Drivers of Observed Excess Alkalinity in the Upper Water Column

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.