Abstract
The most common biomineral produced in the contemporary ocean is calcium carbonate, including the polymorph calcite produced by coccolithophores. The surface waters of the ocean are supersaturated with respect to calcium carbonate. As a result, particulate inorganic carbon (PIC), such as calcite coccoliths, is not expected thermodynamically to dissolve in waters above the lysocline (~4500–6000 m). However, observations indicate that up to 60–80% of calcium carbonate is lost in the upper 500–1000 m of the ocean. This is hypothesized to occur in microenvironments with reduced saturation states, such as zooplankton guts. Using a new application of the highly precise 14C microdiffusion technique, we show that following a period of starvation, up to 38% of ingested calcite dissolves in copepod guts. After continued feeding, our data show the gut becomes increasingly buffered, which limits further dissolution; this has been termed the Tums hypothesis (after the drugstore remedy for stomach acid). As less calcite dissolves in the gut and is instead egested in fecal pellets, the fecal pellet sinking rates double, with corresponding increases in pellet density. Our results empirically demonstrate that zooplankton guts can facilitate calcite dissolution above the chemical lysocline, and that carbon export through fecal pellet production is variable, based on the feeding history of the copepod.
Highlights
The conservative nature of calcium carbonate (CaCO3) in shallow ocean depths is a long-held standard of chemical oceanography
Previous experimental work estimated coccolith calcite dissolution within copepod guts at
Using a novel application of the 14C-microdiffusion technique[10,11], we found an initial 38% dissolution of the coccolith calcite from Pleurochrysis carterae eaten by Acartia tonsa copepods, represented by a significant decrease in the ratio of particulate inorganic carbon to particulate organic carbon (PIC/POC) of initial fecal pellets, relative
Summary
Using a novel application of the 14C-microdiffusion technique[10,11], we found an initial 38% dissolution of the coccolith calcite from Pleurochrysis carterae eaten by Acartia tonsa copepods, represented by a significant decrease in the ratio of particulate inorganic carbon to particulate organic carbon (PIC/POC) of initial fecal pellets, relative. Video-determined sinking rates of the three sets of fecal pellets produced and collected sequentially following an 80 min feeding period showed a doubling in the average sinking rate of fecal pellets collected 5 h after the start of the feeding period, relative to those collected 1.3 h after the start of the feeding period (Fig. 1f) This corresponds to the increase in PIC/POC of fecal pellets collected in a similar time frame from the 14C experiment (Fig. 1c). The density of each video-recorded sinking rate was calculated following a modified Stokes flow equation[14] (Fig. 1g) and each subsequent set of fecal pellets had significantly different mean densities, with densities increasing with time since the feeding period started, again corresponding the increased PIC/POC found in the 14C experiment. Pellet collection, and 14C analyses would further the empirical understanding of the dynamics governing rates of calcite dissolution in zooplankton guts
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