On calcium-to-alkalinity anomalies in the North Pacific, Red Sea, Indian Ocean and Southern Ocean

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Steiner, Zvi, Sarkar, Amit, Liu, Xuewu, Berelson, William M., Adkins, Jess F., Achterberg, Eric P., Sabu, P., Prakash, Satya, Vinaychandran, P. N., Byrne, Robert H., Turchyn, Alexandra V.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2021
• Subjects: Calcium; Alkalinity; Ocean acidification; CaCO3; Alkalinity anomaly; Carbonate cycle; Southern Ocean
• Online Access: https://doi.org/10.1016/j.gca.2021.03.027

An important factor for predicting the effect of increased CO₂ on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO₃). The production and dissolution of CaCO₃ in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (A_T), yet past studies suggest that there could be large discrepancies between calcium and A_T-based balances of the CaCO₃ cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and A_T. We find that there is an excess calcium over A_T anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-A_T anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and A_T data agree well and can be used to reconstruct the marine CaCO₃ cycle. © 2021 Elsevier Ltd. Received 29 May 2020, Revised 25 March 2021, Accepted 26 March 2021, Available online 5 April 2021. This study was funded by a Blavatnik postdoctoral fellowship to ZS, an Isaac Newton Trust grant to AVT and ZS, and ERC StG 307582 (CARBONSINK) to AVT. The cruise on board the ORV Sagar Nidhi was funded by the Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Sciences, India, as the first cruise of the second International Indian Ocean Expedition (IIOE-2). The cruise on board the RV Kilo Moana was funded by NSF Ocean Acidification grant number OCE1220600. We would like to thank the UK Natural Environment Research Council (NERC), the UK Department of Environment, Food and Rural Affairs (DEFRA), and the UK Department of Energy and Climate Change (DECC) for funding the research cruise ...