Abstract
Abstract. Correlation between particulate organic carbon (POC) and calcium carbonate sinking through the deep ocean has led to the idea that ballast provided by calcium carbonate is important for the export of POC from the surface ocean. While this idea is certainly to some extent true, it is worth considering in more nuance, for example, examining the different effects on the aggregation and sinking of POC of small, non-sinking calcite particles like coccoliths and large, rapidly sinking calcite like planktonic foraminiferan tests. We have done that here in a simple experiment carried out in roller tanks that allow particles to sink continuously without being impeded by container walls. Coccoliths were efficiently incorporated into aggregates that formed during the experiment, increasing their sinking speed compared to similarly sized aggregates lacking added calcite ballast. The foraminiferan tests, which sank as fast as 700 m d−1, became associated with only very minor amounts of POC. In addition, when they collided with other, larger, foram-less aggregates, they fragmented them into two smaller, more slowly sinking aggregates. While these effects were certainly exaggerated within the confines of the rolling tanks, they clearly demonstrate that calcium carbonate ballast is not just calcium carbonate ballast – different forms of calcium carbonate ballast have notably different effects on POC aggregation, sinking, and export.
Highlights
The export of particulate organic carbon (POC) to the deep ocean prior to its oxidation back to carbon dioxide (CO2) plays a critical role in the control of atmospheric concentrations of CO2 (Broecker, 1982; Kwon et al, 2009)
This small fraction of material escapes from food webs efficient at retaining and oxidizing organic matter in the upper ocean, largely through the formation and sinking of aggregates larger than 1 mm, including those made primarily of phytoplankton and detritus (Alldredge and Silver, 1988; Kiørboe, 2001; Turner, 2002; Boyd and Trull, 2007; Guidi et al, 2008; Burd and Jackson, 2009; McDonnell and Buesseler, 2010; Ebersbach et al, 2011)
In Tank 7, the foraminiferan tests (Foram) tank which was counted, there were 59 tiny aggregates that consisted of a single foraminiferan test plus a small amount of particulate organic matter
Summary
The export of particulate organic carbon (POC) to the deep ocean prior to its oxidation back to carbon dioxide (CO2) plays a critical role in the control of atmospheric concentrations of CO2 (Broecker, 1982; Kwon et al, 2009). Only a small fraction of the carbon fixed into POC in surface waters is exported below 1500 m and excluded from exchange with the atmosphere for time periods longer than a few hundred years (Suess, 1980; Martin et al, 1987; Antia et al, 2001) This small fraction of material escapes from food webs efficient at retaining and oxidizing organic matter in the upper ocean, largely through the formation and sinking of aggregates larger than 1 mm, including those made primarily of phytoplankton and detritus (Alldredge and Silver, 1988; Kiørboe, 2001; Turner, 2002; Boyd and Trull, 2007; Guidi et al, 2008; Burd and Jackson, 2009; McDonnell and Buesseler, 2010; Ebersbach et al, 2011). Despite being denser than seawater, typical mineral particles in the ocean are too small to sink on their own; their export is dependent upon incorporation (with particulate organic matter) into aggregates larger than 1 mm
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