Evidence for inorganic precipitation of CaCO₃ on suspended solids in the open water of the Red Sea

It is generally accepted that most of the oceanic CaCO₃ production is biogenic, whereas homogeneous, inorganic CaCO₃ nucleation and precipitation from seawater is inhibited by the presence of other seawater constituents, including dissolved organic carbon. Notwithstanding, heterogeneous CaCO₃ precipitation (HCP) from supersaturated seawater onto solid surfaces is well documented, evidence for HCP in the open-ocean settings has not been convincingly demonstrated. In this study, we provide evidence for inorganic CaCO₃ precipitation in the water column of the Red Sea and in the neighboring Gulf of Aqaba. The evidence includes a decrease in alkalinity and dissolved inorganic carbon (DIC) at a 1.7:1 ratio along the southward route of Red Sea deep-water, and an alkalinity deficiency in the Gulf of Aqaba deep-water. These observations are made after correcting alkalinity and DIC for changes in nutrient content and salinity and are therefore not the result of mixing, respiration or photosynthesis. We suggest that the interaction between seawater and suspended solids, providing precipitation nuclei, resulted in HCP and accounted for the above-mentioned observations. We base this suggestion on: 1. time-series measurements in the Gulf of Aqaba, showing abrupt alkalinity decrease following the entrainment of large amounts of solids; 2. incubation experiments confirming that suspension of Gulf of Aqaba sediments in seawater induces a decrease in alkalinity; 3. precipitation of inorganic aragonite needles within the pores of the skeleton of a local coral. Based on the data presented here, we postulate that HCP may occur in parts of the ocean that receive a substantial influx of solid particles, and areas subject to frequent dust storms. Hence, HCP may be an overlooked pathway in the oceanic CaCO₃ cycle.