The imprint of anthropogenic CO2 emissions on Atlantic bluefin tuna otoliths

• We analyzed Atlantic bluefin tuna otolith δ13C and δ18O composition.
• We examined temporal variation on δ13C and δ18O from 1989 to 2010.
• Otolith δ18O remained relatively across the time interval investigated.
• A depletion in δ13C of 0.05‰ yr− 1 was detected in the Mediterranean Sea.
• δ13C depletion was mainly attributed to the uptake of anthropogenically derived CO2.

Otoliths of Atlantic bluefin tuna (Thunnus thynnus) collected from the Mediterranean Sea and North Atlantic Ocean were analyzed to evaluate changes in the seawater isotopic composition over time. We report an annual otolith δ13C record that documents the magnitude of the δ13C depletion in the Mediterranean Sea between 1989 and 2010. Atlantic bluefin tuna in our sample (n = 632) ranged from 1 to 22 years, and otolith material corresponding to the first year of life (back-calculated birth year) was used to reconstruct seawater isotopic composition. Otolith δ18O remained relatively stable between 1989 and 2010, whereas a statistically significant decrease in δ13C was detected across the time interval investigated, with a rate of decline of 0.05‰ yr− 1 (− 0.94‰ depletion throughout the recorded period). The depletion in otolith δ13C over time was associated with the oceanic uptake of anthropogenically derived CO2.