The canopies and roots of seagrass, mangrove, and saltmarsh protect a legacy of buried sedimentary organic carbon from resuspension and remineralisation. This legacy’s value, in terms of mitigating anthropogenic emissions of CO2, is based on total organic carbon (TOC) inventories to a depth likely to be disturbed. However, failure to subtract allochthonous recalcitrant carbon overvalues the storage service. Simply put, burial of oxidation-resistant organics formed outside of the ecosystem provides no additional protection from remineralisation. Here, we assess whether black carbon (BC), an allochthonous and recalcitrant form of organic carbon, is contributing to a significant overestimation of blue carbon stocks. To test this supposition, BC and TOC contents were measured in different types of seagrass and mangrove sediment cores across tropical and temperate regimes, with different histories of air pollution and fire together with a reanalysis of published data from a subtropical system. The results suggest current carbon stock estimates are positively biased, particularly for low-organic-content sandy seagrass environs, by 18 ± 3% (±95% confidence interval) and 43 ± 21% (±95% CI) for the temperate and tropical regions respectively. The higher BC fractions appear to originate from atmospheric deposition and substantially enrich the relatively low TOC fraction within these environs.

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (>100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

Determination of blue carbon sequestration in seagrass sediments over 15 climatic time scales relies on several assumptions, such as no loss of particulate organic 16 carbon (POC) after one or two years, tight coupling between POC loss and CO 2 emissions, 17 no dissolution of carbonates and removal of the stable black carbon (BC) contribution. We 18 tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to 19 capture centennial parameter decay dynamics from two sediment horizons robustly dated 20 as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described 21 for both horizons by near-identical reactivity continuum models. The models predicted 22Additional keywords: sediment geochemistry, diagenesis, carbonate, pyrogenic carbon, 31 methane, sediment isotope tomography 32 Introduction 33Seagrasses, along with mangroves, saltmarsh and seaweeds, are increasingly touted 34 as a significant global carbon sink (McLeod et al. 2011). For seagrass in particular, this 35 service is based on two separate concepts: sedimentary carbon stocks and rates of 36 sedimentary carbon sequestration. The stock or storage service concept, in the mitigation 37 of greenhouse gas emissions, is a scalar concept and conceived at the meadow scale. It has 38 traditionally been estimated by potential carbon loss to mineralisation should it be 39 disturbed over a climatic unit of time (Pendleton et al. 2012). The depth of such 40 disturbance, and the extent of its effect on the carbon stock, is dependent on the type of 41 disturbance (Siikamäki et al. 2013;Gallagher 2017) and independent of the time it took the 42 carbon to accumulate. The sediment found within seagrass beds contains a sizable organic 43 component consisting of a mix of seagrass litter, associated epiphyte and microalgal 44 detritus, and additional inputs from adjacent land activities and fluvial deposition as well 45 as saltmarsh and mangrove ecosystems (Kennedy et al. 2010). In contrast, the carbon 46 sequestration service is a vector concept. Rates of sequestration depend on the balance 47