Sedimentary Environment Controls Carbon Sequestration Potential of Unvegetated Intertidal Estuarine Sediments

Abstract Unvegetated intertidal sediments are increasingly recognized as contributors to coastal carbon storage, yet their organic carbon burial potential remains poorly constrained. This study examines spatial and temporal patterns of carbon accumulation in unvegetated intertidal flats of Ōhiwa Harbor, New Zealand, using surface sediments and three radiocarbon‐dated cores spanning up to ∼7,700 yrs. Within the harbor, five distinct sedimentary facies were identified, each displaying unique sediment characteristics and patterns of organic carbon burial. Mud‐rich, low‐energy facies, including rippled and bioturbated muds, consistently showed higher organic carbon density and burial rates compared to sandy, more dynamic facies. Estimated carbon stocks in the upper meter of sediment range from 44 to 120 t C ha −1 , comparable to or exceeding those of many vegetated coastal habitats. Temporal changes in facies distribution driven by estuarine processes and variations in sediment supply led to significant long‐term fluctuations in organic carbon burial. These results demonstrate that organic carbon storage in unvegetated intertidal flats is highly heterogeneous and controlled by the persistence of fine‐grained depositional environments. A facies‐based framework offers a process‐driven approach to assessing and managing blue‐carbon potential in estuarine systems increasingly altered by climate and land‐use change.