A marine sediment concept defining how particles accumulate, are reworked, and change after burial on the seafloor. It governs depositional processes, physical properties, and post-depositional reactions measured through cores and laboratory analyses. It does not uniquely identify sources or processes without supporting stratigraphic, mineralogical, and geochemical evidence. It materially affects habitat, slope stability, and climate records by controlling the properties and composition of seabed deposits. The concept is generally stable, though analytical resolution and property measurement methods improve over time.

Principle

Alkalinity is a charge‑balance measure linked to the carbonate system: remineralization of organic carbon and anaerobic processes produce bicarbonate raising alkalinity, while precipitation of authigenic carbonate or uptake of carbonate ions reduces alkalinity; salinity and cation release alter carbonate equilibria and therefore alkalinity.

Demonstration

At the sulfate‑methane transition, intense anaerobic oxidation of methane and sulfate reduction can elevate porewater alkalinity, often leading to authigenic carbonate precipitation and visible carbonate nodules in sediments.

Misapplication

Equating alkalinity directly with pH or using alkalinity alone to infer carbonate saturation without simultaneous measures of DIC, Ca2+, Mg2+ and pCO2 can lead to incorrect interpretations.

Consequence

Pore water alkalinity profiles reveal zones of active diagenetic reactions, control carbonate mineral precipitation/dissolution, and are essential for quantifying carbon burial and the response of sediment chemistry to changing organic input or redox conditions.

Reversal

The opposite view would treat alkalinity merely as a bulk tracer unrelated to specific diagenetic reactions; that ignores its mechanistic link to carbonate chemistry and biogeochemical pathways.

Boundary

Applies specifically to alkalinity measured in sediment porewaters, reported in equivalents per volume (e.g., mmol HCO3− eq L−1); does not substitute for measurements of total alkalinity in the overlying water column nor replace detailed carbonate system speciation without complementary data.

Semantic Tension

Tension exists between alkalinity as an analytical titration value (charge balance) and as an ecological/diagenetic indicator of specific processes—both are true but require different interpretative caution.

Synthesis

Pore water alkalinity integrates the net effects of organic matter remineralization and mineral reactions on the carbonate system: it is a practical diagnostic of diagenetic activity, carbonate saturation state, and pathways of inorganic carbon transformation within sediments.