through space and time, which makes it difficult to fully characterize a region without either intensive sampling, or long-term deployment of high-precision instruments. Both of these are difficult in the Arctic, where challenging logistics limit sampling opportunities, and instruments must endure extreme conditions. In this work, we present a high-resolution marine carbon system dataset covering a full Arctic cycle of sea ice growth and melt. We deployed pH and pCO₂ sensors along with a suite of other physical and chemical sensors in a coastal landfast sea ice environment from September 2015 – June 2018. In situ samples were collected to calibrate sensors, and assess the spatial representativeness of the coastal site. Using a diagnostic box model approach, seasonal influencing processes on the marine carbon system at the platform were quantitatively determined. Air-sea gas exchange and biologic respiration/remineralization were dominant in the fall, whereas following sea ice freeze up brine rejection drove pCO₂ to seasonal supersaturation and aragonite to undersaturation. Shortly after polar sunrise in the late winter, the ecosystem at the platform became net autotrophic at very low light levels, driving pCO₂ to undersaturation. As sea ice melted, an under-ice phytoplankton bloom drew down a significant amount of carbon before the open water season, returning the aragonite saturation state to supersaturation and priming the system to uptake CO₂ from the atmosphere during the open water season. These observations show a dynamic system, where biological processes occur at times and rates previously unknown to the literature. These processes will need to be included in future biogeochemical modeling efforts, if we are to properly resolve the current, and future, role of Arctic seas in global biogeochemical cycles.