A method-to-method comparison for direct determination of particulate inorganic carbon fluxes in sediment trap samples

Quantifying gravitationally sinking particulate matter is essential to expanding understanding of flux patterns and pathways of carbon sequestration in the ocean’s biological carbon pump. Sediment traps collected samples of sinking particles in the upper mesopelagic zone of the Porcupine Abyssal Plain in the Atlantic Ocean. Samples were collected on quartz microfiber filters and homologous splits were previously analyzed for particulate inorganic carbon (PIC) after acidification and coulometric detection of carbon. Acidification and subsequent measurement of calcium has been previously used as an alternative method for measuring PIC in biological samples collected at shallow depths, but this method is rarely used for particulate samples collected by sediment traps in the deeper ocean. Parallel filter splits were washed with borate buffer to remove adsorbed sea salts, acidified, and analyzed for calcium and sodium using a Flame Atomic Absorption Spectrometer (AAS). After correcting for residual sea salts, calcium was used to compute PIC and replicate filter splits were averaged to account for split-to-split variation. Split averages were used to compare across the two analytical methods as well as different sediment trap types and deployments. Regression analysis of PIC data from coulometric methods versus AAS methods using calcium yielded a slope of 0.9992 and R² of 0.9046. These results demonstrate that Flame-AAS analysis of calcium offers an inexpensive and straightforward orthogonal method for the measurement of PIC in sinking particulate samples.