Blue carbon refers to the carbon sequestered and stored in salt marsh, mangrove, and seagrass beds (Nellemann, Corcoran, & M., 2009). There has been a growing interest in measuring, mapping, and valuing blue carbon stocks to promote their importance in climate change mitigation (Macreadie et al. 2019). However, the use of total organic carbon (TOC) for sequestration and stock assessments has recently been in question. Traditional assessments have largely failed to remove allochthonous organic recalcitrants from the equation (Gallagher 2014; Gallagher 2015; Gallagher 2017; Chew and Gallagher 2018; Chuan et al. 2020; Gallagher et al. 2020 ). These stable organic forms are not produced by the blue carbon ecosystem; deposition within the ecosystems’ sediments does not afford additional protection from remineralization. Consequently, their presence is not a measurable storage or sequestration service in the mitigation of greenhouse gas emissions. While the argument is unequivocal and recognised by the IPCC as an important blue carbon constraint (Bindoff et al. 2019), data are scarce on their contributions to blue carbon ecosystems.
Arguably, black carbon (BC) is the most ubiquitous of these recalcitrant organic forms. Black carbon also referred to as pyrogenic carbon, comes from the burning of biomass and fossil fuels (Gustafsson et al. 2009). Their recalcitrance to microbial attack comes from the charing effect, the extent of which results in greater degrees of recalcitrance (Binh Thanh et al. 2010). As previously implied, as a constraint on blue carbon mitigation BC production cannot be caused or initiated within the ecosystem. This is certainly the case for seagrasses. For mangroves, historical satellite records of combustion sites indicated that this was unlikely, at least outside deliberate attempts during periods of drought to ignite a build-up of dumped trash adjacent to or inside the mangrove forest (Chew and Gallagher 2018). For saltmarsh, fire can be more common and deliberate but largely restricted to the tall canopies of relatively dry reed and grass-dominated ecosystems (Nyman and Chabreck 1995). As result, BC delivery to these blue carbon ecosystems can come from soil from a history of catchment fires and pollution, or more immediately and directly from atmospheric deposition (Chew and Gallagher 2018).
In addition to BC, attention has been given to the role of the particulate inorganic carbon (PIC) fractions. These calcareous products can be either produced by the ecosystems’ biota or geogenic carbonates, namely coral sands or dolomitic soils that makeup part of the sedimentary matrix. The biotic carbonates originate from the ecosystem’s plant epibionts, benthic macrofauna (e.g. crabs, snails, and bivalves), and more recently found in seagrass leaves of Thalassia spp. (Enriquez and Schubert 2014). There is also evidence for sedimentary carbonate formation under anoxia (Chuan et al. 2020), however, the extent of this process is currently not known. Importantly, PIC has been regarded as the remnants of not a carbon sink but a carbon source process. During the production of PIC, CO2 is formed and expelled into the atmosphere after a chemical redistribution with other dissolved inorganic forms of carbonate and bicarbonate. The extent of which is determined by water body salinity and the atmospheric CO2 (Ware et al. 1992; Saderne et al. 2018).
For recalcitrant organic forms such as BC, the coverage in the literature is still embryonic. However, PIC measurements are gaining traction where distinctions can be made between ecosystems that largely support biogenic PIC (Saderne et al. 2019). Yet, compilations of both BC and PIC remain rare (Gallagher et al. 2020). To increase data coverage, and within the limits of resources, measurements of BC and PIC were made across targeted blue carbon ecosystems of Malaysia and Australia to investigate the sum of black and calcareous blue carbon bias. The wetlands were chosen to expect a large bias in traditional stock estimates from the cumulated fractions of BC and PIC to the TOC stocks. To increase generality, the examples were separated by very different geographic and climatic regions, as well as categories of blue carbon ecosystems where the BC content has not been specifically addressed. That is, examples from the southern limit of Australian rural temperate salt marshes (Tasmania), and a tropical urban seagrass meadow within Malaysia. Both regions are in BC hot spots and underrepresented in our understanding of BC and PIC. Furthermore, we discuss our findings to identify ways in which our study can be extended to motivate further investigations of blue carbon bias as a stock mitigation service by accounting for the sum of both BC, other autochthonous recalcitrants, and PIC contributions.