Accounting for black carbon lowers estimates of blue carbon storage services

Abstract

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.

Blue carbon sequestration dynamics within tropical seagrass sediments: Long-term incubations for changes over climatic scales

“…Clearly, the 382 remaining half of the organic carbon, a seemingly recalcitrant fraction, is more than can be 383 accounted for by the <10% contribution of the BC alone. It is also unlikely in this case that 384 any presence of phytolith occluded carbon was responsible given that the BC methodology 385 may have inadvertently included this form (Chew and Gallagher 2018). What remains is 386 up to speculation; it may consist of bacterial necromass (Burdige 2007 times of over 1000 years (Gewert et al 2015), their amounts as carbon within soils and sediments remain largely unknown.…”

“…Malaysia) and within the penumbra of the near-annual southwest Borneo and Sumatra 110 haze events. These events ostensibly deposit BC into the estuary from peat fires on the 111 southern part of the island as well as from slash-and-burn land-clearing activities (Gaveau 112 et al 2014;Chew and Gallagher 2018). The two bays are both turbid and shallow (1-3 m) 113 and surrounded by mangrove forests with exposed intertidal mud banks.…”

“…Pyrogenic carbon is produced by incomplete combustion of 62 biomass and fossil fuels. It is considered sufficiently stable to be outside the climatic carbon loop (Liang et al 2008; Wang et al 2016), and thus its storage and sequestration 64 within seagrass ecosystem sediments cannot be counted as a greenhouse gas mitigation 65 service (Chew and Gallagher 2018). Mass accumulation rates, the product of 66 sedimentation rates and particulate organic carbon (POC) concentrations, have no assumed 67 significant losses after one to two years within their surface sediments (Cebrian 1999).…”

“…The remaining sediment plug 183 was then dried at 105 °C and the amount of water and sediment was noted to calculate the 184 amount remaining in the mason jars for CO 2 accumulation as dry weight of sediment after 185 correcting for salinity(Lavelle et al 1985). Particulate organic matter (POM), PIC and 186 black organic matter (BOM) from the dried sediment slug was measured gravimetrically 187 by loss on ignition (LOI 0.45g ) in a laboratory furnace (Carbolite CWF 1.8 L;Heiri et al 188 2001;Chew and Gallagher 2018). Additional inter-batch corrections resulting from 189 possible furnace aging and procedural handling differences were performed using in-house 190 local sediment standards taken from the middle of the cores (n = 5) and randomly placed 191 within the furnace.…”

Valuing Carbon Stocks across a Tropical Lagoon after Accounting for Black and Inorganic Carbon: Bulk Density Proxies for monitoring

“…While measurements of carbon within biomass and sediments can be relatively straightforward, questions have been raised on how stock preservation is assessed as a mitigating service in further release of the dominant greenhouse CO 2 from the marine environment. That is, questions of the extent of loss after disturbance, how much is remineralised over climatic scales, whether to include the presence sedimentary allochthonous organic recalcitrants and particulate inorganic carbon (PIC) as an arbiter of carbon stock mitigation services (Chew and Gallagher 2018; Gallagher 2017). For woody biomass, in particular, how much is released as CO 2 could vary with its use, should it be disturbed or harvested (Eong, 1993).…”