Extensive mineralization of terrestrial organic carbon and acidification in a tropical shelf sea

Abstract:

The world’s rivers deliver ~0.5 Pg of terrestrial organic carbon (tOC) to the oceans each year, representing a significant component of the global carbon cycle. A critical but unresolved issue of this large land-to-ocean carbon flux concerns the processing and fate of the tOC in shelf seas, which determines the relevance of these fluxes for CO₂ emissions and their environmental impacts in marine ecosystems. We will present evidence of extensive shelf sea tOC mineralization obtained from a multi-year time-series program in the Sunda Shelf Sea in Southeast Asia. This region receives around 10% of the global flux of terrigenous dissolved organic carbon (tDOC) from the extensive tropical peatlands on Sumatra and Borneo. Using an isotope mass balance approach, we found that 60–70% of the tDOC is mineralized in the inner part of the shelf sea. The CO₂ produced from this tDOC mineralization causes acidification by ~0.09 pH units in the shelf sea, but is ultimately emitted to the atmosphere. Based on a simple exponential decay model and a range of likely water residence times on the shelf upstream and downstream of our site, we further estimate that likely 80–95% of the tDOC is mineralized within the Sunda Shelf Sea, such that only a minor fraction of tDOC is exported to the open Indian Ocean. This is consistent with our experimental data showing that at least 90% of peatland tDOC is photolabile. Because the tDOC export from Southeast Asian peatlands has been heavily perturbed by land-use change, our results point to a significant indirect anthropogenic CO₂ emission and contribution to shelf sea acidification.