Sources of carbon dioxide to the Arctic and its impact on ocean acidification

The Arctic Ocean constitutes a large body of water that is still relatively poorly surveyed due to logistical difficulties, even though the importance of the Arctic Ocean for global climate is widely recognized. The cold waters of the high latitudes have high solubility of gases resulting in high concentrations of carbon dioxide (CO2) and thus a resulting relative low pH. The distribution of CO2, its variability in time and space, as well as its sources, are not known in detail. The Arctic Ocean has wide shelf areas where a number of processes impact the CO2 cycling. These comprise extensive biological activity, both high primary productivity and an active microbial loop within the surface sediment, uptake of CO2 from the atmosphere driven by the increased solubility caused by cooling of the waters flowing in from the Atlantic and Pacific Oceans, and input of total alkalinity and dissolved inorganic and organic carbon by the river runoff. On top of this we have increasing uptake of atmospheric CO2 resulting from the rising atmospheric concentration caused by burning of fossil fuel and deforestation, the so-called anthropogenic CO2. The waters on most shelves flow off into the deep Arctic basins where they penetrate different depth layers depending on their density. Some shelf water has very high density as a result of brine production during sea ice formation, and this water can at places penetrate to several km depths in the central basin while entraining surrounding waters. However, most waters penetrate the upper few hundred meters, i.e., the waters shallower than the Atlantic Layer. In this contribution we utilize data collected during IPY and earlier programs and assess the biochemical production and consumption of CO2 in the shelf seas as well as the air-sea interaction, compute how this transformation impact the acidity of the waters, and illustrate how this signal is exported into the deep central basin. We show that the cold waters of the Arctic shelves has very variable partial pressure of CO2 (pCO2) in the summer; generally lower than atmospheric levels (under-saturated) in the surface waters with high primary productivity, but with over-saturated pCO2 in some surface waters, especially close to the coast and in the river mouths, and with generally over-saturated bottom waters. The high pCO2 bottom waters get its signal from organic matter (both marine and terrestrial) that decays at the sediment surface and its signature reveal that it is brine enriched from sea ice production. The density of this brine enriched water makes it flow out into the deep basin at a depth range of 50 to 250 m. This water with its high pCO2 has a pH minimum that is enough to make it corrosive to most forms of calcium carbonate minerals. Utilizing CFC concentrations we conclude that this corrosive volume has grown since preindustrial times and will further grow to a level where it reaches the surface by the year 2050.