Autonomous 13C measurements in the North Atlantic: A novel approach for identifying patterns and driving factors of the upper ocean carbon cycle

The North Atlantic Ocean plays a major role in climate change not least due to its importance in CO2 uptake and thus natural carbon sequestration. The CO2 concentration in its surface waters, which determines the ocean's CO2 sink/source function, varies on seasonal and interannual timescales and is mainly driven by air-sea gas exchange and biological production/respiration. However, the quantification of these processes is still afflicted with a high degree of uncertainty. In this thesis, a cavity ringdown spectrometer (G2131-i, Picarro, USA), which is able to measure the CO2 mole fraction and it's stable carbon isotope composition, was installed on a VOS line that regularly sails across the subpolar North Atlantic between North America and Europe. From summer 2012 to the end of 2014, two and a half years of d13C(CO2) underway data was obtained along with continuous measurements of temperature, salinity and fCO2. Combined with a discrete sampling program (consisting of DIC, TA, nutrients, Chl a, POM, DOC, d13C(POC) and d15N(PON) samples), the dynamics of the upper North Atlantic Ocean were studied. This analysis comprises interannual variations of fCO2 and d13C(CO2), relative changes of nutrient concentration in comparison with C:N ratios of suspended particle matter, biologically and mixing driven variability in DIC and d13C(DIC) and the fractionation between dissolved CO2 and particulate matter. Based on the variations in DIC, fCO2, DIC, fCO2, d13C(DIC), nitrate, phosphate and silicate, the respective change rates and overall inventory changes due to air-sea gas exchange, net community production and convective mixing were calculated utilizing a box model