Propagation of vent CO2 in a subtropical shallow-water ecosystem assessed by stable carbon isotopes

Carbon cycle of the ocean plays an important role in the global change associated with the emission of CO₂. Anthropogenic CO₂ dissolves in seawater, changes carbon chemistry of the ocean, and affects marine life in different and complicated ways. In this study, we investigated stable carbon isotope systematics of a shallow-water hydrothermal field near the Kueishantao Islet off NE Taiwan, which has vent gas composition dominated by CO₂ and world record breaking low pH hydrothermal fluids. By studying this natural laboratory of ocean acidification, we aim at clarifying to which extent the high dosage of CO₂ propagates in the subtropical shallow-water ecosystem, and how it affects the carbon cycle. Samples of seawater and suspended particles were collected from stations of two nearshore-offshore transects, one with hydrothermal vents at the nearshore end (Transect M, 1230 m long) and the other serving as the baseline (Transect B, 1560 m long). Surface seawater of Transect M showed increasing pH in the offshore direction, from 5.8 at the vent mouths to 7.6 at the most distant station. In contrast, pH of surface water decreased seaward from 8.0 to 7.8 in Transect B. The δ¹³C values of the vent CO₂ averaged −6.4‰, consistent with the range attributed to mantle CO₂. Seawater DIC δ¹³C values of Transect M were ¹³C-depleted (as negative as −2.5‰) at the vent mouths, and became increasingly ¹³C-enriched till 0.7‰ at the most distant station. This pattern is in clear contrast to that of Transect B, the DIC δ¹³C values of which decreased from 0.7 to 0.6 ‰ in the offshore direction. We concluded that the vent CO₂ has propagated in the surface ocean at least >700 m away from the hydrothermal field. Our next step is to explore how the vent CO₂ affects the stable carbon isotopes of particulate organic matter, and to assess the effect of vent CO₂ using quantitative approaches.