The role of particle-triggered calcium carbonate precipitation in the coastal ocean: A significant factor to seawater carbonate chemistry?

The effects of heterogeneous interactions between natural particles and seawater, such as particle-triggered CaCO₃ precipitation, have been extensively studied in laboratory experiments. However, their in-situ effects on the carbon cycle have received surprisingly little attention. Recent studies in the Red Sea suggest these interactions can significantly affect the in-situ seawater carbon and alkalinity budgets. To study these effects in-situ in a broader scale in the coastal ocean, the particle-laden plumes of the Brazos and Mississippi Rivers and their adjacent waters in the northern Gulf of Mexico were sampled in fall of 2017. In both plume areas, dissolved inorganic carbon (DIC) and total alkalinity (TA) were significantly lower than the values expected from conservative mixing and biological effects. The suspended solids concentration in the Brazos plume was exceptionally high due to the floods caused by Hurricane Harvey. Consequently, its DIC and TA deficit after accouting for mixing and biology was approximatly 10 times of that observed in the Mississippi plume, which was unaffected by the hurricane. Particle-triggered chemical precipitation of CaCO₃ most likely ocurred rapidly in both plumes, resulting in removal of DIC and TA from seawater. Interestingly, other interactions between particles and seawater may also occur along with CaCO₃ precipitation, causing a deviation from the TA-to-DIC change ratio of 2, as expected from CaCO₃ precipitation alone. This conclusion have been corroborated by laboratory experiments, which also reveal that cation-exchange and alteration of clay minerals may explain the deviation from the expected TA-to-DIC ratio in CaCO₃ precipitation. Particle-triggered CaCO₃ precipitation and other particle-seawater interactions can thus significantly modify seawater carbonate chemistry comparable to other physical and biogeochemical processes. This may represent an important, but previously overlooked pathway in the marine carbon cycle.