Atlantic Meridional Transect of polyisotopic carbon dioxide: Challenges of ship-based laser spectroscopy and implications for atmosphere-biosphere exchange

During the AMT31 research cruise (Southampton–Montevideo, December 2024), we measured CO₂ polyisotopologues using a tuneable infrared laser direct absorption spectrometer (Aerodyne TILDAS-FD-L2). Dried marine air from an inlet at the bow of the ship was alternated with a working reference every 2 min to correct for instrument drift.

Compared with land-based measurements, ship motion (roll, pitch, heave) was found to deteriorate isotope ratio precision by a factor of 3 to 10 (depending on the sea state). However, after averaging over hourly intervals, precisions better than 0.05 µmol mol^–1 for y(CO₂) and better than 0.03 ‰ for δ(¹³C), δ(¹⁸O) and δ(¹⁷O) were achieved. For the ¹⁷O isotope excess, Δ(¹⁷O), hourly precision was often better than 10 ppm (0.01 ‰), but unfortunately, target tank results showed unexplained day-to-day variability of the order of ±35 ppm.

Preliminary corrections for this day-to-day variability indicate that southern hemisphere δ(¹⁸O) is 1.2–1.8 ‰ higher and Δ(¹⁷O) is about 60 ppm higher than northern hemisphere marine background air. This interhemispheric Δ(¹⁷O) gradient is twice as high as predicted by atmosphere-biosphere exchange models (Koren et al., 2019) and could indicate a stronger than expected influence of the ¹⁷O-enriched stratospheric return flux in austral spring or a stronger biospheric exchange signal in boreal autumn.