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

Aleck Zhaohui Wang1, Eyal Wurgaft1, Mallory Cecile Ringham1, Shuzhen Song2, Timothy Dellapenna3, James H Churchill4, Tanya Rivlin5 and Boaz Lazar6, (1)Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (2)East China Normal University, State Key Laboratory of Estuarine and Coastal Research, Shanghai, China, (3)Texas A&M University at Galveston, Galveston, TX, United States, (4)Woods Hole Oceanographic Institution, Physical Oceanography, Woods Hole, MA, United States, (5)The Interuniversity Institute for Marine Sciences in Eilat, Eilat, Israel, (6)Hebrew University of Jerusalem, Earth Sciences Institute, Jerusalem, Israel
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.