Seasonal variability of the carbon cycle in subantarctic surface water in the South West Pacific

Abstract

Few Southern Hemisphere time series measurements of biogeochemical tracers are available, and this scarcity is a major impediment in understanding the biological and physical processes underlying the oceanic carbon and nutrient cycles in vast parts of the global oceans. We make use of bi‐monthly measurements of carbonate parameters from 1998 to 2010 in upper Subantarctic Surface Water east of New Zealand's South Island at 45.85°S 171.50°E to investigate seasonal cycles and trends in these species and processes controlling their variability. This time series reveals positive trends in salinity normalized dissolved inorganic carbon (sDIC) and the partial pressure of carbon dioxide (pCO2) that are smaller than would be expected from the anthropogenic increase in atmospheric pCO2 alone, possibly due to a decrease of the average temperature over the observational period. The seasonal cycle of pCO2 is dominated by that of DIC, but is substantially modified by the influence of the annual cycle of sea surface temperature. Investigations with a δ13COC‐constrained diagnostic box model suggest that net community production (NCP) is the dominant process controlling the observed seasonal variability in sDIC by removing 1.2 ± 0.7 mol C m−2 yr−1 from the mixed layer. This carbon drawdown, aided by an additional carbon removal due to horizontal transport, is balanced by vertical diffusion, entrainment, and air‐sea gas exchange of CO2. Oceanic pCO2 is below atmospheric pCO2 for nearly the entire year, leading to an annual mean surface ocean pCO2 undersaturation of about 12 µatm and an annual oceanic uptake of CO2 from the atmosphere of 0.9 ± 0.1 mol C m−2 yr−1.