Region-specific LOI-TOC conversion models for marine sediment blue carbon assessment in the Western Irish Sea.

Continental-shelf sediments (< 200 m water depth; ~ 9% of the global seafloor) store large amounts of organic carbon (OC) and accumulate OC annually, but management and accounting applications require regional evidence and transparent uncertainty. Loss-on-ignition (LOI) is a rapid, low-cost proxy for organic matter and OC, but LOI-to- total organic carbon (TOC) conversions vary with sediment type and analytical protocol. This study used multiagency Western Irish Sea data to test whether region- and sediment-specific LOI-to-TOC regressions improve OC estimation relative to a pooled conversion. Paired LOI and TOC measurements were strongly correlated (R2 = 0.948). Structural-break and analysis of covariance (ANCOVA) tests identified changes in the LOI-TOC relationship associated with mud content, iron (Fe) concentration and water depth. Splitting samples into high-mud and low-mud groups produced group-specific models (R2 = 0.97 and 0.72, respectively) and improved fit for high-mud sediments, while the low-mud model retained greater uncertainty. The results support the use of separate sediment-class-specific LOI-to-TOC calibration models for Western Irish Sea sediments and comparable settings, provided that paired LOI-TOC calibration, the range of sediments to which each model applies, and propagated uncertainty are reported. By integrating archived samples, routine geotechnical measurements, grain-size data, LOI results and targeted TOC analyses from multiple survey and research sources, the study also demonstrates how existing marine monitoring and seabed-mapping activities can be leveraged to strengthen national sediment carbon datasets.