Accumulation of mineral ballast on organic aggregates

To address whether the incorporation of suspended minerals drives the sedimentation of particulate organic carbon in the ocean or vice versa, incubations of phytoplankton‐detritus aggregates were set up in rolling tanks containing seawater and suspended clay (illite) or calcium carbonate at concentrations ranging between 10 and 50,000 μg L−1. The suspended minerals were efficiently scavenged by and incorporated into the organic aggregates. The volume and porosity of aggregates decreased with increasing mineral concentrations, and at suspended mineral concentrations higher than 500 μg L−1 the initially medium‐sized aggregates were fragmented into thousands of tiny, dense aggregates. Because radius and density have opposite effects on aggregate sinking rates, the relationship between aggregate particulate organic carbon (POC) to mineral ratio and sinking rate is not a straightforward one that necessarily results in higher sinking rates with increased mineral content. In these experiments the saturating capacity of organic aggregates for mineral particles appeared to be 97 to 98 weight‐percent mineral (i.e., a POC to dry weight ratio of 0.02 to 0.03 μg C μg−1). This parallels the values of 0.05 μg C μg−1 observed for sinking particles in the deep sea and suggests that it is the flux of POC that determines the flux of minerals to the deep and not the other way around.

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