Natural analogue of the rise and dissolution of liquid CO2 in the ocean

The behavior of natural carbon dioxide (CO2) droplets (8–10 mm in diameter) were observed in a seafloor hydrothermal system at the Okinawa Trough. The natural CO2 droplet contain 95–98% of CO2, 2–3% of H2S, and other gas species. The ascending CO2 droplets were tracked by a remotely operated vehicle (ROV), and depth, temperature, salinity, pH and partial pressure of CO2 (pCO2) in seawater near the CO2 droplets were measured during droplet ascent by a conductivity-temperature-depth sensor (CTD) and in situ pH/pCO2 sensor. The visual images of the rising CO2 droplets were recorded with a high definition television camera on the ROV. A mapping survey (400 m × 400 m; 4 horizontal layers) revealed a dominant distribution of low pH area over the natural CO2 venting site. The size and rise rate of CO2 droplets decreased during their ascent in the water column from depths of 1424 to 679 m (a tracking interval of 745 m). The CO2 droplets dissolved gradually to become small flakes of CO2 hydrate while rising, and these ascending flakes were found to disappear at 679 m depth. Although a pH as low as 5 was detected just above the liquid CO2 venting site on the seafloor, no detectable pH depression in the water column ambient to the rising CO2 droplets was observed. The results of the pH mapping survey showed only localized pH depression over the CO2 venting site.