Alkalinity Production Coupled to Pyrite FormationRepresents an Unaccounted Blue Carbon Sink

Abstract

<p>Blue carbon ecosystems, including mangroves, saltmarshes, and seagrasses, mitigate climate<br>change by storing atmospheric carbon. Previous blue carbon research has focused on organic carbon<br>stocks. However, recent studies suggest that lateral inorganic carbon export might be equally important.<br>Lateral export is a long-term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfate<br>reduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in blue carbon<br>ecosystems, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks in<br>mangroves. We quantified pyrite stocks in mangroves, saltmarshes, and seagrasses along a latitudinal<br>gradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades<br>(Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidal<br>amplitude, sediment organic carbon, sediment accumulation rates, rainfall, latitude, temperature, and<br>iron availability. Pyrite stocks were three-times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes<br>(30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated to<br>TAlk export at sites where sulfate reduction was the dominant TAlk producing process. However, pyrite<br>generation could not explain all TAlk outwelling. We present the first global model estimating pyrite<br>stocks in mangroves, giving a first-order estimate of 197 Mg/ha (RMSE = 24 Mg/ha). In mangroves,<br>estimated global TAlk production coupled to pyrite formation (∼3 mol/m<sup>2</sup>/y) is equal to ∼24% of their<br>global carbon burial rate, highlighting the importance of including TAlk export in future blue carbon<br>budgets.</p>