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.
Bibliographical noteFunding Information:
We thank Murielle Dray, Tanya Rivlin and Asaph Rivlin of the Interuniversity Institute of Marine Sciences (IUI), Eilat, Israel, for their invaluable technical assistance in the field, instrumental and analytical aspects of this study. We thank the IUI director, Amatzia Genin and Yoni Shaked from the National Monitoring Program of the Gulf of Eilat (Aqaba) for providing us free access to the monitoring cruises. Sefi Baruch, Timor Katz and Moti Ohavia of the R/V Sam Rothberg helped in water sampling and technical operations. We thank Eugeni Barkan for his valuable help and continuous support. We also thank Adam Levi, Israela Musan, Andrea Istrate, and Evyatar Cohen for their technical help throughout the research. Jacob Silverman from the Israel Oceanographic and Limnological Research kindly read the manuscript and provided many helpful remarks. We thank Yeala Shaked for kindly sharing the processed dust data. Special thanks to Jonathan Erez for his input in discussions on the subject. We thank Anton Eisenhauer from the Marine Geosystems unit (MG) at GEOMAR, Kiel for hosting and helping BL during the coral study, Jan. Fietzke from MG and Magmatic and Hydrothermal Systems group unit (MHS) for providing access and helping in the thin sections lab and microprobe analyses, and Mario Thöner from MHS for conducting the microprobe analyses of the coral. The manuscript benefited substantially from the thorough review and comments provided by Dr. Alfonso Mucci and two anonymous reviewers. The research was funded by The Harry and Sylvia Hoffman Leadership and Responsibility Fellowship from the HUJ, The Levi Eshkol Fellowship from the Ministry of Science and Technology and The University of Calgary CCS Initiative.
© 2016 Elsevier B.V.
- Carbon cycle
- Gulf of Aqaba