Carbon cycle and ocean acidification in the northern North Atlantic

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Ingunn Skjelvan

Uni Research and University of Bergen

Station M is one of the FixO3 observatories providing data related to the marine carbon cycle and ocean acidification. The site is located in the northern North Atlantic at 660N 20E, approximately 200 nautical miles from the Norwegian coast, and its history dates back to the late 1940’s when there was a need for improved weather forecasts due to the increased air traffic between Europe and North America. More than a dozen weather ships were established in the north Atlantic and one of these was situated at Station M, from where oceanographic and meteorological data was monitored since 1948. Hydrographic data from the deep water at Station M is among the longest time series in the world, and based on these data it was discovered that in the mid 80ties the Norwegian Sea deep water started warming from a relative stable low temperature.

In 2011 autonomous instrumentation replaced the weather ship at Station M and since then partial pressure of carbon dioxide (pCO2) and pH have been measured continuously in the surface water as well as in the mixed layer. A variety of data, such as air temperature, dissolved oxygen, and CO2 in sea surface and atmosphere, is publicly available in real time at http://talos.nodc. no:8080/stasjonm/index.html?lang=en. This data is used to study ocean acidification and its temporal variability. The term ocean acidification describes the ongoing decrease in pH of seawater and depletion of carbonate concentration of the Earth’s oceans, caused by uptake of CO2 from the atmosphere. The acidification affects the ecosystems in the oceans, but to what degree and in which form is not yet fully understood. Thus it is important to monitor trends both on annual and seasonal scale to keep track of the acidification process. Ocean acidification is detected by monitoring the inorganic carbon system in sea water. pCO2 and pH measurements from Station M are used to calculate the saturation state for carbonate in its two crystal forms calcite and aragonite (ΩCa and ΩAr).These two parameters indicate how “corrosive” the sea water is for carbonate, either in form of planktonic skeleton or solid carbonate deposits on the sea floor. Saturation state of carbonate, Ω, larger than 1, indicates that the water is over saturated with respect to carbonate, while Ω less than 1 indicates under saturation of carbonate. When Ω is well above 1 there is sufficient amount of carbonate for the organisms to form shells and skeletons, but when Ω is approaching 1 the environment is getting harsh and it is difficult for organisms to maintain sound and solid shells.

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Figure: Station M – temporal variability of aragonite saturation state (ΩAr), which changes with temperature and biological production.

The figure shows the temporal variability of aragonite saturation state (ΩAr), which changes with temperature and biological production. In summer, the ΩAr values are at a maximum (between 2.85 and 3.05), when surface water is at the warmest and primary production is reasonable large. This means that the abundancy of aragonite at the site is large during summer season and organisms build shells and skeletons as usual. In fall the temperature decreases, the mixed layer deepens, and ΩAr decreases towards winter concentrations. During wintertime, when the surface temperature is at a minimum, the ΩAr in surface water is also at its minimum of approximately 1.8, meaning that there is still sufficient amount of aragonite for organisms dependent on this substitute to survive, but the amount has decreased a lot since summertime. The current time series is too short to determine interannual trends; however, other studies in the northern North Atlantic have shown that the surface value of ΩAr is decreasing over years. Therefore it is of vital importance to continue the observations of the carbon cycle to determine the changes in the ocean acidification in the future. From other studies it is known that the transition zone between over saturated and under saturated water with respect to aragonite is found at depths between 1800 and 2000 m in the eastern part of northern North Atlantic. It is expected that this zone will shoal in the future, which will have implications for the carbonate dependent organisms living in that area.