Lateral Coastal Blue Carbon Export Fuels a Deep-Ocean Carbon Sink in an Oligotrophic Warm Pool

Abstract

Coastal blue carbon (CBC) ecosystems export organic matter offshore, but their contribution to deep-ocean carbon sequestration remains insufficiently quantified. We measured depth-resolved particulate organic carbon (POC) fluxes, delta 13C signatures, and environmental DNA (eDNA) from sediment traps deployed at 150, 500, and 1,000 m in the western North Pacific warm pool. POC fluxes at 1,000 m were two- to four-fold higher than predicted by classical attenuation models, indicating efficient deep-water transfer driven by both vertical export and lateral inputs. eDNA from sinking particles revealed macroalgae, mangrove, and seagrass taxa throughout the mesopelagic and bathypelagic layers, demonstrating long-distance transport of CBC material. delta 13C values showed predominantly marine-derived POC, but with measurable CBC contributions persisting at depth. These observations identify a previously underappreciated pathway linking coastal vegetation to deep-ocean carbon storage in oligotrophic gyre systems. They also highlight the need for ground-truth biological carbon pump (BCP) measurements, particularly for oceanic island countries with small land areas yet large exclusive economic zones (EEZs), where accurate carbon accounting depends on in situ validation of model-based estimates.