Drivers of Observed Excess Alkalinity in the Upper Water Column

Aimee Coggins1, Paul Richard Halloran2, Andrew J. Watson1, Samar Khatiwala3 and Michael Salter4, (1)University of Exeter, Exeter, United Kingdom, (2)Met Office Hadley Centre, Exeter, United Kingdom, (3)University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, (4)University of Exeter, United Kingdom
The global ocean is an important sink of atmospheric CO2 having taken up ~40% of anthropogenic emissions. Alkalinity is a key component of the oceanic carbonate system and controls the capacity of seawater to hold additional CO2. Understanding what drives oceanic alkalinity distributions is essential for improving the scientific communities’ ability to project future ocean CO2 uptake. Although the carbon system is well understood, observations of ‘higher-than-expected’ alkalinity in the upper 1000m have been the cause of significant debate in the last few decades. A potential driver of the upper ocean ‘excess’ alkalinity is the dissolution of calcium carbonate (CaCO3). However, thermodynamically dissolution of the CaCO3 polymorphs conventionally thought to be the most common (calcite and aragonite) occurs at much deeper depths. Therefore, shallow dissolution requires another explanation, such as a source of less stable CaCO3. Marine bony fish continuously produce intestinal CaCO3 pellets as a by-product of their normal physiology, estimated to account for up to 15% of global CaCO3 production. Primarily formed of magnesium-rich polymorphs, this fish CaCO­3 is predicted to be relatively soluble and could explain at least a part of the shallow ‘excess’ alkalinity. We attempt to explain the differences between expected, observed and modelled alkalinity through a series of simplified model experiments which aim to evaluate the contribution of high-magnesium calcites, alongside other drivers such as fresh water forcing, on ocean alkalinity distributions.