The seasonal cycle of δ³C_DIC in the North Atlantic subpolar gyre

Abstract. This study introduces for the first time the δ¹³C_DIC seasonality in the North Atlantic subpolar gyre (NASPG) using δ¹³C_DIC data obtained in 2005–2006 and 2010–2012 with dissolved inorganic carbon (DIC) and nutrient observations. On the seasonal scale, the NASPG is characterized by higher δ¹³C_DIC values during summer than during winter, with a seasonal amplitude between 0.70 ± 0.10‰ (August 2010–March 2011) and 0.77 ± 0.07‰ (2005–2006). This is mainly attributed to photosynthetic activity in summer and to a deep remineralization process during winter convection, sometimes influenced by ocean dynamics and carbonate pumps. There is also a strong and negative linear relationship between δ¹³C_DIC and DIC during all seasons. Winter data also showed a large decrease in δ¹³C_DIC associated with an increase in DIC between 2006 and 2011–2012, but the observed time rates (−0.04‰ yr⁻¹and +1.7 μmol kg⁻¹ yr⁻¹) are much larger than the expected anthropogenic signal.