An Estimate of the Global Marine Carbon Flux of Cold-Water Corals

Abstract. Marine calcification constitutes an important component of the global carbon cycle by sequestering carbon. During skeletal growth corals accumulate carbon, with numerous studies have highlighted their potential contribution to long-term carbon storage. This study compiles data from 32 published studies attempting to provide first estimates of the annual global carbon accumulation through cold-water corals. Carbon accumulation rates from individual reef systems were averaged and calculated up to global scale. As the worldwide coral covered area remains incompletely constrained, two different approaches were used, based on (i) coral mound density and (ii) regional coral area percentages, to asses the global suitable area. In addition, the proportion of living coral cover and the percentage of exposed mounds were taken into account. The global carbon accumulation was estimated using Monte-Carlo uncertainty propagation. The resulting simulations yielded probability distributions for the global carbon accumulation, from which the mean, median and 5th and 95th percentiles were derived. The mound-density-based approach resulted in medians, 5th and 95th percentiles of 0.003 (8.7x10^-5–0.079) Gt C yr^-1, whereas the area-based approach yielded 0.0022 (2.0x10^-5–0.086) Gt C yr^-1. The calculated means are approximately one magnitude higher than the corresponding medians, reflecting the asymmetrical and strongly right-skewed distribution. Consequently, most Monte Carlo realizations produced relatively low carbon accumulations, whereas only a small number of parameter combinations resulted in substantially larger values. The uncertainty spans approximately four orders of magnitude, emphasizing strongly the need for additional observational data to better constrain the parameters. The estimated global carbon accumulation of cold-water corals is approximately one magnitude lower than published estimates for tropical corals and two orders of magnitudes lower than the pelagic carbon burial. Compared with other carbon reservoirs cold-water corals therefore likely play a subordinate role in the global carbon cycle, but may locally be of importance. The estimated accumulation corresponds to a burial of 0.01 % (mound-density-based) or 0.076 % (area-based) of the global annual oceanic net uptake of CO₂. As anthropogenic CO₂ emissions continue to rise, cold-water coral ecosystems are increasingly threatened by ocean acidification and warming. However, the implications of these environmental changes for the global carbon cycle are expected to be considerably greater for tropical coral reefs and pelagic carbonate-producing systems.