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This work describes a new, generally applicable method to comprehensively characterize the inorganic carbon system of aqueous solutions. The method requires only simple spectrophotometric measurements and is appropriate for not only open‐ocean seawater (where convenient assumptions and approximations may be made) but also the more challenging case of freshwaters. The overall approach is to (1) measure pH in the field at the time of sample collection and (2) measure sample pH, carbonate alkalinity (A_C), and total alkalinity (A_T) later in the laboratory. All required equipment is inexpensive and portable. The paired laboratory measurements of pH andA_Ccan be used to obtain the concentration of total inorganic carbon (C_T). ThisC_Tcan in turn be paired with the field pH measurements to comprehensively characterize carbon‐system parameters in the sampled water body at in situ conditions. To our knowledge, this method is the first to spectrophotometrically measureA_Cand thus the first to completely characterizeC_Tand the carbon system of freshwaters using spectrophotometric measurements only. The concurrent measurements ofA_CandA_Tcan also be used to partition alkalinity into its carbonate and noncarbonate components. This work additionally describes how to quantitatively correct for artifacts that may arise (especially in freshwater samples) from using HgCl₂to halt respiration in sample bottles. The use of these methods is illustrated using samples collected from the Snake River (Idaho, USA) before and during the 2020 spring flow.

We have designed, built, tested, and deployed a novel device to extract porewater from deep‐sea sediments in situ, constructed to work with a standard multicorer. Despite the importance of porewater measurements for numerous applications, many sampling artifacts can bias data and interpretation during traditional porewater processing from shipboard‐processed cores. A well‐documented artifact occurs in deep‐sea porewater when carbonate precipitates during core recovery as a function of temperature and pressure changes, while porewater is in contact with sediment grains before filtration, thereby lowering porewater alkalinity and dissolved inorganic carbon (DIC). Here, we present a novel device built to obviate these sampling artifacts by filtering porewater in situ on the seafloor, with a focus near the sediment–water interface on cm‐scale resolution, to obtain accurate porewater profiles. We document 1–10% alkalinity loss in shipboard‐processed sediment cores compared to porewater filtered in situ, at depths of 1600–3200 m. We also show that alkalinity loss is a function of both weight % sedimentary CaCO₃and water column depth. The average ratio of alkalinity loss to DIC loss in shipboard‐processed sediment cores relative to in situ porewater is 2.2, consistent with the signal expected from carbonate precipitation. In addition to collecting porewater for defining natural profiles, we also conducted the first in situ dissolution experiments within the sediment column using isotopically labeled calcite. We present evidence of successful deployments of this device on and adjacent to the Cocos Ridge in the Eastern Equatorial Pacific across a range of depths and calcite saturation states.